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Community and Q&A

Is it really possible to acheive Net Zero construction in northern VT

Jay Hersh | Posted in Mechanicals on

A friend recently sent me a copy of the Nov 2010 issue of Green Energy Times (www.greenenergytimes.org) and right on the cover is an article titled “Imagine No Fossil Fuels to Heat Your Home” which outlines a project by Murphy’s CellTech in which a home it states has R-40 walls was built using an R-85 insulated high temperature storage tank that can hold water up to 205F. It doesn’t say what size they used, but another ad in the paper says the tanks come in sizes up to 5000 gallons from Thermal Storage Solutions and that they can store large amounts of heat collected before heating season for use during heating season.

Having previously run the numbers (5000 gallon tank, water at 8lb/gal, a BTU heats 1lb of water 1F) even if the water in the thermal store is 200F working with a radiant floor operating at 100F (i.e. 100F delta T) that comes out to only 4.0 Million BTU total storage capacity.

When I spoke with John Unger Murphy of Murphy’s Cell-Tech he indicated they design for 5BTU/sf/hr. I’ve run scenarios in REM/Design with R-50 walls, R -60 Roof and Triple Pane windows and even for a 12F design point (which is 18F above what REM/Design uses for Hyde Park, VT) and can’t see how one can get to a heat loss that low. The best -6F loading, even for a house oriented and designed to have a significant heating component derived from passive solar was 7BTU/sf/hr. Even if I could achieve 5BTU/sf/hr loss and only consider the 2100sf above grade that comes out to about the 10,000BTU/hr.

The REM/Design thermal modeling tool indicates that for a building insulated as I plan (R-40 SIP walls, R-50 SIP roof, ACH of 1.0 at 50 Pascals) at the design load temperature of -6F it uses for my Vermont location I’d need somewhere around 25,000 to 27,000 BTU/hr and 46 Million BTU total for the entire heating season. Admittedly that per hour BTU load level, which is based on a worst case load of -6F, is higher than the load one would expect most of the time from looking at the table of NOAA Vermont weather normals for the average highs and lows for Dec to March, but the annual BTU usage is determined based on total degree days taken from these tables, not the worst case loading, and should be pretty accurate. So it would seem that even a 5000gal thermal store like this can only hold at best about 1/10th of the total heating season load. During the worst part of the winter that amounts to maybe 7 days or less worth of heating capacity .

The above assumes one wouldn’t be able to replenish any of the heat lost during winter, but that’s not really an outrageous assumption. Due to the extremely low sun angles and short day lengths (see attachment of azimuthal sun positions for various times of year) combined with the scant number of sunny days (5 per month on average) in this part of Vermont I don’t think it’s feasible to collect enough solar thermal energy when it is sunny to replenish the store. For a bank of 10 evacuated tubes which can collect 1750BTU/hr at peak efficiency to collect approximately 500,000BTU (the amount that a house which looses even only 10,000BTU/hr would need each day) and assuming 6 hours of sunshine would require 22 collector banks. That’s assuming optimal collection efficiency across the entire 6hrs on days when the sun shines 12hrs or less.

Even at peak efficiency a bank of 22 collectors would just keep up with a single day’s usage at an average to low load. That doesn’t allow for charging the thermal store above and beyond usage. To do that would require at least twice the number of collector banks. Even then it ignores the fact this part of Vermont has on average 5 sunny days per month, and 5 partly sunny days per month. The rest are cloudy. To account for the fact that only 1 day in 6 is sunny in order to recharge the store you’d need a much, much larger number of collectors.

Prior to reading that article, but after conversing in some of the GBA forums with folks like Martin Holladay who lives fairly close to where my lot is, and doing the math along the lines shown above I’d basically resigned myself to having to use a fair amount of some type of fossil fuel. Either around 500 gallons propane or 350gal heating oil to heat even such a fairly well insulated house designed to gain a fair amount of heat from passive solar as I plan to build.

But when I read stuff like this article in Green Energy Times I can’t help but to keep wondering if I’m just not getting something and there is some way to cost effectively make more of a reduction in fossil fuel usage than I’ve resigned myself to or if it’s all just hype and companies like these are fishing for folks who don’t care about calculating the cost vs benefit and are willing to spend top dollar so they can feel good about themselves for not using any fossil fuels or .

Is Green Energy Times just a clean energy industry infotisement (or is it adformation?) disguising itself as an informative publication?

Are there really any feasible approaches to actually storing enough solar thermal heat collected in sunnier times to get one through the cold, grey winter days in northern Vermont?

I keep wanting to believe that there are but unless I’m doing something wrong the math just doesn’t seem to add up…

Thanks,
Jay Hersh

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Replies

  1. Damon Lane | | #1

    Jay, I think you are correct in your analysis that neither storing thermal energy nor solar heating during the depths of winter will get you a 0 fossil fuel home. If you could, I think those would be among the most direct and real "net zero" strategies, but they aren't the only ones. I have heard from the builder that the South Farm homes in Hinesburg are performing better than modeled and are slightly net positive. Granted Hinesburg is probably not as severe as Hyde Park, and the PV + geothermal solution is very expensive. They become net positive on an annual basis by generating more electricity from the PV in summer than the geothermal heat pumps use in winter (of course all annual energy is exceeded by the annual PV output, but we're focused on heating here), but they surely don't meet their energy needs in winter. The grid is providing the seasonal storage here. Avoiding those two technologies and using a wood or pellet stove would give you a house that has extremely low energy use with the small heating demand met by a local renewable source.

  2. Jay Hersh | | #2

    Unless I missed something the article itself doesn't mention anything about geothermal heat pumps. Are the South Farm homes in Hinesburg also known as South Village? The article doesn't mention the town just "the developers of South Village".

    The only actual references to materials used are R-40 cellulose for the walls, "evacuated tube solar collectors that will store the heat in a large, heavily insulated tank built by a local company called Thermal Storage Solutions" and a "photovoltaic array ... placed on the roof and sized to accommodate our expected electric needs".

    If a geothermal system is involved as well then this article is somewhat misleading because it really does seem to imply that a solar thermal system and a heat store tank alone can eliminate the need for any fossil fuels. Since geothermal systems are pretty pricey that's sort of a big "forgot to tell ya"....

  3. TJ Elder | | #3

    How cold do you think it would get if you had a well-insulated home (R-40 walls, etc), well air sealed, passive solar design, and simply chose to use no heating?

    Here's a basic observation: a high performance thermal enclosure is not useful if you don't maintain a delta-T. The demand for heat drops as the delta-T shrinks. That would result from either the weather being less cold, or the house being less warm. The point being, you don't need a heroic thermal enclosure to use less heat, or an elaborate heat storage strategy involving R-85 insulation around 5,000 gallons of water.

    After all, this is a green building forum, and how green is it to insulate thousands of gallons of water to R-85? Would that require a truckload of styrofoam? A bird's nest is infinitely greener than a Passive House despite having no air sealing and minimal insulation. Fossil fuel demand: zero. Styrofoam: zero.

  4. GBA Editor
    Martin Holladay | | #4

    Jay,
    Damon gave you good advice.

    You can't use solar thermal heat as your only source of heat in northern Vermont. If you want a heat source with minimal global warming impact, choose firewood or wood pellets.

    If you want a net-zero energy house, heat your house with electricity (for example, using a Mitsubishi HyperHeat ductless minisplit) and size your PV array to meet your annual electricity needs. (For most superinsulated homes, that means an array in the range of 5 kW to 9 kW.)

    If you are a purist, and you think that using the electricity grid for storage is cheating, you'll have to put on a sweater. Or a snowmobile suit.

  5. Jay Hersh | | #5

    To be fair I should qualify my initial post by saying that John Murphy didn't write the article. It was written by Steve & Meagan Roy. It does however start on the front of the publication and where it continues in the interior an ad for Murphy's Cell-Tech is right underneath it. That was probably the publisher's idea...

    I'm not completely sure of Thomas J's point other than that I should keep the thermostat turned down. It being a vacation home at first I will actually be leaving it set back 15-20F lower than when it's occupied and installing a temperature warning phone notification system.

    Regarding Martin's comments, I'm not a purist at all. We're definitely running line and connecting to the grid. We won't be putting in solar PV right away but hope to phase it in. I think we may actually do it somewhat backwards. By that I mean installing a backup battery bank that charges off the grid first, then adding the PV capacity later on. The idea behind this is to have 2 panels, one that's grid only the other that's backup or grid.

    Since I'm pretty sure we'll be going with a smallish solar thermal system (three 10 evacuated tube banks) with a drainback tank that will allow passive pre-heating of incoming well water going to DHW use. We will also (regrettably :-) have fossil fuels as the backup heat source. My thinking behind the battery backup on the 2nd panel is that by connecting the heating system (burner & pumps), solar thermal system circulation pumps, and well pump to it this would allow the heating & DHW to remain operational during power outages of moderate duration.

    Before you ask, no I haven't completely thought this one through yet, it's still very much a work in progress and I need to run numbers on the power consumption of the various pumps to determine whether it can be done without needing a huge battery bank.....

  6. Keith Gustafson | | #6

    I am NOT an expert, but it seems like either your math our your assumptions are off.
    I am heating a 2800 sq ft 70's ark with 750 sq ft of glass[!] far less insulation[~R30 roof ~r19 wall in foam] and excellent windows, 4 leaky sliders and a couple hundred sq ft of R3 overhang[brrr] and live in mass[design ~0] a wife who insists on 69F 24/7 and we are on schedule to burn around 500 gallons of oil for the season[91 percent buderus]

    my math says your load is coming in at 360 gallons or so...in a smaller house.....with twice the insulation.......hopefully less glass....... not twice the Delta T

    Most people considering what you are looking at would be more flexible on what temp it is in the house at 3AM {IOW peak load]

    Maybe calcs made for furnace sizing[max design temp] are not great for this exercise.

    I was at a guys house 20 years ago, active solar, tall skinny, all south facing glass, big spine of rock down the middle, claimed he vented most of the time

    I am wondering if it might be interesting to try and reverse engineer a house local to yourself and see how their actual energy usage compares to the theory

    NOT trying to get in anyone's hair, but it is frustrating to think your goals are not achievable, and very expensive to overbuild

    Oh, and windows are everything. I am going quad pane

  7. Jay Hersh | | #7

    Did you say you're in Massachusetts? Where about? The climate between MA & VT actually differs a lot more than you think.

    My current home is near Concord, MA. It's a 1948 dormered cape around 2400sf with leaky windows, doors, foundation and little or no real insulation. I used to use 1350 gal heating oil with the original 1948 Weil-Mclain boiler. I switched to a Buderus condensing gas boiler (94% AFUE) with a an outdoor reset controller a couple of years ago and cut my heating bills in half. I now use around 115Million BTU per year as opposed to 188Million BTU per year with the oil. This amounts to a 39% reduction in load but natural gas is cheaper per BTU than oil where I live which is why the overall cost reduction was greater.

    The number of heating degree days (NOAA avg from 1971 to 2000) where my MA home is comes out to 6370. The number of heating degree days where my VT home is being built is around 8900. That's a 40% higher heating requirement. While I modeled for a peak load of -6F (that's what REM/Design uses for the part of VT I'm building in) I know it got down to -27F on my lot several weeks ago and has reached -10F a couple of other times since.

    The annual heating load I calculated with REM/Design of 46Million BTU/year is 60% less than the number of BTUs my MA house uses in a climate that requires 40% more BTU per heating season on average. REM/Design assumes 68F and I've set up my model with setback thermostats as a parameter. So I don't think that the REM/Design numbers are off.

    My VT house will have another 1440sf unfinished below grade space with R-30 insulation separating it from the upstairs. Overall total window area is 465sf with 240sf of that south facing.

    Is the 2800sf of your house total or just above grade?
    I've been reading Daniel Chiras' "The Solar House" and he talks a bit about houses that were designed (mostly back in the 70s) so that they can gain a lot of solar heat during the day and over heat even in winter, but don't hold the heat and lose it fast at night. I'm not saying your house is like that but most designers today say that if you're having to vent then your passive solar house isn't properly designed.

    Also, you need to consider that not only is the part of VT I'm building in 40% colder it also has less sunshine in the winter. Only 5 sunny days per month on average in Jan, Feb & March. I've been up to my lot several times this winter and think I saw the sun for a total of about 15min. I checked the NOAA listings for Boston and Burlington (see http://lwf.ncdc.noaa.gov/oa/climate/online/ccd/cldy.html) and MA has close to twice as many sunny (Clear in NOAA parlance) days with avg for Jan, Feb & March being 8.3 days per month in winter than my part of VT. So not only do you lose more heat there, you have much less opportunity to collect any to replace what you lose.

    BTW I discussed this in another Q&A on here but for my house design I modeled the heat usage for Inline 325 casement windows (which have low U but high SHGC and therefore good net passive solar heat gain) to compare double pane and triple pane. The recovery time for the cost differential was around 40 years. I also found a similar trade off for significant increases in insulation though I may still use higher levels of that.

    Where I found the real bang for the buck is in reducing air infiltration. Dropping a house from ACH 2.0 at 50 pascal to ACH 1.0 had a whopping 18% head load reduction. I plan to make sure the SIPs get sealed well, as well as the sill plate and shoe plate joins and all the window and door installations. My spec for any contractor I will consider will be that they use a blower door test and fog test (run the blower in reverse and place a fog machine in various rooms) to locate and seal leaks.

    If I find the time I might try to model up my MA house in REM/Design and see how well the numbers it spits out correlate with what my actual tracking of the energy usage here is...

  8. Keith Gustafson | | #8

    Jay

    you are from Concord, so you will know what a Deck house is. Yeah, 750 sq ft of glass, most of it facing WNW, south side fully shaded. Solar is there, but not a big factor in my heat load. I am on the north shore. I get the environmental difference vs Vermont, but something is still striking me wrong

    My house is a classic straight raised ranch, the ESE [front] bermed into the ground, uninsulated concrete. About 2850 total sq feet, no unconditioned space under the roof.
    This was a house built when oil was 19 cents a gallon. I did a heat loss as built and it comes back as basically unheatable...... It is much better now, but I have a ways to go yet.

    The house I was speaking of was not really a passive solar, it was designed to be truly 'solar' It was a long time ago so I cannot recall all the details[I was there to look at a car, not the house]

    How are you accounting for the unconditioned space WRT insulation? Would seem a tricky detail, as you are not really R30 away from -6, but R30 away from R'x' away from -6. That space would probably sit at some intermediate temp, significantly altering the max heatloss.

    One thing Deck got right was the way they did windows, the big fixed glass and small opening component. It is much easier/cheaper to build well insulated fixed glass than the opening part. My house has about 120 square feet of opening windows which figure to lose me about 4200 btu at 70 degree delta, while my entire roof loses 4400. Increasing these windows to R5 from R2 saves me more BTU's than adding another R30 on the roof would

    It is all about the windows

    Let me speak frankly:

    C'mon, payback?

    Are you doing this for payback or because you LOVE IT?

    Do you ask what the payback is on the fancy Italian tile?

    If you really want to do this thing, really Net Zero, it is because you want to, not because it makes the most sense. Total project cost is important, but payback is just a silly number. Besides if Libya blows up its oil wells, that payback might go from 40 years to 6 months in a hurry

    Let's face it, double pane glass is so......1958.

    Pull out your numbers and look at design heatloss for roof wall and windows, and ask where are you going to get the improvement you need to meet your goal?

    That 465 feet of glass would lose nearly 11500btu at design[68+6] with pedestrian lo E [R3] windows,I'll bet that is your single biggest loss, by far. Cut them in half with R6 windows, and you still lose more than the resistance losses in your roof or walls[I would guess] close to what you are losing combined.
    Again, I am not trying to mess with you, I think you can do this thing, even in Vermont

  9. Jay Hersh | | #9

    Hi Keith,

    MA weather norm data far 1971-2000 is at http://cdo.ncdc.noaa.gov/climatenormals/clim81/MAnorm.pdf. You didn't say where on the north shore (I keep my sailboat up in Salem and have friends in Beverly so I'm familiar with part of it. NOAA has heating degree day data for this area for Marblehead and Middleton. The former comes in at 5704, the latter at 5868. So those numbers are another ~10% less than Bedford (which being next to Concord serves as a stand in). Being on the coast definitely has a noticeable effect.

    Also it's that much more of a difference between where you are and where I'm building in VT. About 2000 degree days per year which is over 34% more than at Middleton. That's a significant difference.

    I didn't mention that the unconditioned space is not uninsulated, just unheated. We're probably going to use R-20 ICFs. Initial estimates seem to indicate that because of the large reduction in labor over conventional form poured foundations, the cost is within 10% of building a standard foundation then framing inside it but you end up with an insulated foundation.

    Even without any heat down there the temperature differential between the basement and main floor shouldn't be too large because two of the basement walls, the south and west, are walk out walls which will have the Inline 325 windows chosen for high ER (net passive solar gain) so it will gain some passive heating in the winter.

    The Inline 325 windows I'm leaning toward using are outward opening casements for the windows that open and fixed glass for the picture window in the west gable end wall opposite the balcony which faces towards the mountain views. Details at http://www.inlinefiberglass.com/products/325casement.html

    For the casements the dual pane low E hard coat windows have U=.31, SHGC=.5 and an ER of 30 while the triple low E hard coat x 2 has U=.22 and SHGC=.39 with an ER of 35. The triple panes run about 30% more $ and yield 17% better performance. For the number of windows on the house as designed going from double to triple panes would, at current estimates, raise the cost by $5400 tax included and, according to thermal modeling, save $170/year. That's a 32 year payback. Not clear I'll even have the windows that long.

    Just to be thorough I ran the thermal modeling with 2 scenarios. Both had the triple pane windows. One was a more "standard" level of SIP insulation with R-26 walls and R-33 roof. The other was R-40 walls, R-52 roof. The model indicates that for this scenario the annual heating savings was $250 but the cost differential based on actual price quotes from the SIP provider I'm planning on using is $2200. That's a 9 year payback.

    Then just for yucks I redid the same comparison on the double pane windows with the two different wall and roof insulation levels and the result came out the same, which is good because it would make me less confident in the modeling SW if it hadn't.

    Basically what it comes down to is that EPS is pretty cheap and most any house has significantly more sf of wall than window. The total window sf of the house is 16%. The price difference between the R-33 EPS SIP I was originally thinking of using on the roof and an R-52 (even one being custom made for me) is only $0.60/sf. Similar cost differences hold for getting the walls from R-26 to R-40. So even though it might seem at first glance that windows are the weak link in the chain due to their inherently low insulating values, the cost to improve their performance comes at a bit of a premium when compared to things like using higher insulation and building a tighter enclosure.

    Don't get me wrong, going from Inline double panes at U=.31 (R-3.1) to U=.22 (R-4.5) would make about an 11% difference in the overall heat loss which comes out to about 4.5Million BTU/yr. It's just that putting the same amount $5400 that it would cost to go from double pane to triple pane windows into $2200 of better insulation yields 6.8Million BTU/yr savings. If I take the other $3200 and spend it on having a good contractor take the time to reduce air infiltration from ACH 2.0 to ACH 1.0 that's another 10.4Million BTU/yr. Together that totals 17.2Million BTU/yr reduction which is a better bang for the buck IMO....

    Right now I have pretty high confidence in the results of the modeling SW. What it's telling me is pretty consistent with what I've read in various places on the internet and with feedback from an expert in this forum who lives pretty close to where I'm building.

    BTW as a side note, it's interesting that you mention fancy Italian tile for 2 reasons:
    1) Something like that is an aesthetic choice. There is also an aesthetic choice which is a significant part of the considerations in which window we're choosing. The visual transmittance of the double panes is 16% higher than the triple panes. I really don't want to build a house in VT with great views of Mt. Mansfield only to then have windows so deeply tinted that it spoils the view. We might still consider mixing window types and using triples on the north and east sides and staying with the higher VT doubles on the south and west sides

    2) Even though I used "fancy Italian tile" in renovating my upstairs bathroom it doesn't mean I'm not cost conscious. I first saw the Alba Chiara tile I used at a place between Watertown Center and Arsenal Mall that was getting $28/sf. I did some looking around and found the EXACT same tile in Marlborough for $8/sf. I even borrowed a sample from one store and brought it to the other to compare them side by side and they looked exactly the same.

    Wondering how did I could be sure they were in fact the same material? I actually researched where marble and granite are quarried from. There are really only somewhere between 100-200 active quarries worldwide and for many you can find out what color variations come from many of them. I learned that tiles are cut from larger blocks that are quarried whole. There's always natural variations within the seams the blocks are taken from. Somewhere along the chain, either the quarry, the wholesaler, or the retailer makes up some name to market the tiles or slabs under. So it's quite possible to find marble or granite from the same quarry being sold under different names for significantly different prices. But I digress....

  10. Keith Gustafson | | #10

    orth shore (I keep my sailboat up in Salem and have friends in Beverly so I'm familiar with part of it. NOAA has heating degree day data for this area for Marblehead and Middleton. The former comes in at 5704, the latter at 5868. So those numbers are another ~10% less than Bedford (which being next to Concord serves as a stand in). Being on the coast definitely has a noticeable effect.

    Also it's that much more of a difference between where you are and where I'm building in VT. About 2000 degree days per year which is over 34% more than at Middleton. That's a significant difference.
    >>>>>>>

    I GET the difference, really I do
    >>>>>>>>>>>>

    >>>>>>>>>>
    Just to be thorough I ran the thermal modeling with 2 scenarios. Both had the triple pane windows. One was a more "standard" level of SIP insulation with R-26 walls and R-33 roof. The other was R-40 walls, R-52 roof. The model indicates that for this scenario the annual heating savings was $250 but the cost differential based on actual price quotes from the SIP provider I'm planning on using is $2200. That's a 9 year payback.
    >>>>>>>>>>>>>

    Really, R33 roof is not even in the conversation if you are talking about trying to do zero energy R50 is a start, not really an upgrade.

    >>>>>>>>>

    >>>>>>>>>>>>>>

    Is my math off? 465sqft/2100 sq ft looks to be like 22 percent WFA, not even in the Energy Star mix. An easy fix is reducing some of the non useful glass.

    >>>>>>>>>>

    Don't get me wrong, going from Inline double panes at U=.31 (R-3.1) to U=.22 (R-4.5) would make about an 11% difference in the overall heat loss which comes out to about 4.5Million BTU/yr. It's just that putting the same amount $5400 that it would cost to go from double pane to triple pane windows into $2200 of better insulation yields 6.8Million BTU/yr savings. If I take the other $3200 and spend it on having a good contractor take the time to reduce air infiltration from ACH 2.0 to ACH 1.0 that's another 10.4Million BTU/yr. Together that totals 17.2Million BTU/yr reduction which is a better bang for the buck IMO....

    Right now I have pretty high confidence in the results of the modeling SW. What it's telling me is pretty consistent with what I've read in various places on the internet and with feedback from an expert in this forum who lives pretty close to where I'm building.
    >>>>>>>>>

    I think modeling software disconnects you from the specifics of what you are doing. I did my first heatloss calcs in 1986 with a flair pen and pocket calculator.

    >>>>>>>>>>>>
    The visual transmittance of the double panes is 16% higher than the triple panes. I really don't want to build a house in VT with great views of Mt. Mansfield only to then have windows so deeply tinted that it spoils the view. We might still consider mixing window types and using triples on the north and east sides and staying with the higher VT doubles on the south and west sides

    >>>>>>>>>>>>>>>>>>>

    NOT TRUE

    I was worried about this myself with 4 panes of glass. Glass does NOT get tinted. I would have to point out to you which panes are 4 and which are 2. What happens is they get slowly more mirror like. When you put a lamp right below it the reflection is more, when it is dusk the reflection of the room happens a little bit sooner.

    >>>>>>>>>>>>>>>>>>>

    2) Even though I used "fancy Italian tile" in renovating my upstairs bathroom it doesn't mean I'm not cost conscious. I first saw the Alba Chiara tile I used at a place between Watertown Center and Arsenal Mall that was getting $28/sf. I did some looking around and found the EXACT same tile in Marlborough for $8/sf. I even borrowed a sample from one store and brought it to the other to
    >>>>>>>>>>

    Man after my own heart, my local tile place kicked butt over the fancy place for the same stuff. Likewise the internet, nothing better than saving a hundred bucks per toilet with free shipping.....

    I want to try and show you that it really is all about the windows.

    I don't know what your plans are, so I will just use the number you tell me. I am going to pretend the rest:

    2100 sq ft 2 story, 44x24 12 pitch roof

    1500 sq ft roof R50
    1847 sq ft walls R40
    1050 sq ft floor R30
    465 sq ft windows R3

    Design case Delta T: 74 Degrees

    I get much higher infiltration numbers, so I must be making a mistake in that calc. Leave it that you can do much better than 1 ACH, probably .3 at least, with an HRV.

    sq ft X Delta T for these[comes in handy later]
    roof 111000
    walls 136678
    floor 73500
    windows 32550

    With those R values the heat loss through surfaces are:

    roof 2220
    walls 3416
    floor 2590
    windows 11470

    for a total of 19696

    58 percent is going through the windows

    If you increase the R vales for the roof, walls and floor to something reasonable, let's say R200[ 44 inches of eps] Use the sqft*DT:

    roof 555
    walls 683
    floor 367
    windows11470

    for a total of 13075 btu/hr

    Now, if we did something silly, and had R6 windows and left the rest alone:

    roof 2220
    walls 3416
    floor 2590
    windows 5425

    for a total of 13651 btu/hr at design

    If your unconditioned space is at least airtight, you will not lose that much through the floor. R30 is pretty low for that spot.Otherwise, the walls could bump up to R50 and floor to r50 gives us: ( I reread your post and realize the walls downstairs are insulated, really doesn't affect the gist of my post]
    roof 2220
    walls 2734
    floor 1470
    windows 5425

    total 11849

    Drop your WFA to .16 [336 sq ft of glass]

    window loss is 4144

    total design case resistance heat loss: 10568 btu/hr

    now with enough thermal mass to store the passive heat, a few tubes on the roof with some storage, I would think you could fire up the wood stove a couple times a winte.

    I get numbers like 31k btu loss at 1ACH, at 74Delta T so maybe I am doing something wrong, 23254 cubic foot volume.

    It is always all about the windooooooooooooooooooows

    I will leave you alone now.....

    edited for a bit more clarity

  11. Bob Coleman | | #11

    pardon my interruption, but fantastic post Keith, including the calc examples and thought process!

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