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Community and Q&A

How thick an SIP should I use?

GBA Editor | Posted in Green Building Techniques on

I’m designing a house to be built in VT just north of Stowe. We had started out considering log homes but after investigating more now think Timber Frame is a better way to go because of insulation considerations. We’re looking into using SIPs (EPS, polyurethane or Neopor) outside of the frame but haven’t ruled out a spray on approach using low density polyurethane or cellulose.

The house will be in an open field and we plan to site it to maximize passive solar absorption by facing the high ceiling great room which will have a 24ft wide full gable window wall facing south. The house is basically a dormered cape with a bump out on the south side for the great room which makes it have a center part that’s approx 24×32 with wings on either side that are approx 15×24. So the footprint is about 1500sf plus or minus. The wings will also have windows on the south as will the upstairs dormers.

We are looking into the best way to heat the place above and beyond the passive solar. We considered electric radiant subfloor and solar PV but are leaning against this because solar PV is just too expensive still. So we’re thinking of doing a hydronic radiant subfloor system with solar thermal. Even though we think we’d need at least 300sf of solar thermal and probably about 1000gal of water for the thermal mass heat sink it’s still cheaper than solar PV. Not sure about whether we’d use propane or some type of electric coil heater for the backup but we will have a wood stove in the great room and are considering using a type that can heat air in a manner that it can be blown through ducts to the upstairs rooms which will have walls bordering on the great room.

We’d have a separate solar thermal domestic HW system of around 30sf for a 50 gal system probably using a Superstor tank with electric backup.

I’ve read a couple of articles about passive solar designing and they all seem to call for R values of R-45 or so for the walls and over R-50 for the roof. In order to do that with EPS SIPs (which is what almost every timber frame place in VT I’ve talked with uses) we’d be looking at 12 inch thick walls.

The catch with using 12in walls is that unless I go with a slab foundation I’d have to do something like use pilasters on the foundation walls in order to accommodate the extra thickness of the SIP and still make sure that the posts of the frame sit over the top of something that can support them. I’ve discussed this with the excavator I’m thinking of using and he didn’t see it as much of a problem.

However when I’ve discussed this with the timber frame folks they all seem to think that this is over kill and have been trying to steer me away from this. They all use 6in SIP standard which gives R values in the mid 20s. I My impression is that they’re just resistant because it’s not the way they usually do things and they’re not really familiar with designing a house that can minimize or eliminate fossil fuels.

So my question is does it really make sense to go with the extra thickness for the SIPs (or other insulation)?

We’re willing to spend a little more $ to try to really reduce the need for fossil fuel heating so in that regard the extra insulation doesn’t need to reach payback, but we don’t want to spend what could be a lot of extra $ to go with super high insulation values if it really won’t provide much return at all.



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  1. wjrobinson | | #1

    I know two timberframe builders. They both use sheet foam in layers outside the frame taped. The homes are quite tight because of this. But they don't try to get to R values equal to what you want. Passive House does, thick walls with cellulose and or foam will. Foam is expensive. Going to say R- 40 to R-60 in foam is very expensive.

    Martin or someone can point you to the many discussions here that you may find interest in.

    One had to do with when does it make sense to use PV energy verses higher insulation levels.

    Did a search of "solar" didn't find exactly what you want to know... but this post is interesting.

    A Contrarian View of Passive Solar Design

  2. GBA Editor
    Martin Holladay | | #2

    Your instincts are good. Of course it's better to build R-40 walls than R-21 walls. If you can afford the thicker foundation and thicker SIPs, that's the way to go.

    Anyone building a new house in Vermont with walls that are less than R-40 and a ceiling/roof that is less than R-60 is being short-sighted.

    Don't worry about ducting your wood stove heat. If you build a very tight envelope (insist on blower-door testing, and set a goal in your construction specs), and if you have R-40 walls and an R-60 roof, your upstairs rooms will be warm enough, even with just a woodstove in one downstairs room. (Unless, of course, your house is very large.)

  3. Brian O' Hanlon | | #3

    Martin, Jay,

    When I come across a question such as this one - it always throws back onto that whole long debate about the windows as a part of the entire assembly of a good house. If I have time I will convert the table I referred to here, into Imperial R-values.

    But for any designer/builder looking to go ahead with a project at the moment, it is important to have an idea of where the cost balance between window and wall expense is going to end up - and how that impacts the performance of the envelope.

    I don't know the climate in Vermont, and how that impacts on the design. But there is also those issues about the price one is willing to pay for window frames, and to get best SHGC's and so forth. As I suggested in this item,

    To look at the geometry of your building form. Perhaps one might be able to start with the cube form, and then compress it down - so you can add very high R-values for a larger roof and floor area - and then the wall performance isn't as critical. Then elongate the building form on the east-west axis as I said, and begin to work on costings for the windows.

    There are probably a couple of different options to get where Jay wants to go. But choosing the right option may depend on the choice of floor, wall, roof and windows. Choosing the easiest root to fruit, and the best for the climate zone in question in Vermont. A whole lot of legwork and consideration there to be weighed up.

  4. user-626934 | | #4

    I'm a bit biased, but if you really want to get everything "dialed in" in order to optimize both performance and economy, then you should consider working with a good energy consultant who can run an energy model of the many different construction permutations you might be considering.

  5. Jay | | #5

    Thanks for the info. Interesting article from Peter Powell. Now re-thinking extending the gable overhang. Also his indication that you can be off from facing due south and that he recommends being just west of south works well for us because the best mountain views from our lot are to the SW.

    You wouldn't happen to know how to get a hold of Peter. I googled him and tried a White Pages search and can't find any info on him. Just a listing in the AIA directory with no contact info.


  6. Brian O' Hanlon | | #6

    John Semmelhack says,

    I'm a bit biased, but if you really want to get everything "dialed in" in order to optimize both performance and economy, then you should consider working with a good energy consultant who can run an energy model of the many different construction permutations you might be considering.

    My own training many years ago was in architecture, and we never quite got to the stage for residential, where the mass market felt it was worth obtaining the services of an architect. It happened moreso in the 2000's here in Ireland as the value of land and buildings constructed on plots shot through the roof. Ironically, it was when property became most expensive, that people here found themselves most willing to invest a few extra Euro in the services of an architect.

    The crucial point being, that when homes retailed on the market at €500K, then the €25k the architect commanded in fees up to design and permit stage, was considered only a single figure percentage. Today, as the value of residential has halved and more, the services of an architect cost exactly the same, and have become a double digit percentage of the value again. Ironically, the architect is finding a lot less commission work in residential, as a result.

    I again, and I would emphasize the need for a good energy consultant. In Ireland, the energy consultants are often brought in by the contractor, after the design work has been finished and the tender price has been agreed. The contractor has guaranteed a certain performance to their client, and it is the contractor who will hire the energy consultant.

    The good thing about that arrangement, is there may not be any architect driving the construction phase of the project. So it allows the contractor a good degree of flexibility to attain high performance levels on the energy conservation side of it. My own concern, is that often, when the architect is driving the construction phase of the project, more emphasis may be placed on internal finishes and visual aspects. My suspicion is that many architects will skew the budget in favour of island kitchens and fancy light fixtures.

    It has come as a reality check to many architects to have to consider thermal performance, construction durability, air quality and interior comfort levels. I salute Mr. Jay and his effort to achieve the standards he is talking about with the fabric of the building. It may well be, that with the right approach to construction management and collaboration, that a very high standard is obtainable - that will repay handsomely to the bottom line, and environmentally, within a short space of time. Best of luck.

  7. Jesse Thompson | | #7

    The SIP builders are most likely trying to steer you away from R-40 to R-60 SIPs because they are worried it will get too expensive for your budget. That's why we build our R-40 to R-60 houses out of other systems than SIPs, because they are usually correct...

  8. Jay | | #8

    Hi Jesse,

    what type of building of building systems do you use?

    The SIP builders I've been talking with weren't trying to steer me away from doing SIPs over R40, they were steering me away from anything over 6in EPS, which at R 3.6 or so per inch I believe comes out to only the low 20s.

    I suppose at this point I should step back a moment and say that if the materials were priced linearly and there was a linear relationship between their performance and their price, i.e. a 12in EPS SIP was twice the cost of a 6in EPS SIP but reduced your heating costs by 50% over a 6in EPS SIP, then it would really be a matter of considering the extra cost up front knowing that you'd recover it all over time.

    However it seems to me that the cost curve versus performance (where by performance I don't really mean R value but overall reduction in heat loss) is more like an asymptotic in that the performance increases flatten out at some point despite spending a lot more for higher end materials.

    We're not averse to spending more to go with higher levels of insulation, windows that do a better job of letting passive solar heat in but not out, siting the house to take advantage of passive heating, etc.. even if we don't see a dollar for dollar return on the savings in heating costs. So for example if spending $10,000 more produced only a $7500 return in heating costs but lowered our fossil fuel usage and CO2 production that's not unacceptable. But if the $10,000 only produced a return of $2500 in heating cost reduction then you're getting into a point of diminishing returns.

    I'm convinced that a lot of the builders and affiliated designers we've talked to so far that build with SIP and Timber Frame construction deal mostly with customers that want to keep their building costs in line with the going rate. So they're choosing materials that let them build homes that are more energy efficient than old fashioned 2x4 stick construction while still trying to be cost competitive with the prevailing construction prices in their area.

    I'm equally convinced that it's possible for us to go further than what those builders are doing without driving the cost of building up dramatically. While the passive home guidelines I've seen call for R45 walls and R60 roofs it's not a requirement for us to go to that level. We know there are lots of ways to build a house that can minimize heating costs the challenge for us really is to try to find out more about the materials and find the price point that lets us get the most efficient home without getting too far down the slope of diminishing returns.

    thanks again

  9. Brian O' Hanlon | | #9

    Jesse says:

    The SIP builders are most likely trying to steer you away from R-40 to R-60 SIPs because they are worried it will get too expensive for your budget. That's why we build our R-40 to R-60 houses out of other systems than SIPs, because they are usually correct...

    For your own interest, the SIP technology started to arrive into the British Isles circa 2006, when the energy regulations for buildings pushed it, so that achieving compliance with traditional masonry techniques became difficult. The traditional masonry techniques were great for durability, but along the dimension of energy performance, now appear to be dated. The SIP panel took a significant amount of risk off of the contractor's shoulders. The contractor is able to plug into a high output industrialised building materials process, which assures him he more than complied with energy conservation regulations. In plain language, it made the pain go away. Of course, by keeping depths of panel down to six inches, it hardly affected his usual building process at all - the same as if he was still using a masonry inner leaf.

    But the funny thing is, the passive house designers I spoke to in Ireland, did not express the greatest degree of confidence in the SIP construction technique for exactly the same reasons Jesse Thompson indicated. Many of the passive designers and builders here in Ireland, bi-passed the SIP frame alternative, and worked with other methods. It is no skin off the SIP manufacturer's nose, as they are still doing well through the larger contracts of 100-200 homes at a time. SIP framing certainly does have it's niche over here and I would not be reluctant to use them on a large job.

  10. Brett Moyer | | #10


    There are many ways you can achieve a R-40 wall assembly:
    - Double Walls
    - Larsen Truss Walls
    - I Joist Walls & other engineered lumber walls
    - Spray foam shell encased walls
    - Exterior rigid foam sheathed walls
    - SIPS

    Check out this Fine Homebuilding Article (you may have to have a subscription):

    I have ran cost comparisons for many wall assemblies. For new construction, double-stud walls usually win the cost battle. It is, however, extremely important that you hire a builder who understands air sealing.

  11. Brian O' Hanlon | | #11

    Brett, thankyou for that run-down list of methods. It's nice to gain the input of someone who is really familiar with the building processes in housing. Thanks again, B.

  12. Jay | | #12

    yes Brett, thanks for the pointer to that. It was a nice summary of designs.

    Turns out I'd actually thought up the double wall idea on my own. I was basically thinking of trying something like going with the standard 6in SIP that the contractors are used to using and then building an inner 6in SIP wall in between the 8in deep timber posts. That would get finished with tongue & groove pine boards with finish molding along the top of it where it joined the posts. This would leave the inner walls flush with the inside surface of the posts. Alternately I thought about going with log siding and covering the posts over. Either way it would give it a log cabin type of look and have the added benefit of creating space for a wiring channel.

    When I floated that one by the contractors and designers I've been considering working with they also pooh-poohed it....

    I need to do some cost comparison on polyurethane SIPs. I know that somebody in or near VT makes them. If the price isn't too outlandish for them then 6in polyurethane SIP on the walls and 8-10 in on the roof might be the way to go since all of these guys are used to working with materials of those dimensions.

    The other quandry I have is spec'ing the heating system. I'm assuming that in VT even with R40 walls and an R60 roof with 15-20% window surface area on the south walls and stone tile floors (probably blue or green vermont slate) I'll need to put in some kind of heating system. My plan is to go with solar thermal and hydronic radiant subfloor.

    I've read that for solar thermal you need to size the collectors at about 20% of the building's footprint. That's around 300sf of evacuated tube collector or about 10 banks of 30 tubes. There's several grand right there....

    On top of that I figure I need thermal mass to store the heat collected in the day to heat the water in the radiant subfloor at night. The question is how much?

    I read one article about a guy in Amherst, Wisconsin that put 2ft of sand below his slab floor, which is a huge thermal mass. He runs his solar thermal in late summer to heat that up so that then during the heating season the heat he collects sort of tops that off on sunny days to average out the lack of heat coming into the whole system on sunny days. I discussed this with my excavator and he thought it was doable but might be sort of pricey to get that much sand trucked in.

    We want to do a full foundation with a two car garage in it because as a home beer, wine, and cider maker as well as hardy New England type that makes lots of preserves we want some cold storage space. So that means no slab to serve as at least a partial heat sink. The foundation would be unheated and have a 2 car garage.

    For the heating system on the other floors we'd put subfloor pex on the floor above with insulation below it. I might go with baseboard or electric radiant subfloor in the rooms over the garage.

    The thermal sink would have to go in the basement. We're currently thinking of either a septic tank installed in an insulated enclosure (relatively inexpensive and readily available) or just having the concrete guys cast a square cistern with insulation in it into the basement floor that we can put an insulated top over. I also considered an indoor fish pond. We have an outdoor one at our current home that's over 2000 gal.

    If we located that in the great room and used a black liner like I have on my pond it would absorb solar heat directly (my pond definitely does) as well as serving as a great coffee table (if I put a glass top over it) and adding some serious ambiance. That's a lot of weight to put on main deck floor though and I think we'd need to put more heat into it than the fish could handle in order to have a high enough delta T to transfer the heat out of it later at night.

    Dreaming aside, the big question is how much solar thermal collecting capacity and thermal storage mass do I really need for a 2400-2500sf passive house built to the R40:R60 insulation spec?

    My current house, built in 1949, is 2400 sf and has never used more than an average of 9 therms of natural gas a day to heat in the coldest months since we put in our Buderus high efficiency condensing gas boiler with the outdoor temperature setback controller. We replaced an old oil burner (but left all the existing baseboard radiators in place) with that and cut our heating bills almost in half to around $1800 per year from around $3200.

    Based on calculations I ran using the 900,000 BTU per day (9 therms) of my current poorly insulated house I came up with a figure of 1800 gallons of thermal mass. Given the big difference in insulation between my current house and what we plan to build it would seem that this is definitely too high. I was thinking 1000 gal might be more reasonable, but I'm not sure if that might even be too high.

    While I intend to over-design the system a little (and probably put in a backup heating system just in case) I don't want to massively over design it since that would be a waste of money.

    Does anyone here have any experience for how much heating capacity I might really need to supplement the passive heating given a house designed to the R40:R60 spec and located in a cold climate like northern vermont?

    Thanks again in advance for the help. This forum rocks!

  13. Doug McEvers | | #13


    A shot in the dark on your last question. I would say the house you are considering would lose no more than 200 Btu's per hour per degree F. 200 x the design temperature for your area would be a rough hourly maximum heat loss in Btu's. Much depends on the window quality and your infiltration rate target.

  14. wjrobinson | | #14

    Jay... you could read here for weeks..... and get yourself so full of ideas that your head splits open.

    Suggestion. Hire someone to design and build your home.

    There are many who could do a worthy job.

    Trying to figure this all out by asking a million questions here will not do it. But you will be able to at least ask some great questions as you search for designers and contractors.

    You need a budget.
    You need a timeframe.
    You need an action plan.
    Square footage desired.
    A location to purchase, price, orientation available.
    etc etc etc

    What you can do now;

    Start a notebook.
    Write a preliminary outline, objectives, ideas, wants verses needs, wife verses you... and lots more.
    Take a look at your bank roll and monthly available to pay all costs.

    Paying for an average of R-50 in urethane over timberframe complete is going to cost what... $600,000 for your dream home? Have you asked... for complete costs with finished payed for land, driveway, landscaping, well, septic................?

    just some quick thoughts... hope I am helping.

  15. Jay | | #15

    I already have the lot and it's perfectly situated for placing a passive house on it. It's a wide open meadow with south and west exposures. I have a timeframe. I want to turn dirt next spring and try to have the place usable by the end of fall 2011.

    I have a ballpark budget, but it's not set in stone. That's because we're willing to spend more *IF* we think that building a passive home is feasible without it breaking the bank. I've seen several designers and builders say that a passive home can be built for only 10% more than a standard one. Typical costs for standard construction run around $100-150 per sf where I'm building.

    Basically I've found that the problem with stating a budget up front to builders is that they'll always find a way to spend every $ of it. This is especially true if they think you don't have a clue as to what it is you're doing.

    I prefer to try to work from the ground up. I've served as the GC on several major additions and renovations to my current and past homes. On a few I've done quite a bit of the work myself especially when various contractors proved to be unreliable. One electrician left me in the lurch and didn't return calls for 5 weeks after doing the rough work and as a result held the whole project up by almost two months. I'd hired this guy 4 times before and he'd always been reliable and done good work. Go figure...

    Others have just proved clueless, telling me certain things couldn't be done the way I wanted them to get done because they were just too lazy to be bothered. The way one plumber in particular got himself fired was memorable. I ordered a jacuzzi tub and stall shower pan through him and was having him install it. He told me the tub would take 4-6 weeks to come in and the pan 1-2. When the tub showed up there was no pan in sight. He then told me that would be another 1-2 weeks because it was out of stock. When I told him that was unacceptable he said he'd tried to find one. He showed up the next day with one that was damaged to the point of being unusable. So I called up the supplier he was using (JW Webb) and asked about the order and found out that they only first placed the order for the shower pan the day after the tub got delivered. In other words he lied to me. The final straw was that even though I'd made it clear to him that I was using a glass and chrome wall mount sink and wanted him to use chrome fittings for all the visible drain parts he installed them with PVC sticking out of the wall. When I asked about this he said what I wanted wasn't possible which I knew wasn't true. So after going down to the local ACE HW and finding all the fittings I asked him to use I fired him on the spot because I was tired of paying someone who just didn't want to be bothered with actually having to do the job I was paying him over $125/hr for. I've since found a much better plumber but unfortunately I'm not building the new house anywhere near where I currently live.

    On this project I considered doing a bit of the work on the shell when it was initially going to be a log home. When we learned about passive design and decided to change our plan to go with a timber frame and what will most likely be SIPs outside it I recognized the importance of having someone experienced do this. I'll probably be doing most or all of the interior finish work myself.

    So far, without looking too hard, I've found over a dozen different timber frame builders that work in the area I want to build. As I've indicted many of them seem to not want to be bothered with doing something a little different than the way they usually do things. Had I listened to them without looking further into what's involved in building a passive home and asking the types of questions they'd have talked me out of even considering the possibility of building to the passive spec.

    Thee guys I've spoken with don't do complete turnkey. They only want to erect the frame. Some will also put up the SIPs if you buy them from them. One or two will do the tongue & groove under the roof SIPs for extra $. They specialize and want to do their part of the project and then move on. And since it doesn't seem like many have any experience building to the passive spec it doesn't appear to be possible to find someone that can give me a complete cost to do something they've never done before.

    So I'm lining up an excavator for the septic, access road, driveway and utility trench. I'll be getting a well guy. I'm also looking into the contractor to build the main deck and windows. Then one other to do the roofing. Basically as far as I can tell from he friends we have up that way that's not at all an unusual way to get a project done especially if you want to keep a handle on the cost of the project. There are a few turn key builders, though none that do passive spec construction, but they all seem to gravitate towards pricey, high end projects usually for people with unlimited budgets in and around Stowe itself.

    So the questions I'm asking aren't random. They're very directed towards trying to learn what I need to know in order to choose the right designer and contractors and avoid a repeat of the Joe the Plumber incident. Understanding the design specification, getting a handle on what materials are needed to build that design and how much labor is involved, and finding out what those materials cost and what an appropriate labor rate for the work is makes a big difference in choosing contractors who know what it is that they're doing and I've definitely been taking lots of notes towards that end.

    If you have a specific recommendation as to a designer or builder working in the Stowe VT area that has experience building homes to passive spec then please let me know. So far I haven't come across any so I'm trying to learn what I need to in order to pick one that seems knowledgeable enough to take on something new who I can work with. There do seem to be one or two of those...


  16. GBA Editor
    Martin Holladay | | #16

    My own two cents:
    1. Forget the sand bed. In spite of the existence of a few enthusiastic proponents of the sand bed schemes, monitoring data from these houses show that the sand beds never perform the way the builders claim they will. The sand beds certainly never get hot enough for seasonal heat storage.

    2. Forget the solar thermal system for space heating using a huge water tank. While these systems work (unlike the sand bed systems), the money they cost could be much better invested elsewhere. The real tragedy, for me, is to see a $20,000 solar thermal system installed in a house with double-pane windows. Makes me want to cry. Buy the triple-glazed windows first! After all, here in Vermont, we basically get no sunshine from November 1 to February 10 anyway. (I'm exaggerating only slightly. Sometimes we get three or four days each month with one or two hours of useful sun each day.)

    3. If you want low energy bills, concentrate on building a very tight, very well insulated envelope with the best triple-glazed windows you can afford. And make sure that your south-facing windows have high-solar-gain glazing, not the low-solar-gain glazing that is being touted by lazy window merchants from Florida to Minnesota.

  17. Brian O' Hanlon | | #17

    @ Martin,

    To what extent should we discount the hybrid approach in certain climte zones? Here in Ireland in a temperate oceanic type of environment, where we don't reach the minus 40 degree Celsius (what is minus 40 in Fahrenheit scale?), we have had some success with hybrid passive house construction. Bill Quigley of Nutech in Ireland published a good article on his system in Construct Ireland magazine. I know that Jonathan Jennings of Kingspan Century homes here (who sell a lot of SIP panels in the UK/Ireland), also came up with a kind of hybrid system. Those are just two kinds of hybrid systems I know of. I just said I would mention them in passing, in case any of your readers want to investigate what is going on, here in Europe. We don't have any sand bed type of dwellings being built here, but some of the solar panel with air distribution hybrid types are achieving passive house certification, unless I am mistaken. In any case, they are no flash in the pan.

    In Ireland we have seen solutions which combine solar panels and passive house type of construction. It is not the purist approach to building a passive house, but meets the same standards with regards to energy consumption per meter squared and so forth. Many of them use solar panels, and distibute the heat from the solar panel via an air heating system. This means, that you can achieve passive house type of performance, where you don't have the ideal orientation on your site. The designers of these kinds of systems claim also, that you can back away from the frontier of installing high performance windows, demand high levels of air tightness and require large areas of glazing. These hybrid systems are quite interesting, but in fairness they are as much mechanical as they are passive. Someone's joke recently about the windows being part of the mechanical contract in a purist passive house spring to my mind.

    Jay's suggestion to link up the solar panels with a radiant floor water heating system is a different type of option, to that where air is the distribution medium. I agree, it is a lot of expense for a system such as that.

    @ Jay,

    Thanks for sharing those experiences about working with various contractors by the way. I would really like to see anecdotes like the above compiled into some kind of blog site. A bit like the one which Ty Newell set up for his Equinox house in Urbana, Illinois. I can see Jay that you are very experienced in dealing with site conditions. It is nice to hear you articulate those experiences in writing. Best of luck, B.

  18. adkjac | | #18

    Jay.... welcome to the world of a GC. Sub troubles can be your biggest nightmare as a GC. Some GCs have way less trouble than you are posting about. Some more. When I find a worthy sub it makes my day for weeks... a get a forever smile.

    The best around here are busy, medium priced, organised, steady hard workers with even tempers and good home lives. My best and favorites are worker owners.

    Do you have an email Jay you would post?

  19. Kevin Dickson | | #19


    Your comment about triple glazing caught my attention. Can you steer me to an affordable supplier of high SHGC triple pane windows?

  20. GBA Editor
    Martin Holladay | | #20

    -40 F = -40 C
    Easy to remember.

  21. GBA Editor
    Martin Holladay | | #21

    Thermotech Fiberglass, Inline, Accurate Dorwin, or Duxton.
    These are Canadian manufacturers of windows with pultruded fiberglass frames.
    Read these articles:
    Choosing Triple-Glazed Windows
    Passivhaus Windows
    Serious Windows vs. Thermotech, Fibertec, Inline, Accurate Dorwin

  22. Brian O' Hanlon | | #22

    -30 F = -34 C
    -20 F = -29 C
    -10 F = -23 C
    0 F = -18 C
    10 F = -12 C
    20 F = -7 C
    30 F = -1 C
    40 F = 4 C
    50 F = 10 C
    60 F = 16 C
    70 F = 21 C

    Okay, I think I have a handle on this Fahrenheit business now. Anything below 30 F in Ireland is considered a show stopper. The entire country simply grinds to a halt, and no body leaves their homes. Well that is an exaggeration - but surfice to say, we aren't set up to deal with cold conditions. That is why our building standards and codes were always so easy to meet.

    Anything above 70 F, is considered overheating of the living space during the heating season, and a waste of energy in our 'Building Energy Rating' software. Somewhere between 60 and 70 F, is considered okay for the non-living area zones during the home-occupied hours of the heating season. If you can tolerate below 70F in your living room, well and good.

    But the trouble with many of our homes here in the British Isles, is that they are currently over-heating to an excessive amount, and thus burning through too much fuel. That is what our energy rating scheme was designed to fix. Though the tax credits and such, have not been set up in anything like the way they do in north America. We still rely on fairly inefficient grant systems to try and stimulate a retro-fitting industry here. There are grants to install room and WH thermostats etc.

  23. Graham Mink | | #23


    I am currently building a very effiicent house in Stowe that will have R-40 walls and R-60 roof insulation. Unfortunately I do not have a million dollar budget so this is not an easy task. I was in the same place you are last November when I purchased the lot that we are building on. Being new to the process I spent a lot of time looking at all of the different technolgies and possibilities out there. Rather than get into all of that on this forum if you would like to send me an email I could tell you about my personal experience and show you the house that is currently being built. Next week the roof will be going on. You can reach me at gmink21 (at) homail (dot) com. I wish you luck in your project.


  24. Graham Mink | | #24

    that should be hotmail dot com. will get you a totally different type of information!

  25. Kevin Dickson | | #25

    Back to the SIP thickness issue:
    Many parts of the country are already at R-48 for the ceiling (roof) as minimum code. A 12" EPS SIP doesn't even cut it any more (R-45 per Murus).

    I did a quick search and found an 8" PU SIP with an R value of 56 from Earthcore in Boulder:

    I like this solution a LOT. I'm planning infill redevelopment in Denver, and the height restrictions can be annoying. This SIP saves the most headroom by far of any other option, and is dead easy to air seal. Cost is unknown just yet, but a SIP can often span the roof without trusses, beams or rafters, causing more savings.

  26. Brian O' Hanlon | | #26

    By the way, climate zone dot com, has a really neat table of weather characteristics for every zone in the world. I am looking here at the Vermont statistics and I will print one for Ireland also, and have a look at them side by side later on. Maybe this is a good way, in which we can have a discussion - while still being aware of our different locations on the globe?

  27. Mark Attard | | #27

    There is another option to obtain the R-value that you are looking for, which is to use polyurethane panels. A 6" panel has an R-value of 42 and a 8" panel has an R-value of 56. Either of these panels can be used for wall or roof application. Urethane panels also have other advantages such as less risk of de-lamination, greater structural integrity.

  28. Brian O' Hanlon | | #28

    @ Mark,

    Good suggestion. Phenolic foam panels are the premium grade insulation panels sold over here in Ireland also. Kingspan Kooltherm range offers thermal conductivity of 0.024 w/m2k, and better conductivity values in greater thicknesses. So when I do the quick conversions for a 6-inch layer of Kingspan Kooltherm, I come out between R-35 and R40.

    That must be one heck of a panel to get to R-42. But when I use the 0.21 w/m2k conductivity figure for a 200mm panel (8-inch), I come out with a good R-52 insulating layer. I think the phenolic foam that Kingspan use is some kind of polyurethane, and I have yet to specify myself. But I know it has gone into lots and lots of retrofits here in Ireland.

    Out of curiosity, what is the conductivity rating on the best polyurethane panels in the US?

  29. Brian O' Hanlon | | #29

    Some conversion factors here btw, in case anyone can source the thermal conductivities of polyurethane panels.

    1.0 Btu in /(ft² h F) = 0.14419346 W/(mK)
    1.0 W/(mK) = 6.935127284 Btu in /(ft² h F)

    1.0 Btu/(ft h F) = 1.730321522 W/(mK)
    1.0 W/(mK) = 0.577927274 Btu in /(ft² h F)

    1.0 ft² h F / (Btu in) = 0.14419346 W/(mK)
    1.0 W/(mK) = 0.14419346 ft² h F / (Btu in)

  30. GBA Editor
    Martin Holladay | | #30

    Polyurethane SIPs have an R-value of about R-7 per inch.

    By the way, Brian, you have the time to share many very long posts. You also ask a great many questions, many of which can be answered by about 30 seconds of Googling. The next time a question pops into your head, you might just try to Google it.

  31. Brian O' Hanlon | | #31

    Martin, points well taken. I do appreciate it.

  32. Jay | | #32

    Hi Martin, Brian, et. al.

    This is an awesome discussion. It's been super helpful.

    Let me explain some thoughts and considerations about this project so folks understand it and can comment as to whether what I outline below makes sense.

    We're initially going to be using the house as a vacation home. When we're there we'll be using the fireplace a bit, but that requires tending. We know there's lots of cloudy days in Vermont, especially in the winter, but even on cloudy days solar thermal can extract some heat. When we're not there the heat requirement is less because we'll be setting the thermostats back, probably to 50F. The one concern we have is that the place stay warm enough when we're not there that pipes don't freeze. Now onto the design consideration discussion. Advance warning, another long post coming with a bit of brainstorming but maybe some of my ideas will be new and/or inspire folks...

    I've been in touch with a couple in north central CT that has built a home along similar lines as I'm thinking (see They went with a 3ft sand bed under a slab. The heating costs they reported for March & April were about 40-45% lower than the costs to heat my comparably sized, poorly insulated (built 1949, renovate mid 70s) current home near Concord, MA where the climate has almost exactly the same number of degree days as that of the Schaller home and is about the same size.

    I was a little surprised that their heating costs weren't lower, but they told me they didn't get their solar thermal system installed until until late January and operational until Feb. That means they didn't have the sand bed getting heated in the late summer and fall. Not sure how much of a difference that would make. I'm personally not convinced about the sand bed (earthbox they called it) approach though.

    Regarding windows, I'm strongly considering the triple panes. I haven't priced them yet, but I have taken a quick look at the Unilux products. I know there are other manufacturers but looking into them is still on the to do list. Thanks to those who posted info about other manufacturers, I'm noting that and will look them up

    The EarthCore SIPs mentioned look like they use a urethane core. In the learning curve I'd found out that urethane has around R7 per inch. That means if I use urethane SIPs I can get the R values I want in a 6in SIP which means being able to build things the way the guys I've been speaking with are used. One home package I was looking at has prices posted right online and quotes just under $100K for the timbers and 6in EPS SIPs all erected for a 3000sf home with a layout similar to what I want. The SIPs are $20K of that. So the question is how much more would the urethane SIPs cost. The guy who designed has a 2400sf version of the same house (which is the size we prefer) that presumably would cost less than they've quoted for the 3000sf.

    Ideally I'd like to find a VT SIP supplier. Seems like shipping across the country runs up the cost and creates CO2. That's counter to the spirit of the thing and I'd like to help the local economy since coming in from out of state and planning to retire there it seems like a way to get off on the right foot. I think there is one in Jeffersonville (R-Tight) but I was a little confused by their website since in some places it mentions urethane panels are available (like their online PDF installation manual) but in others they talk only about EPS. Other SIP manufacturers in the area I've checked with so far only do EPS or Neopor (which is around R5 per inch).

    WRT Solar thermal, I don't think the cost of a large thermal mass + solar system need get up to $20K. My thought was that to create the water thermal storage I could use one of two approaches. Have the excavators building the foundation install a 5-6ft square footprint cistern with a 5ft high wall by building forms with rebar and pouring concrete over insulation in the floor and then insulating it on the outside. If they put lag bolts into the walls I can bolt down a SIP scrap with a rubber sheet as a liner on the bottom to serve as an insulated top. Alternately I can use a 1000 gal concrete septic tank (around $500-700), used dairy tank (it is VT after all so I can probably find one), or plastic farm type tank. If this is built into a strong (6x6 posts) frame and set up on 6in blocking (to create an air gap) over a reinforced portion of the floor it should be fine. To insulate it I'd seal all around with spray foam (a case or two of Great Stuff would probably work for that) or SIP scraps if there are cutouts from the windows. With a system like this I can use something like baseboard radiators immersed in the tank as the heat exchanger. They're available off the shelf at HW stores for not too much $. So I think the thermal storage system can be built for $1500-$2000.

    I've recently bought evacuated tube solar thermal collectors for $200 for a bank of 10 that are 1sf each. Price included the manifold and mounting rack. That's about $20/tube. The guy I got these from has more and said if he runs out I can order them wholesale from the overseas manufacturers for even less. I'm assuming I'd need 20% of the footprint of the house, which I'm figuring would be around 1500sf, so that's 300sf. If I can get them at the same price that's about $6000, less if I go the wholesale route.

    The tubes I bought are spec'd at 174.65 BTU/h per tube. So in the perfect world where I got full performance I'd get about 52,500 BTU per hour. That would raise 1000 gallons of water about 6F (1BTU raises 1lb water 1F and water is 8.35lbs per gal) in an hour. January is probably the worst month due to the low sun angle and cold temps. If I got 4hrs a day of full capacity in that month then I could theoretically get about 24F a day.

    Heat Transfer Products (makes of Superstor) says their tanks lose only 1/2F per hour and they're pretty small. So let's use that as a reference. For a large tank like a septic tank the ratio of surface area to volume goes down as the size increases. Heat is lost through the surface so thanks to the laws of physics the heat loss is lower for a bigger tank. So I'm thinking that using a well insulated septic tank for the thermal tank would lose no more than 1/2F per hour. Perhaps less. Other than standing loss the thermal tank loses heat by transferring it to the heating system. The differential controller would only circulate the heating system fluid through the exchanger when the temperature difference was high enough. So once heated the thermal tank probably shouldn't drop below around 100F. Even at 4hrs a day that should produce enough heat to provide the additional heating needs of a well insulated passive solar house.

    If I use the large thermal storage tank in a drainback type system where it serves as the drainback tank too then the water getting stored in it would get heated directly thus avoiding the transfer loss of having the thermal storage heated by exchange from some working fluid. This is simpler than using a glycol system since it doesn't have to hold pressure which means I can do the plumbing myself. I'd split the feed to the solar thermal collectors since if I put them inline with each other the ones at the end of the line will have low or no delta T and that loses collection efficiency in them. Splitting the feed means the inlet temperature is the same for all units and that maximizes the delta T. I'd probably need a few pumps in parallel.

    My basic plan is to design a system with thermostats, differential temperature controllers connected to pumps and check valves that will draw heat from the best available source depending on the circumstances. The idea is to use a single Superstor SSE hot water tank with a built in electric backup heater. That tank will store hot water that goes to the domestic supply as well as through the radiant subfloor. The inlet to the tank would be well water that would pass through a heat exchanger in the thermal storage tank on it's way in. I also think I'll set it up to pass through the fireplace, but more on that later. The output would split, one side having a one way check valve would go to the domestic hot water lines.

    The other side would go to a loop for the radiant subfloor. A manifold will split the radiant feed so that I can have separate zones each with it's own pump controlled by a thermostat. This should be feasible because radiant subfloor (or even baseboard for that matter) shouldn't need the temperature of the water being sent through it to be any hotter than what I'd want for domestic hot water. The feeds would combine on the return and the return line would be passed through a heat exchanger inside the thermal storage tank. That way the water that returns to the Superstor will get heated up so it won't make the built in electric backup kick in inadvertently. While this loop is running it would serve to draw the heat the radiant floors need out of the thermal storage tank without degrading the amount of hot water available. In the worst case I might need to add some low capacity electric point of use heaters in the kitchen and bathrooms to supplement things.

    I'm still thinking through how to incorporate the fireplace into all this. It's going to be located in the middle of the house. Most likely a brick box built around a low or zero clearance wood stove and sited over a reinforced part of the floor with a wood stove type chimney pipe. That keeps the weight down, should be cheaper to build, but still provides some thermal mass to hold heat. If I get something like the high efficiency one built into my current house it can do a good job of heat the air which can be combined with a ventilation system so that it heats the entire house. I also think I can build an old timey baking oven into it like the colonials had, which the wife will like. It is New England after all.

    I think the fireplace would be used only to pre-heat the domestic hot water inlet. I'd actually route the cold incoming well water through the fireplace first before routing it through a heat exchanger in the thermal mass tank. If the fireplace isn't in use it's not likely to be colder than the incoming well water and won't have much effect on the temperature of the room. If the fireplace is in use and it heats the water hotter than that in the thermal storage tank then it will passively transfer heat into the thermal tank, at least when the domestic hot water is being drawn on. If the fireplace isn't in use or doesn't heat the incoming water as hot as what's in the thermal storage tank then the inlet water will draw heat out of the thermal tank on its way to the Superstor.

    This should work because when we're in the house and the fireplace is running it will heat the air in the house and the radiant heating system shouldn't kick on much if at all. But when we're in the house we'll be using domestic hot water and the fireplace can supply that, especially for cloudy days. It won't have much effect on the thermal storage tank but could give it a little boost. The electric heater built into the SuperStor SSE would back this all up.

    One other thing I'm considering is using DC circulation pumps run off solar PV for the solar collectors and radiant flooring system. Combined with a battery backup charger hooked into the AC system to keep the batteries topped up when the power is on even if power goes out for a bit between the solar PV and the power stored in the batteries the heating system could still operate. Because I plan to split the lines to feed the collectors in parallel I expect to need a few pumps, especially if I do go with the DC pumps since I understand they don't have us much oomph to them. Not sure if this is overkill as it might not be very helpful if there is little or no sun and without AC the electric backup heater wouldn't be operational since it needs more power than can be generated by the amount of solar PV I'm willing to buy right now.

    In summary I think that with careful of the control systems and plumbing I can have a "tiered" system consisting of passive solar, supplemented by solar thermal for heat and DHW, backed up by wood heating which is also designed to cover the DHW needs, and finally the backup heater built into the SuperStor tank to serve both heat and DHW needs in case all else fails.

    At first thought this may sound pricey, but I don't think it will be.

    I spent $12K to replace my old oil burner in my current house with a high efficiency Buderus condensing gas boiler. That included taking the old boiler and tank out as well as installing an indirect hot water system using a Superstor tank. The Buderus boiler itself was around $5500 of that cost though. Even a standard gas boiler is probably going to be $1-2K.

    I think the solar thermal system would be close to a wash, or not too much more, when compared with the cost of a conventional heating system since the radiant sub floor, pumps, etc. would be needed for that anyway. The lot is fairly large and 3/4 wooded, so I'm going to also be putting in a fireplace or wood stove located in the center of the great room. I intend to design this so it can heat water with it as well as doing passive or forced hot air to the upper floors and radiant within the room itself.

    This design completely eliminates the need for a traditional heating system which costs several thousand dollars. A SuperStor SSC-80SE (with integrated electric backup) runs under $1500. The Phoenix (which have an integrated gas fired backup) are quite a bit more $ and I'm not really sure why because for standard hot water heaters the price difference between an electric system and a gas one are not very big. So I'd probably go with the Contender with electric backup.

    By using the solar thermal mass tank as the drainback and running the water in it directly through the collectors I won't need a separate drainback tank. I also don't have to plumb for high pressure like with a glycol system. The collectors being ground mounted and the tank being in the basement means that the head would be less than 10 ft for how we're planning the site. For the fireplace diversion loop to pre-heat the inlet water no pump is needed and I think I can design a very simple but effective heat transfer approach (like a baseboard radiator operating in reverse) with very low cost. I'd need a few differential controllers and some extra pumps for all this but the prices on them are not too bad.

    So I think that not including the costs of the radiant subfloor installation, which I'd have no matter what the heat source would be, I can keep the cost of this design at or below $10,000 and provide close to all my heat and HW needs. Taking into account that this design eliminates a traditional heating system the extra up front cost of doesn't appear to be that bad although it doesn't include the extra cost of the insulation upgrade to R40:R60.

    This all assumes there isn't anything important I have failed to consider, but on the whole I actually think this is a relatively straight forward design approach. The floor is open for comments :-)...

    BTW Thanks to Brian for the pointer to climate-zone. Useful info there that I'd been meaning to google.

    PS Those wanting to reach me direct can use the user name ecohome at the domain doctorbeer dot com (you'll have to edit it for the proper format but doing it this way reduces spam bots at least a little). This is a temp forwarding email I created on my domain so I'll likely nuke it if spam starts piling up but at least for now you can try me at that address.

    PPS I'm also tinkering with designs to run a low temperature differential stirling engine off of a solar thermal collector and use it to drive a generator to create electricity. That experiment is in the early stages but if it works then it can make electricity in the summer....

  33. Riversong | | #33


    As your posts reach book length, you're going to get fewer responses.

    Here's my advice from 30 years of designing and building super-insulated, healthy passive solar homes:

    If you want an affordable house with a responsible ecological footprint, forget the timber frame and the SIPs envelope. Those are both very expensive systems with large resource footprints.

    If you want an efficient, durable and healthy home use a double-frame envelope insulated with dense-pack cellulose.

    Separate the garage from the house to avoid fire and toxic exhaust issues.

    The best passive solar designs are the simplest. Use a slab-on-grade for thermal storage and avoid excessive glazing. Keep the envelope geometry simple and design appropriate overhangs for solar shading (as well as weather protection). Vent the roof and use the air-tight drywall approach.

    Don't overthink your mechanical systems - complex systems are costly, rarely work as expected, and have many more potential failure nodes.

    A "passive" or "passive solar" house should be just that - a shelter that functions without active intervention, sophisticated mechanicals, or unnecessary energy inputs.

  34. GBA Editor
    Martin Holladay | | #34

    It seems you've got your own agenda, but I'll repeat my advice:
    1. Move the triple-glazed windows from the "maybe" list to the "must have" list.

    2. The sand bed is a waste of money. If it includes PEX tubing and a circulating pump, it will also waste electrical energy.

    3. The solar thermal system you describe will easily cost more than $20,000 once it is installed. Large rubber-lined concrete tanks installed in basements will leak. When they do, draining the tank and patching the rubber is a huge pain. Many people have walked the path that you think you are inventing, and they have lived to regret it.

    4. When solar thermal equipment dealers tell you that their systems can collect useful energy "even on cloudy days," they're talking about partly cloudy days in July. In California. NOT cloudy days in December in Vermont, when the sun barely rises above the treetops at noon and the clouds hug the earth like a great looming gray rug for months at a time. (And when your solar collectors will be covered by snow.) Ain't no solar energy there, bub.

    5. If you expect your house to stay above freezing when you're gone, it can be done. It's challenging, but it can be done. You will succeed not because of a solar thermal system, which won't provide any useful thermal energy when you need it most, but only if you do a superb job of air sealing and if you install a superinsulated envelope. Remember, the thermometer can be below zero for a week. Even at noon.

  35. Kevin Dickson, MSME | | #35

    There's a company in Boulder throwing a few million in R&D at the evac tube/Stirling Engine concept. Nothing readily available yet, but I would suggest letting them learn all the painful lessons first.

    It's a seductive idea because it theoretically has a better ROI than PV right now. It's a dead end, however, because PV will soon be cheaper, and will always be more reliable with less maintenance.

    Speaking of simplicity, when you build a home with a great envelope, there is very little money to be saved with highly efficient mechanical systems. ( for example how much does a 98% boiler save compared to an 86% boiler when your total yearly heat bill is only $200?) Therefore, go cheap and reliable.

  36. Jay | | #36

    I had contacted the Boulder company and they said they weren't on the market yet. I was actually going to build something myself. The beauty of displacement type stirling engines is they're pretty simple from a mechanical standpoint. I'm looking into taking the electronics from a standard gasoline generator and replacing the drive mechanism with the stirling engine.

    I've been looking at solar PV for close to 10 years now and I keep hearing how the prices are going to drop dramatically and yet while they've come down some they still run about $5k per kWh. Worse if you want battery backup. And if you're talking simplicity well solar PV isn't really it. You need an inverter and a grid tie connector and with batteries a charge controller too. All complex electronics with many different manufacturers without long track records to judge reliability. Add in the fact that currently multiple panels hook up to a single inverter (rather than each having their own built in) and if the output drops on one panel it drops the output of all the panels using the same inverter.

    Thermal is very simple. The stirling engine design has been around for well over a century. A well built one lasts decades with little or no maintenance. And it can be hooked to an AC generator which avoids the loss on DC to AC conversion. Even if solar PV drops in half I think a stirling engine based generator could still be commercially successful because it's based on combining mature technologies and can be built very cheaply especially when done in large quantities. So I think it's premature to write that approach off.

    My Buderus high efficiency boiler is pretty reliable so far after 2 years but you make a good point about the cost difference. With my current poorly insulated home I did do the cost benefit analysis to determine if the extra price for the higher efficiency was worth it and both the standard gas boilers and the higher efficiency ones calculated an 11 year payback. My current system is running ahead of that because the spread between natural gas & heating oil is higher than my original estimates.

    You make a very good point about a higher efficiency home that uses less energy to start having a cost savings on the difference that is much less and makes the payback time on the cost difference for the higher efficiency unit longer if I go that route.

    thanks for pointing that out. I'll run numbers on that. Cost isn't the bottom line or I wouldn't be looking into this at all. I want to try to do my part to build something for the long term that uses less fossil fuel and makes less CO2, but I'm not approaching it with the need to maximize those goals no matter what the cost so you're right that it is something I should consider.

  37. Kevin Dickson | | #37

    Don't forget that if you don't grid tie the output of the Stirling Engine, you will throw away all the electricity you generate in the summer once the batteries are charged.

    First Solar keeps hitting all their cost goals:
    $1.23/kw in 2007
    $1.08 in 2008
    $0.87 in 2009

    I can see a trend there, whereas the parts and labor for a Stirling engine aren't doing that until someone throws billions at the manufacturing process, not just millions.

  38. wjrobinson | | #38

    Jay.... put your plans together and share the pdf's bro. And when this baby is all up and you have the fire lit... I am all for a visit and a ski nearby. I want to see all this technology and design in person and in action.

  39. Jay | | #39

    Which type of stirling engine are you thinking of? There are several designs. Probably the most well suited for use with solar thermal is the low temperature differential. Take a look at to see what these look like in action. Building an engine like this is ridiculously simple. I've done it at home with parts from the hardware store. Not the engine I'd want to use long term, but it demonstrates the simplicity of it. I think we're talking more like tens of thousands, not billions, to get the engine itself into production, but maybe I'm a dreamer. The low differential engine will work on temperature differentials as low as 40F. The hardest part is the electronics, but it would be really straightforward to adapt this engine to the chassis and electronics of a standard gasoline powered generator and create AC from it which, with a grid tie controller, can be put back on the grid via net metering. My neighbor designs and builds un-interruptable backup power systems for Global 1000 companies so we're going to sit down at some point over a few beers and review the feasibility of this.

    I've actually been drawing plans up. I'm still fiddling with the designs a bit but think we've settled on a basic layout. I'm still trying to decide on roof design, and the SW I'm using has some bugs which make it a pain. After many iterations we keep coming back to the layout shown here ( so the final plan will probably be along these lines. We have to go back to the lot and review the terrain to determine the final siting and that is a critical factor in the layout. The reason is because it determines where the garage goes. The garage's location dictates where the stairs can go. It also dictates the location of the utility room which in turn determines the plumbing layout and thus the location of the bath rooms & Kitchen, which I want to keep together.

    You'll also see a draft of the water and heating systems design. I have been researching the issues with heating water through the fireplace and know safety is a big consideration. Because of the potential safety issues with pressure on the water pipes I'm really planning on using something like the masonry stove shown here ( or

    These types of units (or something built from brick situated around something like this ) are appealing because the wife would like a baking oven in it. I want to forego use of a thermacoil or similar heat exchanger from being in the firebox itself. So something with a big thermal mass and good sized hot air heating chamber which could accommodate the installation of a baseboard radiator in it to function in "reverse" to heat the water seems like a better approach. It doesn't eliminate the need for pressure relief but it's functioning in a part of the stove with lower temperatures should reduce the potential for problems.

    I'd originally considered trying to incorporate the wood stove water heating into transferring heat to the solar thermal mass tank. The thinking behind that is that when we're using the stove to heat the house we're not running the radiant floor heating system. The stove puts out excess BTUs that go up the chimney, so why not try to capture some of that and put it back into the thermal mass at a time when we're not drawing heat out of it?

    The design diagram posted doesn't incorporate that because I'd seen one web site saying that wood stoves just won't work to provide the heat for a radiant floor heating system, but I'm not sure what the considerations for that statement were and need to go back and review. I'd agree that if I was trying to use the wood stove to provide heat to the radiant floor system directly to the fluid circulated through it at the time it was demanding the heat then it probably wouldn't work well. However I think that in a highly insulated house where the stove can provide all the home's heating needs when it is in use that it wouldn't hurt to try to utilize it to move some of that heat back into the thermal storage to reduce a need for it later on. Still thinking through this idea...


  40. GBA Editor
    Martin Holladay | | #40

    You are an optimist. "Building an engine like this is ridiculously simple. ... it would be really straightforward to adapt this engine to the chassis and electronics of a standard gasoline powered generator and create AC from it which, with a grid tie controller, can be put back on the grid via net metering."

    Well, go ahead. Trust me -- people living off the grid have been tinkering with ways to generate electricity dependably for at least 80 years, and all of these homemade backyard gadgets have dependability problems. (I'm old enough to remember the Mother Earth News headlines from the early 1970s -- "Build Your Own Wind Generator!!" Right.)

    However, I'll grant you one thing: it's fairly easy to pull hot water from your wood stove. Put a stainless-steel coil in your firebox, and hook it to a tank on the floor above the woodstove. (This is a thermosyphon system -- no pump required.) Don't forget the pressure-temperature relief valve, or you have just built a bomb. Such systems have no moving parts and are extremely dependable.

  41. Jay | | #41

    Hi Martin,

    I know I'm not the first person to consider a lot of these designs. I do appreciate the healthy skepticism. I've worked on a lot ideas that don't pan out. One has to be optimistic to even try looking into a lot of these things.

    I do think a lot has changed in the last 80 years. In fact even in the last 20 years the availability of materials and knowledge needed to build things has changed dramatically.

    In the 1970s you couldn't buy a small generator inexpensively. Now I can buy a 2000 watt generator off the shelf at Home Depot for $219. In the 1970s I'm not sure you could even buy evacuated solar tubes that can heat water to 165F at all. Now I can get 10 of them with a rack and manifold for $200.

    Likewise for small generators, power inverters, battery chargers, etc. All widely available COTS now and not that much money.

    Same goes for lots of HW. In the late 70s I had a part time job while in high school working at a local hardware store. They had paint, wallpaper, and some stuff to fix the toilet handle when it broke. If you wanted to buy the types of stuff you'd need to do all your own plumbing work they'd tell you to hire a plumber. Now I can get MAPP gas, plumbing fittings, etc. real cheap in a couple of different stores.

    I've fiddled with a low temperature stirling engine model and built a small one myself. I know that's a limited amount of experience but this design literally has less than 2 dozen parts. My bicycle is more complicated and I've seen plenty of designs as well as kits being sold which people actually use to create power from riding their bicycles...

    I also know some machinists that could build me a prototype with very tight tolerances for a couple of grand or less. About the only parts on this that would suffer significant wear are the bearing races and rods and it would be easy enough to sell a maintenance kit with those parts that a homeowner could use to replace them in a matter of minutes.

    My personal take is that the short coming of the approaches people have taken so far is that they haven't kept it simple enough and I think it's really only a matter of time before someone pulls these things together in a way that is cost effective and makes a commercially available product.

    About the wood stove heating, I'll have to reconsider the thermo siphon approach. The problem with it is the tank needs to be located relatively close to the stove and the floor plan we currently have doesn't accommodate this well. Might need to rethink the floor plan....

  42. user-659915 | | #42

    Martin, Robert et al. have given you excellent advice but you seem intent on following your own path. It's your money and your mistakes to make, but bear in mind that you're only borrowing all the essential resources to build this house from the earth. They're not really yours to squander.

    I'll add one more comment though you'll probably ignore this also. If you rely on a high-mass masonry fireplace to heat what is initially at least a vacation home, when you turn up for a long winter weekend and fire up that baby it'll start giving out useful heat about the time you're packing to leave on Monday morning. Use a stove.

  43. mike | | #43


    i would strongly recommend talking to an architect. your kitchen will be a major fail, for starters.

  44. Jay | | #44

    Hi James,

    I'm not ignoring any advice. For example I have a line on a couple of designers and do plan to hire one to do some consulting on this project. I really do appreciate people's comments but I am also mindful that things can change pretty rapidly these days. What might have been sound advice in the recent past might not be true in all circumstances anymore. For something as complex as a house there is often more than one good approach and advice from even the most experienced experts can sometimes conflict. I'm still brainstorming and haven't ruled anything in or out as final and don't expect to until after having consulted with the designer I decide to go with.

    One place where I've gotten conflicting information (not necessarily in this forum) is that several experienced builders told me that trying to build to passive spec is not worth the extra $. I'd begun this thread to try to find out if that was true. If I'd just gone with the advice of the several people that had told me that without questioning it was still sound and considering other approaches we wouldn't be having this discussion at all.

    I just got what is a reasonably close estimate from one of the builders we're strongly considering for the design we plan to build. I say close because it is based on a preliminary, not final design. The quote was $24.5K for 7-3/8in R26 walls and 11-5/8in R-33 roof EPS SIPs. That cost is just the SIPs themselves.

    I also tracked down a VT Urethane SIP manufacturer just this morning and spoke with them about Urethane SIP pricing. I didn't tell them the prices I'd gotten from the other builder. Based on their base pricing it looks like going with Urethane I can do 6.5in R-38 walls and 8.25in R-50 roof for $32.8K. That's only 1/3 more for 46% higher R value in the walls and 51% in the roof.

    So unless I'm missing something it would seem to me that the cost is worth the benefits of using the Urethane SIPs. Maybe it won't pay back $ for $ but I think at the least I'd achieve some savings that would make it less of a price premium to utilize the higher insulation values. That's in contradiction to advice I'd previously gotten and I'm not sure all reading this will agree that it is worth the cost.

    It's also not the R-40 walls and R-60 roof the Passive House spec calls for, but it's not far off. It is about twice the insulation for only 1/3 more money. And it seems like building with the Urethane SIPs would be cheaper to do than trying to do a 16in I joist roof with dense pack cellulose like I've seen in the passive house designs. That and the sizes on these is consistent with what the builders and designers I've talked to are used to working with and wouldn't require adjusting the design the way thicker walls would.

    So this is basically the type of cost benefit trade-off that I'm exploring and whether or not I follow every piece of advice given here IMO kicking things around has been worthwhile and I do greatly appreciate hearing people's recommendations.

  45. Jay | | #45

    Hi Mike,

    I'm curious as to what you see as the short coming on the kitchen. Is your comment based on looking at the layout I posted on my website? (It's not a final plan)

    This layout was derived from plans of one of the log homes we were originally considering and we saw lots of photos of this layout in log homes that were built and even toured one that used it. Granted that none of those were trying to achieve something approaching a passive design as we hope to.

    What would you change?

  46. mike | | #46

    as a caveat, i'm a minimalist and an extremely picky person. these are just my somewhat-educated but extremely biased initial thoughts...

    workshop won't be effective
    door to tv room should be flipped, oddly proportioned room
    entry not laid out well
    main floor bath not laid out well
    not enough room between kitchen island and sink, kitchen layout isn't efficient
    dropping off groceries to kitchen will be a pain (entry and then around great room, dining to kitchen)
    master bath not laid out well, 2 doors may pose issues, you'll hate the vanity.
    master and 2nd bedrooms oddly proportioned. also, have you cut section? according to the massing models, they look to have ginormously high ceiling if using SIPS w/ hot roof.

    basically, i would say there is a lot of wasted and redundant space, that you could conceivably get away with about 2/3 or less if designed correctly. working with an architect could bring the cost down by utilizing a more efficient layout.

    also, i have major qualms about the height/proportions of the house, especially in regards to the roof.

  47. user-659915 | | #47

    Mike - "basically, i would say there is a lot of wasted and redundant space, that you could conceivably get away with about 2/3 or less if designed correctly. working with an architect could bring the cost down by utilizing a more efficient layout."

    Agreed. Working with a design professional might also help you see the project as an integrated whole rather than a collection of features. And shopping for bits and pieces at this stage will not only make you crazy, you will also leave out some important connect-the-dots that will cost you big-time later on. Sign up early with a reliable contractor experienced in putting it all together. They'll do the shopping for you and they'll do a much better job. Contracting your own house, especially a high-performance one, is about as smart as a DIY root canal.

  48. Jay | | #48

    I know the workshop layout isn't optimal but my current workshop is 11x12 with work benches on opposite sides. If I put the workbenches along one wall then it's effectively about the same working space as my current one. I mostly use it for home repair and tinkering so it's not a critical shortcoming.

    Thanks for pointing out the doors. I need to go over that again and make sure they flip the right way. I will be working with a designer in the not too distant future.

    Our current TV room was once a garage and it's basically the same size. The large screen TV is hung up on the long wall.

    The main floor bath was designed around my wife wanting a closet on the main entry hall (even though I don't expect to use it) and a pantry cabinet off the kitchen. We have our laundry in the basement of the current house but I shoe-horned it into the main floor bath on this design thinking the utility room might eventually have thermal mass storage tank for solar and battery bank for Solar PV. I could swap the entry hall closet and sink but I laid it out this way because I wasn't sure about the closet door being on top of the entry door. It is a little weird to have the entry to that bath off the entry hall but having it off the kitchen complicated the pantry location. I've pushed those squares around a lot but see your point and might push them around again.

    The kitchen island location is approximate. I hear you about the grocery thing. We currently have a detached garage so we're sort of used to carrying the groceries in.

    Our current house's master bath used to be 2 small separate ones, one opening only to the master BR the other to the hall. They were both closets. I combined them into one big one that's about the size of the one in the plan. Works for us since other than when we have company it's just me and my wife. We think current house plans have way too many bathrooms. Our current one doesn't have the vanity right off the master BR door though. I should probably push things around again in that one too...

    The design program I'm using is a little bit clunky on the roof building. It doesn't really look quite right. If you want to see what this is patterned after go to this page and scroll down to the Tellico Forest and Tellico Lake models. That might give you a better idea of the roof and proportions...

    thanks for the suggestions..

  49. Jay | | #49

    Hi James,

    We closed on the lot just 2 weeks ago. The plan I posted is a draft made up with a $99 design program as something to use as the starting point for when we do hire the designer. We haven't hired a designer yet because
    1) we're still getting recommendations on designers to go with and interviewing them
    2) still brainstorming to decide what the design parameters will be.

    it gets pricey to bounce ideas off someone when they're charging you by the hour, and it's frustrating to them if they quote you a project price and you're still kicking around ideas without having narrowed them down at least a little.

    We're in the stage of ball park pricing to decide on whether certain design aspects are even worth considering or if they're way out of the price range. I know that sounds a little backward compared to how most people do things, but I don't want to start off with a fixed number and just kick things off the table to meet it. At least in my mind that approach assumes all possible design aspects have equal merit.

    Instead we prefer to look at things we'd like to have, get a ball park idea for what doing them will cost, rule out those which show themselves to be financially unfeasable and then make cost benefit or persona choice trade offs to decide among those that seem doable as to which ones we consider our priorities.

    Here's an example. My wife would love to have a soapstone masonry stove. I just got a price quote on a beautiful one for $24K not including the foundation it needs to sit on. I've also seen kits that could get built for less $. Going with just a wood stove is also more cost effective. The cost difference between the latter approaches and the pricey soapstone stove would more than cover the cost of using Urethane SIPs instead of EPS.

    But until I understood some of the considerations of each of these things (which this forum has been a good source of info for) I didn't know that and couldn't make that consideration until I got a ball park price on these things. Certainly a designer might have known some of this, but there's a world of stuff out there it might be nice to have in a house. We're building from scratch and have the ability to consider a lot of different approaches and materials.

    When I built the mantle for my current inset wood stove I poked around on the internet and in some forums before finding a great type of marble whose colors perfectly matched the room. If I hadn't looked I'd have gone with the black that the local store stocked. The great thing about the internet is that so much more is available in the way of materials, ideas, designs, etc. But you need to do some searching, asking questions, looking for prices, seeking advice and recommendations, etc. We feel that it's only after finding what the world of possibilities available is that we can work to narrow it down so that working with a designer is more efficient.

    We're still at the stage of exploring the world of possibilities and winnowing them some and appreciate the help we're getting from folks in doing so.


  50. kevin omeara | | #50

    I am glad you discovered the same thing I did when we got bids on SIP panels of different thicknesses. We started with 6" EPS panels, and on a whim I had my GC get the price on the 12 1/2" thick "fat boy' panels. I has assumeed that they would be twice a much sinc ethey were twice as thick. To our great surprise they were only 10-15% more, so it became a "no brainer". The hidden extra cost od for the bigger lumber splines that connect them and the extra thick foundation walls, but still not all that much more.
    Also to consider the aging process on the different "fills" used in the SIP panels. EPS is rated at R4/in and PU is R7/in however over time this changes. I had read a report testing a 30 year old EPS sample that was remoced during roof remodeling that still came out to R4/ loss. However PU does loose some of it's R va;lue over time. MY energy consultant advised to consider it closer to R5.6/in when for lorg term analysis.
    The 12 1/2 inch EPS SIP panel still doesn't get you to your desired R60 for your roof, however there are a couple of ways to get around this. In discussions with various manufacturers they were willing to make custom SIP panels with 16" of EPS....however they are not UL rated so that could create headaches if your building inspector requires it. We instead added another 4" EPS foam sheet and another 1/2"OSB sheet on top, using special long screws that the manufacturer provided, which also had the side benefit of reducing the thermal bridging caused by the lumber connecting the SIP panels.

  51. GBA Editor
    Martin Holladay | | #51

    Nice solution. If you can afford it, that's the way to go -- SIPs with added exterior rigid foam.

  52. Jay | | #52

    Thanks for the info Kevin. Maybe I shouldn't write off the EPS entirely.

    Were you building over a slab foundation or a full basement?
    We want to do a full basement and the trouble I see with the 12in EPS for the walls to get to R-48 is the thickness complicates the design. From what I understand urethane has the added benefit of greater fire resistance over EPS though. I only got info on the price differential between 6.5in EPS and 12.25in EPS from one company because of the 3 I had been making inquiries of only one even made the 12.25. They quoted me $1.40 per sf higher which is about 36%. That's why I started looking harder into urethane.

    BTW is it really 4/in for EPS or is that rounded up? I've seen it listed as 3.75 by some but 4 by others. Urethane is apparently 6.75 per in. I'm starting to realize it's probably better to work in actual R values for specific panel thickness and compare those directly on price and performance.

    Getting as high as R-60 on the roof is still the kicker as no one makes a single product that achieves this. You have an interesting solution.
    What part of the country are you in?
    Does your design handle high snow loads or is that not a consideration?

    Do you recall which manufacturers said they'd make 16in SIPs?
    One approach could be to go with 8.25in R-50 urethane on the walls to get the R-50 and a 16in EPS on roof to get the R-60. Wonder if there's any issues using one for the walls and the other on the roof.

    The mix & match approach could avoid the design complication of having to account for 12in SIPs and the timber frame post supports on the basement foundation and still get me to the target R value ranges.

    thanks again

  53. Jesse Thompson | | #53

    Two things:

    R value isn't the only issue with your choice of foam inside SIP panels, the EPS will have less damaging blowing agents than the PU panels, see this GBA article:

    Also, SIPs are much better suited for roofs than walls because you don't typically cut big holes in roofs and then throw away large chunks of OSB and foam. Much lower construction waste is a clear advantage to wood framed walls. If construction speed is desired, then off-site panelization can offer similar advantages, see Michael Chandler's posted techniques here at GBA for an example.

  54. Jay | | #54

    Hi Jesse,

    I've seen some incredibly vitriolic debates between EPS SIP proponents and PU SIP proponents on various sites and don't want to start one here. I think there they are both good products and I have no bias one way or the other. I plan to use whichever product provides me the best compromise at maximizing the ability to get a sufficiently high enough total R value without costing me an excessive premium.

    By that I mean that if it costs 33% more to get 46% greater R and achieve R-38 instead of R-26 as compared to costing 60% more to get to R-48 that's OK. I'm not seeking only to maximize the R value. I'm looking to do it cost effectively but am willing to pay more, even if it doesn't have a 100% payback, up to a certain extent.

    I've already considered the waste issue, but thanks for the advice. All the panel guys I've talked to so far say they'll work with the designer I choose to optimize the panel size selection for the building plan and pre-cut the SIPs to minimize waste. The designers I've been talking to all seem to do this. So I anticipate that there should be minimal waste.

  55. Jesse Thompson | | #55


    If you aren't simply looking for pocketbook economic payback, then external factors like the damage the blowing agents in your foam panels cause should be relevant. These are serious issues, we need to not make our individual houses cheap to heat but fry our atmosphere in doing so.

    As to minimizing waste, it's worth asking your SIP supplier what they do with the sections of foam panels they cut out for window opening. Optimizing and minimizing may be their goals, but I wouldn't assume that there isn't a tremendous amount of waste inherent in the process that ends up as a cost they pass on to you just because they don't have other production capabilities. Bensonwood has stopped using SIPs for walls for this reason (among others).

  56. wjrobinson | | #56

    Some SIP builders stick frame window and door opening areas avoiding waste.

  57. Harry | | #57

    You seems to be a DIYer. You may find this site helpful (, especially the solar water system which can be self built. That would cut your cost by a lot and perhaps make your radiant floor heating option more economical. Of' course the reliability and durability is your own responsibility.
    Also, if you may also find recycled EPS/XPS foam boards at discounted price, but quality and inspection is on your own. You see where I'm going here. Many things can be done cheap, but there is trade-offs and risks to everything.
    Good luck.


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