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Q&A Spotlight

How to Build Efficiently in Massachusetts

Plans call for a slab-on-grade foundation with radiant heat and double-stud walls. Comments?

Do radiant floors make sense for superinsulated homes? According to many energy experts, investments in a well-sealed and well-insulated thermal envelope make more sense than investments in expensive heat distribution systems.


Image Credit: Fine Homebuilding

Noah Kaput and his wife seem to be off to a good start in planning their 2,100-sq. ft. house in Massachusetts.

Key elements of the design include:

  • A 5-in. thick slab-on-grade, insulated with 4 in. of rigid foam insulation.
  • Windows in the south-facing wall with an area equal to 13% of the first-floor area.
  • 10½-inch-thick double-stud walls, insulated with cellulose.
  • A tankless on-demand heater for domestic hot water.
  • Radiant-floor hydronic heat distribution on both the first and second floors, with hot water supplied by a propane-fired water heater.

Kaput has still not decided on which windows to use; he would love to have triple-glazed units but is leery of the high cost.

“I don’t really know what I’m doing,” Kaput writes in Q&A post at GreenBuildingAdvisor, “so any and all advice would be appreciated before we start and royally screw this up!”

It’s an open invitation to critique a house-in-the-making, and the subject of this week’s Q&A Spotlight.

First, skip the radiant floor heating system

Radiant-floor heat has some advantages, but with the labor to install the tubing, plus cost of the tubing, pumps, controllers, and manifolds, it’s costlier than other distribution systems.

GBA senior editor Martin Holladay would choose another option. “It’s a very expensive way to heat a house,” he tells Kaput. “If you improve your thermal envelope, you can install a much cheaper heating system.”

Without in-floor hydronic heat, Kaput replies, the slab will have to be eliminated so “we’re not walking around on a cold stone [floor] all winter.” And if the slab goes, it will have to be replaced with a crawl space and first floor deck, and the house will have…

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25 Comments

  1. Doug McEvers | | #1

    MA Build
    2,100 sf in a 6500 hdd climate with the specifications noted would have an annual heating load of 13.65 million Btu's if you get the house to 1.5 ach50 or less. This is about 150 gallons of propane or 4,000 kWh with electric heat. I agree with other comments, concentrate your efforts on the building envelope and go with a simple heating system.

  2. Paul W | | #2

    Air quality and comfort
    Doesn't a forced air system come with a lower air quality than a radiant heat system? Isn't a forced air system a less comfortable heat system than a radiant heat system? Or are those concerns just marketing hype? Does the calculation change if we add in a basement to the house?

  3. GBA Editor
    Martin Holladay | | #3

    Response to Paul W
    Paul,
    Q. "Doesn't a forced air system come with a lower air quality than a radiant heat system?"

    A. Not that I know of. Why should it? Of course, if the system is defective -- for example, if a contractor installs leaky return ductwork in a moldy crawl space -- it's certainly possible to have a problem. But the problem is due to contractor error, not the forced-air system. After all, it's also possible to install a radiant-floor heating system that leaks and causes mold and rot. Any poorly designed or defective heating system can cause problems.

    Q. "Isn't a forced air system a less comfortable heat system than a radiant heat system?"

    A. It shouldn't be. Of course, any heating system can be badly designed. It's certainly possible to design a hydronic heating system that is uncomfortable.

    Q. "Are those concerns just marketing hype?"

    A. If you read them in an ad, I would guess they are.

    Q. "Does the calculation change if we add in a basement to the house?"

    A. Are you asking whether the fact that a home has a basement should dictate the type of heating distribution system? If that's what you are asking, I would say no. If your home has a basement, you can use either a forced-air or hydronic distribution -- your choice.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Personal experience,
    Personal experience, typical two story home with finished full basement. Basement walls are drywall over ICF. Basement floor is radiant from Triangle Tube condensing unit. Entire home is at one tempetature shot with infrared temp gun. Basement space is much less humid and much more livable space than any typical home built decades ago. Slab also has poly and 2" of foam under it.

    Adding PEX tubing to a slab is not a big project and if DIY installed, inexpensive compared to the $70,000 kitchen I just worked on.

    In a superinsulated home, I bet a low temp radiant heated slab is more comfortable than an inheated one.

  5. Paul W | | #5

    Forced Air Musings
    Martin,

    Martin,

    I guess my thinking on the issue of air quality is that Ductwork is a collection system for pet hair and dander, smoke, dust, pollen and other air borne pollutants. It is a system that can not easily be cleaned, maintained by the homeowner on their own. That means hiring a company to come in and clean and inspect the system regularly. According to the National Air Duct Cleaning Association website, the EPA estimates a duct cleaning to cost between $450 and $1000, noticeably, the NADCA website lacks information on how often these cleanings should be done, other than to say, "it depends". While we all hope that a home owner is going to do maintenance on schedule, do it well, and do it consistently, that's probably not the best assumption to make about human behavior.

    Whole house HEPA Filtration systems are of limited use, because the small particles that these systems are designed to collect are the least likely to make it out of the room and to the filter. They also add extra cost to the system.

    With regard to heat "comfort", I couldn't find any sort of metric to describe this experience. From my own limited experience, I just notice that I have less problems with feeling cold/chilled, dry cracked skin, and upper respiratory problems when I spend time with relatives that have radiant heat than I do in my own home with forced air, even though the relatives keep their thermostat several degrees lower. I know there are other factors that go into this, and quite a bit of subjective observation that limits the value of the anecdotal evidence. I wish I had a better way to objectively quantify these observations.

    I'm not sure where I was going with the basement question either. I'm sure I had a great reason for asking it at the time. :)

  6. GBA Editor
    Martin Holladay | | #6

    Response to Paul W
    Paul,
    In a well-designed superinsulated house, ducts will be sized properly, relatively short, and made from galvanized steel.

    I think homeowners worry too much about duct cleaning. In some cases, cleaning ducts can even make things worse than leaving them alone. Include a filter at your furnace and change the filter regularly. Every now and then, you can lift your registers and stick a vacuum-cleaner hose into your duct as far as the hose will go, if you want.

    However, I'll admit that one of the chief benefits of ductless minisplit systems is that such systems have no ducts.

  7. Expert Member
    Dana Dorsett | | #7

    At very low heat loads and superinsulation...
    Heating with a decent-HSPF mini-split/multi-split air source heat pump is cheaper than heating with oil or condensing propane in central mass, even in places with 20cent/kwh electricity. It's sufficiently cheaper that even keeping the indoor temps 5F warmer for additional comfort - say 73F instead of 68F with the radiant (could get sweaty for some :-) ), it would still have a lower operating expense. The 99% design temps for central MA are around 0F, and there are several mini-splits out there that are better than break-even COP at -10F (and rated to run at those temps) and many with operational temps of -5F.

    At 20cents/kwh and an average COP of 2.3 (conservatively estemated) you get ~40,000BTU/$. With condensing propane you'd get about 80,000BTU/gallon, so unless you're expecting propane to average below $2/gallon and electricity to go higher than 20 cents, the heat pump is the better deal. This year in Worcester MA electricity has averaged15-16 cents range, and propane prices have averaged north of $3/gallon- it's not even close. In towns with town-operated utilities and even lower electricity prices the difference in price per BTU-delivered is more like a 2 to 1, condensing propane/mini-split. Similar arguments can be made for oil.

    Putting the money that would have been spent on the radiant into (well-subsidized in MA) grid-tied photovoltaics makes the cost of operation even cheaper. And since the same mini-split provides air conditioning & dehumidification, it's a better consolidation of mechanical systems that you can't get with a condensing HW combi system.

    If it's not a fairly open floor plan some consideration has to be taken into account for the differences in air temp between the room with the mini-split heads and those without. Putting the heads in the lossiest spaces makes for the smallest delta-T, but sometimes upgrading the windows of the rooms without to lower the loss is enough to keep it well bounded even at design condition.

    It's well worth doing the math or simulation on the passive solar design for choosing those south facing windows, and at 13% of floor area, sticking with the slab for it's thermal mass is probably going to be important to keep from overheating on sunny winter days.

  8. James Morgan | | #8

    Paraphrase for Paul W
    "I guess my thinking on the issue of air quality is that Ductwork is a collection system for pet hair and dander, smoke, dust, pollen and other air borne pollutants that traps these irritants on an easily changed particulate filter. Radiant heating on the other hand is a system for retaining and continuously circulating these products within the room air volume via convection currents to keep them available for absorption by human bronchia."

    I honestly don't know which version of the air-quality theory has more truth to it but it would be useful if someone can bring some actual data to the discussion. Disclosure, I live in a climate where summer a/c is, if not an absolute necessity, then highly desirable. As we also have cold winters and radiant cooling in a humid climate is not an option, I confess a bias toward a single system which can blow both hot and cold as needed.

  9. GBA Editor
    Martin Holladay | | #9

    Response to James Morgan
    James,
    Like you, I welcome data. Most people -- especially those with alarmist descriptions of the dangers of ductwork -- rely on anecdote and conjecture.

    Here are the important variables, I think:

    1. What level of house-cleaning occurs on a regular basis? Homes that are rarely cleaned with a vacuum cleaner (especially those with many wet dogs) are different from homes that are kept clean.

    2. Are the occupants sensitive? Some people have robust health and are never bothered by mold, dander, or pet hair. Other people sneeze all the time, and are extremely sensitive to these irritants.

    I think ducts are a minor factor compared to these other two factors. But for people like you in a climate that needs cooling: if you are worried about the cleanliness of your ducts, then by all means choose ductless minisplit units.

  10. James Morgan | | #10

    Well I never
    Well I never thought of cleaning my wet dogs with a vacuum cleaner.....

    But seriously, the only source of ductwork contamination we worry about is the result of leaky ducts in a contaminated crawl space. This is obviously an unacceptable condition from multiple standpoints and one we can easily fix. It's certainly arguable that the air in a home which is regularly passed through a filter by the hvac system is going to be cleaner than in one that is not.

    Talking of multiple factors, I believe that apart from the relative scarcity of flying insects, one of the reasons the Brits are happy to leave their windows open much of the year is that the outside air is regularly swept clean of particulates by the frequent rains. Here in the US south at least this is not the case and one of the unsung benefits of mechanical air systems is protecting the home from airborne dust and pollen. If you're looking for high levels of indoor dust contamination, try a naturally-vented home along a North Carolina dirt road in the summer.

  11. GBA Editor
    Martin Holladay | | #11

    Response to James
    James,
    You're right: if you're cleaning dogs with a vacuum cleaner, it's best to wait until the dogs are dry.

    (I have corrected my misplaced clause... so anyone who wonders at your reference will just be confused now.)

  12. Noah Kaput | | #12

    my house!
    Holy helpful! This is great! To clarify, I should have mentioned that we are actually on the south coast of Massachusetts and therefore a little more temperate than other parts of the state. I'm not sure how to start responding so I'll just go ahead...
    In regards to the on demand water heater vs. a flat plate heat exchanger coupled with the hot water heater: i got similar prices for the on demand and the heat exchanger component and figured that when the heating system wasn't on for 6-7 months, the on demand would be more efficient than keeping all that water heated all summer for our occasional use.
    As for the envelope, we were planning on following any and all air sealing methods that we could find in an effort to make things as tight as possible including a designed, continuous air barrier. An HRV system will handle the air quality issues.
    As for the heat... I have no delusions about the toasty toes phenomenon, and that is not my aim, in fact after being informed by Martin that a well insulated slab will be comfortable with any heat source, I am open to anything. My starting point was simply that I have always enjoyed hydronic radiant heat in all the houses I've been in that had it. Most of these houses are 2x6 conventional framing, fiberglass batts, no thought to air-sealing, Anderson windows wherever they look nice, type of homes and they were very comfortable. As an added benefit, the system was silent and invisible which I feel is also important when considering the overall comfort of a new home. I was planning on installing the tubing myself and thereby lowering the cost a bit. For $7-8k we could have this system, installed, and I feel that when kept at a constant low operating temp it would be quite comfortable. The zones would be broken up so that those with more solar activity could be at different temperatures.
    I do like the idea of the ductless mini-split, but I hate how they look, and I don't want to hear them or be in the line of their breezes. It would also seem that given the relatively open layout of the first floor and the not-so-open 2nd floor we would probably need 3 units and the installed cost would be close to that of the radiant system. The benefits of the mini-split would be that we would also have (somewhat) central a/c and we could use electricity. I agree with Dana that it is the most cost effective way to go, especially with the p/v incentives here in MA. We do plan on having a grid-tied 5Kw system so it would make sense.
    As for the triple pane windows, what is the payback on those badboys? I couldn't find any info regarding that, probably because there are so many variables in the rest of the house design etc. It looks like they would cost $8-10k (at least) more for our house and I wonder if that is the best place to put that amount of money.
    Well, that's probably enough for this post! Thanks again for all the info and ideas!

  13. Peter Hastings 4C | | #13

    Payback on triple-pane windows
    " As for the triple pane windows, what is the payback on those badboys? "

    You get to sit next to the windows, drapes open, summer or winter and look at the view in comfort.

  14. Expert Member
    Dana Dorsett | | #14

    Minisplits are not as noisy or breezy as you might think...
    Minisplits with continuously variable ECM-drive blowers on the interior units are remarkably quiet at low & mid-speeds, and the air motion pretty small (just don't stick it over your bed or kitchen table, etc.) It is by no means as noisy as the ducted hot-air furnace or air conditioning systems you might be thinking of. Even the outdoor units are also nearly silent at part load- scroll compressors work with a rolling action, with a fraction of the vibration of reciprocating refrigeration compressors. At 5AM on the coldest night of the year it would be louder than your refrigerator, but still nothing to write home about. During the lower heating loads of daylight hours they're pretty whispery.

    You COULD screen them visually with wall valances too, which has no down-side except for using the remote control from the far side of the room.

    But if you really hate the concept of mini-splits under all circumstances, in your location and heat loads a Daikin Altherma air-to-water hydronic heat pump (built on their mini-split technology) could provide you both heat & HW with a radiant floor, and you'd likely average over 2.5 for a COP anywhere from Buzzards Bay to the Cape & Islands. What you don't get with the Altherma is latent-cooling (dehumidification), which would be the lions-share of the air conditioning load. IIRC you can get sensible cooling with it via the radiant slab.

    "Payback" on things like triple-pane windows or any envelope upgrade has at least a couple of parts, aren't always easily quantified. The initial cost & size of the mechanical systems that come with the higher-efficiency envelope are now smaller due to the lower loads. But at some low load the cost-deltas on the mechanicals are also pretty small. And the cost of the energy savings is a very squishy number, affected by both the anticipated fuel/utility pricing over a couple of decades, as well as the efficiency of the mechanical systems. The payback will be very different if you're talking about heating with 12cent electricity in a a mini-split with an average COP of 2.5 vs. a 95% condensing propane boiler.

    Then there's the "what if"s. If it costs $10K more for the triple panes and it saves you $100/year in operating costs compared to the pretty-good U0.30 double-panes, is that a better value than adding 2KW of PV (even un-subsidised)? If you shrunk the total glazed area by 25% to achieve the same heat loss numbers- would that lower your wintertime solar gain and end up INCREASING the net energy use of the building? If you changed all south facing glass to high solar gain 2-pane goods, would that end up over heating the place on sunny days (even if you shrunk it by 25%)? Again, do the math on the passive solar, particularly with south facing glass that's 13% of the floor area! With sufficiently massive slab and high-gain double pane would take a big chunk off the average heating load and energy use if you design the summertime solar-rejection/shading part well.

  15. Robert Brown | | #15

    slab radiant expensive?
    I'm curious about the idea that slab radiant is "expensive". You can pipe a slab for less than $1/sq ft pretty easily, with DIY pipe installation, and not much more than that installed. the controls can be pretty simple as well in many cases. It may be more than forced air, but slab radiant is the absolute cheapest form of radiant and in most cases it's cheaper than most alternatives you'd consider as well. and while the toes may not be toasty, I can say the radiant floor in my superinsulated shell is certainly COMFORTABLE, which is kind of the point, right?

    A few panel radiators or a low-cost radiant ceiling on the upper floor and you have a finish system that is competitive with (not cheaper than, but but ridiculously more expensive) forced air.

    Now, you still need to consider air for air quality and perhaps cooling, We are using the Daikin Altherma in our shop, with ERV ventilation and radiant cooling (and dehumidification from the altherma chilled water as well) but that is admittedly "bleeding edge" tech in the residential market. there are many hydronic coil/fan combos out there that could be used if cooling was necessary though, and they are fairly cheap. You certainly can get the latent load out.. the altherma can spit out 45 degree water, plenty good for latent removal.

    The minisplit vs altherma choice varies home by home. If you only have a couple of rooms the minisplits might make sense. Otherwise a hydronic system w/altherma will certainly be a comfort upgrade, even without toasty toes, in the winter at least. it opens the door for vast improvements in cooling comfort as well (thinking no minimum airspeeds, perfect latent vs sensible modulation, etc).

    I do think heat pumps are the way to go if you aren't on natural gas right now though, OR if you are and you want to do cooling as well.

  16. GBA Editor
    Martin Holladay | | #16

    Response to Robert Brown
    Robert,
    Aficionados of in-floor hydronic heat often compare the cost of DIY radiant systems with contractor-installed forced-air systems.

    We really don't have to argue about this. All it takes is a few phone calls for most builders: get some bids. All over the country, the answer is the same: a boiler and in-floor PEX is much more expensive than a furnace and ducts. And with the hydronic system, you have no way to provide air conditioning.

    As far as the Altherma is concerned, here is real-world cost info from GBA readers:

    https://www.greenbuildingadvisor.com/community/forum/mechanicals/14216/seeking-air-water-heat-pump
    Mike O'Brien: "First Daikin Altherma system installed in Portland looks good, however, I got a quote of $22,000 to install a system in our house, and because it is not yet elegible for the 30% Federal renewables tax credit, it's an expensive way to reduce electric energy use. I have to say I don't understand where the high price comes from, the components are pretty straightforward to install."

    https://www.greenbuildingadvisor.com/blogs/dept/design-matters/ground-source-heat-pumps-part-2-rules-thumb
    Nick Lehto: "I got a quote on an Altherma for $36K installed in an 1800sqft house. It was $4k more than a gshp after you take CT state rebates into account. IMO they are both too expensive. If you have a superinsulated house its tough to argue against the mini splits at $1500 a pop."

  17. Andrew Marani | | #17

    I just went through this
    I just went through this review (radiant vs ducted) on a house near Baltimore, MD. We are just about done design and hope to start construction this winter. 2300 sf, 10" double stud walls with cellulose, 2" of mineral wool on the exterior (great article on that one), triple pane windows.

    Our current house has radiant on the first floor so I went there right off the bat for the new house. We ended up dropping the radiant floors (and the active solar heating). After spending lots of time looking at how to set up the system, I asked the engineer what happens if we drop the radiant and change over to all electric ducted system with geothermal for heating and cooling? Everything got simple and mechanical system costs dropped by at least $10,000 (guessing here, we never reached the pricing stage). Budget pricing from two drilling companies for geothermal wells and pipe for 2 1/2 tons came in under $6,000. Latest design tweeks now put the house under 2 tons.

    I am ducting a supply and return into every room, which adds cost. There was a good discussion here about just ducting heat and AC to the core of the house, massively simplifying the duct system. The backup study showing that it works was good, but I am too chicken to try it.

    As far as dirty ducts go, I think it is something that sounds like a problem but in real life is not. There is a filter after the return duct, so any air moving through the supply duct is filtered. Really good 1" thick filters are available for $10 bucks or so at any big box store. I would rather move the house air through some dirty ducts followed by a good filter than not filter it at all by using a mini split (or radiant). And we do make some dust: two kids, two dogs and a cat, plus the temporary additions brought in from the exterior by that group.

  18. Charles Eichenlaub | | #18

    Skip poly vapor barrier in double wall system
    Thanks for the info..........I too was planning on using radiant in-floor heat with a super insulated home. I'm building a double wall system with cellulose (8-10" of insulation). So, now I'll look at a forced air system or splits.
    I am building on a commercial/residential lot that is only 25' wide. I can build on the lot line IF I have a one our fire rated wall. Nu-Wool cellulose has a 2 1/2 hour rated firewall system that includes 5/8" drywall on both sides. Both sides need to be taped. I will probably put Tyvek on the outside with an air channel topped with cementboard siding. The guys at Building Science say to put poly in the middle of the double wall system as an air barrier. Can I skip that if we seal the exterior drywall really well?

  19. GBA Editor
    Martin Holladay | | #19

    Response to Charles
    Charles,
    You wrote, "The guys at Building Science say to put poly in the middle of the double wall system as an air barrier."

    Really? Cite your source, please! I'm surprised -- Joe Lstiburek has been bad-mouthing poly for more than 20 years.

  20. Charles Eichenlaub | | #20

    Joe's Poly
    It is strange to see Joe and Poly in the same place. Read it and weep.
    Enclosures That Work
    High R-Value Wall Assembly-04: Double Stud Wall Construction

    http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/high-r-wall-04-double-stud-wall-construction?topic=resources/high-r-value

  21. GBA Editor
    Martin Holladay | | #21

    Second response to Charles Eichenlaub
    Charles,
    I know that plenty of people in New England build double-stud walls insulated with dense-packed cellulose WITHOUT the poly.

    I just e-mailed Joe Lstiburek to ask about his logic. I'll post more info when I know more.

  22. Charles Eichenlaub | | #22

    Nu-Wool
    To add fuel to the fire, NuWool strongly advises against the use of a poly vapor barrier. Not sure what they suggest to use for a more aggressive apporach to air infiltration.
    http://www.nuwool.com/Products/Wallseal/VaporRetarder.aspx

  23. GBA Editor
    Martin Holladay | | #23

    Third response to Charles Eichenlaub
    Charles,
    I just got an e-mail from Joe. It turns out that the detail is a straw dog -- in other words, it is a detail that really isn't built in the real world, but is one that Joe drew just so he could shoot it down as dangerous.

    Joe wrote to me, "The Big red note says 'some assemblies not recommended.' The ones with poly, for instance."

    OK, Joe -- I get it. "Don't build it this way."

    Charles: My advice is, omit the poly and you'll be fine, as long as you can find a contractor who knows how to dense-pack thick wall assemblies.

  24. James Magner | | #24

    Splits and radiant
    I recently completed a heavily insulated house on the south shore of Mass and grappled with the same issues. Ultimately I went with mini splits coupled with a wood stove/boiler that can also heat water to be sent through pex in the slab on the ground level. I am very happy with the performance of the system and have no reservations about opting for the spits. I honestly can not tell when they are operating unless I put my hand up to them. The operating costs have been impressive and even though I thought I would need the wood heater during extended cold snaps I no longer believe that to be the case. I think the splits could handle anything this climate throws at us.
    Installing the radiant system myself made it an easy decision, as its only costs were that of the materials, but I am also very impressed at the heating ability of the stove/boiler. It is certainly a product I think has a useful place in our homes. Follow this link to see the product I installed. http://www.hydro-to-heat-convertor.com/freestanding.html I am not attempting to push any particular manufacturer or product here, I just thought it was relevant to show what I tried to describe. That being said, I am not aware of the relative efficiency and/or "cleanliness" of wood being burned efficiently as a heat source vs. more conventional home heating fuels (e.g. propane, natural gas, oil, etc) when they are being burned efficiently. I am sure the idea is to not burn anything if possible, but could someone speak to how wood compares to other fuels?

  25. GBA Editor
    Martin Holladay | | #25

    Response to James Magner
    James,
    For more information on burning wood, see Should Green Homes Burn Wood?

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