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  1. #1
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    Default Sub-Slab Insulation--bragging point or pointless.

    I've been custom building in an allegedly cold climate since 1989 and haven't yet seen a practical dollars-and-cents or comfort reason to add sub-slab insulation unless the basement floor had embedded hydronic heating.

    Yes, it's a given if you consider yourself a cutting edge . green geek. But with the average homeowner moving every seven years or so, natural gas prices plummeting and likely to remain so, and heating systems ever tighter and more efficient, the payback period for sub-slab insulation, if you can even calculate it, must be in the decades.

    Though I worship at Dr. Lstiburek's temple, I'm leaving out the "10" of his "10-20-40-60 rule." For now, at least. Am I a lone apostate?
    "The fatal flaw of all revolutionaries is that they know how to tear things down but don’t have a f**king clue about how to build anything." Jim Goad

  2. #2
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    The Passivhaus guys go nuts with that stuff and I don't know why.
    "First we finish the game, then we’ll deal with the Armada!"

    Sir Frances Drake

  3. #3
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Quote Originally Posted by worthy View Post
    But with the average homeowner moving every seven years or so, natural gas prices plummeting and likely to remain so, and heating systems ever tighter and more efficient, the payback period for sub-slab insulation, if you can even calculate it, must be in the decades.
    Building offers many such questions, and solutions; are you building for the 7 year owner or the next 70 years of owners? Not saying I wholeheartedly agree with that 10" below, but if you build for the average occupancy one would assume that MDF is all the better you need for trim [unless you want woodtones in which case the vinyl wrapped foam products would make more sense than a faux paint job], and click-lock flooring over L240... who knows after 7 years w/o window screens maybe the occupants will re-develop a resistance to skeeter bites or a new fondness for bats.
    “I find the curiosity of our men with respect to this animal is pretty much satisfied.”
    ~ Meriwether Lewis

  4. #4
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Have you ever done an energy model on your houses? Shouldn't be too hard to make an economic determination that way.

    Chris Corson makes a good argument - there are plenty of ways to beef up the other insulation later, but pretty impossible to put more in sub-slab afterwards.

  5. #5
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    This posting by the redoubtable Martin Holladay cites both Canadian and US studies pointing to savings for insulating basement walls. No mention of slabs. (And just seven years after the Canadian study, Canadian natural gas costs have been halved to 3.57 C$/Gj.)


    are you building for the 7 year owner or the next 70 years of owners?

    An energy payback period of 70 years is of no benefit to the initial buyer, let alone the builder. And as an infill builder who has made a career demolishing 30-50 year old homes to replace them with new homes, l'm a sceptic of long payback periods.

    Have you ever done an energy model on your houses?

    Martin Holladay's article on energy modeling critics has convinced me that, unless it's your obsession, energy modelling is a massive waste of time and money that's typically not even close to accurate.

    Finally, I see that BSC 511 says:

    "Although the energy savings of sub-slab insulation are not as significant as basement wall insulation, such insulations do offer a significant improvement in comfort and moisture damage resistance (including against summertime condensation)."

    I'm still waiting to hear what is the heat loss from an uninsulated basement floor.
    Last edited by worthy; 07-17-2012 at 12:34 PM.
    "The fatal flaw of all revolutionaries is that they know how to tear things down but don’t have a f**king clue about how to build anything." Jim Goad

  6. #6
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Quote Originally Posted by worthy View Post

    An energy payback period of 70 years is of no benefit to the initial buyer, let alone the builder. And as an infill builder who has made a career demolishing 30-50 year old homes to replace them with new homes, l'm a sceptic of long payback periods.


    I'm still waiting to hear what is the heat loss from an uninsulated basement floor.
    Later first; depends on how well built/insulated the rest of the structure is. Te better the rest is the greater the loss thru the floor relative to the whole. If the windows are left open year round it could be deemed negligible. As for the former, obviously that's one approach and many take it. Two years ago I was working on the assumption that one of the many doomsday/end of the world scenarios would pan out. Glad I moved and changed my name now ; I share concerns with Seibert over the problems with foam[s] in the environment and am not convinced of their full life cycle value but have not been able to convince the OneWorld Gov to mandate [conditioned] 62°/83°... yet.
    “I find the curiosity of our men with respect to this animal is pretty much satisfied.”
    ~ Meriwether Lewis

  7. #7
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Scan211.pdf
    Quote Originally Posted by Dancing Dan View Post
    Have you ever done an energy model on your houses? Shouldn't be too hard to make an economic determination that way.
    Short answer is yes, we do energy modeling.

    Considering 90+% of the projects we do have unconditioned basements, when I look at the consumption due to slabs it is generally very little because it is outside of the envelope.

    Much to my surprise this morning, I ran this report on a small, 2 bed, 2 story house which is a slab on grade, and found the loss consumption of energy through the slabs to be almost as much as the above grade walls.

    To all the PH folks out there, sorry I picked on you. I like you now.
    "First we finish the game, then we’ll deal with the Armada!"

    Sir Frances Drake

  8. #8
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Yeah, I'm not sure Worthy is interpreting Martin accurately - I'll send him a link to this thread and see if he wants to weigh in.

  9. #9
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Some comments on this thread confuse insulating a basement floor with insulating a slab on grade; the two issues aren't the same.

    In most northern climates, a continuous horizontal layer of rigid foam makes a lot of sense when building a new building with a slab-on-grade foundation. However, installing continuous insulation under a basement slab is less cost-effective.

    Here's what I recently wrote in response to a question on the topic posted on the GBA website:

    "Insulating an existing basement floor will rarely be high on the list of cost-effective retrofit options. But if you are aiming for the Passivhaus standard or willing to spend the big bucks required for a deep-energy retrofit, then it may make sense to insulate your existing basement slab."

    Needless to say, if you are installing hydronic tubing in your slab, you ALWAYS need a continuous layer of horizontal insulation underneath.

  10. #10
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    worthy,

    Feel free to ignore the money and energy being wasted by leaving out sub-slab insulation if you like, but if you build in Toronto there are a whole host of other important reasons to insulate your slabs.

    For me, the personal experience of spending time at my sister's house in northern Vermont with an occupied un-insulated daylight basement has taught me enough. The house is well insulated for the 80s (2x8 fiberglass walls, truss roof), but the bedrooms on the lower level there have really unpleasant moisture, mold and mildew issues that are almost impossible to fix. The concrete slab and walls sit at soil temperature all summer and winter long. They're slick with moisture in the humid summer leading to constantly running high $ draw dehumidifiers. In the winter the rooms are uncomfortable no matter what the thermostat says. It turns out 70 degree air is no match for 45 degree radiant surfaces surrounding you on two sides...

    In contrast, the buildings we build with R-10 to R-20+ under the slab and walls are beautifully dry below grade. They don't stink like we're used to basements smelling, they just feel clean and dry. It's a tremendous improvement over what people are used to with the typical existing buildings in our area. Honestly, I would never build a house without insulation and vapor barrier between wet soil and concrete for the quality and comfort issues alone.

    When you add on the substantial money that energy models show are flowing out of typical below grade areas of a building, there's no doubt in my mind.

    Seriously, why would you trust Dr. Joe on all the rest of 10-20-40-60 but not on the 10 part?
    Jesse Thompson
    Kaplan Thompson Architects
    http://www.kaplanthompson.com/
    Portland, ME

  11. #11
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    I'm still suspicious you're not actually looking for real data, just a reason to justify your heresy to Dr. Joe, but here are some energy modeling numbers off a current project.

    3,000 SF house in Portland, Maine. 7,300 HDD. R-30 or so walls, R-50 code roof, R-20 basement walls, 1.0 ACH50 proposed leakage. Modeled in PHPP.

    Slab R-1: 86,500 kBTU / year losses.
    Slab R-10: 12,580 kBTU / year losses.
    Slab R-20: 8,473 kBTU / year losses.

    This assumes an interior basement temp of 68, which is higher than most people run in our area if only used for storage, which will magnify the losses. If I turn the house temp down to 55 degrees in the model instead of 68 the slab losses are as follows:

    Slab R-1: 27,260 kBTU / year losses.
    Slab R-10: 4,000 kBTU / year losses.
    Slab R-20: 2,670 kBTU / year losses.

    Those are big numbers for the uninsulated slabs with fuel oil at 140 kBTU / gallon and natural gas at 100 kBTU / CCF...
    Last edited by jesse@kaplanthompson.com; 07-17-2012 at 10:21 AM.
    Jesse Thompson
    Kaplan Thompson Architects
    http://www.kaplanthompson.com/
    Portland, ME

  12. #12
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    I see my attachment got buried under Dan's quote. If you go to the left of Dan's quote in my post you can click on the attachment and it shows you the consumption summary. quite compelling.
    "First we finish the game, then we’ll deal with the Armada!"

    Sir Frances Drake

  13. #13
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    Ted S.

    A good illustration of the importance of insulating a slab. Of which I never doubted. But I am questioning the savings from insulating the floor of a full basement foundation, as Martin pointed out.

    JT

    I've always insulated basements. Since 2004, I've used either: 1" of XPS followed by high-density fg; or three inches of ccspf. These basements are dry and require only moderate dehumidification.

    By contrast, my current (temporary) residence was built in 1964 with the only insulation being about four inches of blown in the attic. Two dehumidifiers in the 1,850 sf eight foot-high basement are yielding two gallons daily!

    None of the prescriptive "compliance packages" in the Ontario Building Code for my heating zone of less than 5,000 HDD (Toronto's ten-year average is under 4,100 HDD) calls for insulation below a conventional basement floor.

    However, your Portland, Maine example with 7,300 HDD is apparently another matter. Your modelling--to the extent it's reliable--certainly gives one grounds for considering sub-slab insulation in a colder climate.
    Last edited by worthy; 07-17-2012 at 02:04 PM.
    "The fatal flaw of all revolutionaries is that they know how to tear things down but don’t have a f**king clue about how to build anything." Jim Goad

  14. #14
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    If you are 8' down and the constant temp of the soil is 50°-55°F, how much energy are you losing with a comfort level of 65°F? Is the cost justified? I can see it for slab-on-grade, but for deep basements without in-slab radiant does it make that much difference? When the foam degrades, will it cause cavitation and ultimately cracks that would be bigger headaches?

    Phil
    It's better to try and fail, than fail to try.

  15. #15
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    Default Re: Sub-Slab Insulation--bragging point or pointless.

    It's interesting that we couch our discussion about how much insulation is justified, in part, on recent trends in the price of fossil fuels. In that regard, a couple of points (hopefully not too tangential):

    One point is that the Passive House geeks would argue that even if natural gas were free, it's a bad idea to burn it because there are serious issues about what it's doing to the atmosphere. I don't want to get into creationism versus global warming or Al Gore versus Dick Cheney, but assuming for the sake of argument that CO2 emissions have the potential to really harm the living conditions of our grandchildren, that would cast this insulation issue in a different light. Passive House people want carbon neutral, not cheapest path.

    But even if you want cheapest path - it's worth considering that the latest drop in natural gas may not reflect the underlying resource reality. We've all seen the market do crazy things. One point two million new homes in 2007, 350 thousand new homes in 2011 -- that's the power of the banking system, government, and interest rates, coupled with the market's tendency to overshoot. It happened to us, it also happens in oil and gas. There is huge investment in tracking and a big increase in production right now. But is that investment cost-effective, and are those price declines real? There's no way to know, really. The oil and gas industry has surplus dollars to invest in production, precisely because the supply has been tight and their profits have gone up. Now they're flooding the market with product that is cheap. But whether that price is real, or just the fallout from another speculative bubble, we do not know.

    Meanwhile alternatives to fossil fuel are stressed by the fossil fuel industry's ability to over invest and over-produce, in the short term. If the same investment capability was backing, say, insulation manufacture or solar panel manufacture, we could look at prices and say "hey, why put in a heating system when insulation is free?" ... but it happens the shoe is not on that foot.

    Long run, it's a gamble either way. The nice thing about gambling in the direction of a sustainable house is, if you guess wrong, hey, you missed out on the easy path of cheap methane. If you guess right, well, those people in the methane-heated under-insulated houses are crying and you're sleeping in.

    I guess it's a crapshoot on the money angle. Also on the save the planet angle -- although the stakes there are much higher. But on the other hand, what can one person do?

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