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New construction insulation plan and conflicting advice

dfvellone | Posted in General Questions on

My location is upstate NY, climate zone 6. New construction is timberframe with full dimension 2×5 exterior insulation frame that gets sheathed with 2) 1″ layers xps, and dense pack cellulose in stud bays. Painted sheetrock provides air barrier and I’ll use Intello-Plus smart vapor retarder where walls get wood paneled. Ive gotten a few insulation quotes and a couple installers are strongly recommending closed cell foam in the walls instead of the cellulose explaining that it prevents moisture issues more effectively…but then I need an exhaust fan devoted to removing moist air. The claim is that “the latest building science indicates that in my climate foam is superior”. My plan outlined above was recommended by my engineer who specializes in passive solar and energy efficient design and my research seemed to validate it as well. The installers are contradicting this advice and citing the latest building science. My eyes are starting to glaze over and my frustration build regarding some of the complexities and conflicting advice. I apologize for the length of the post. I’d like to use cellulose. Is my original plan a sound one?

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  1. Jon_Lawrence | | #1


    I assume your wall stack up from the exterior is as follows, excluding the finish 1) 2 layers of 1" XPS foam, 2) sheathing, 3) 2x6 stud bays filled with dense pack cellulose 4) painted gypsum or wood paneling with Intellio-Plus. If I am correct then my observations are as follows:

    1) I believe your exterior foam r-value is a little low, r-10ish. In Zone 6 with a 2x6 wall you need a minimum of r-11.25 on the exterior and it should comprise at least 36% of the total wall r-value. XPS, unless recycled, has a high global warming potential (as does ccSP unless it uses Solstice as the blowing agent). Alternatively, you could look at regular EPS or graphite infused EPS (Neopor) which gets you a slightly higher r-value than regular EPS. Polyiso is another low GWP option, but its effective r-value drops once temps get below 40 degrees.
    2) Have you looked at installing an ERV as opposed to exhaust fans? This is the time to do it.
    3) Good luck with gypsum as an air barrier, it takes a lot of attention to detail to get it right You might want to consider Zip OSB or a peel and stick membrane over regular OSB or plywood, which can act as your VB too.
    4) In a sense your contractors are correct about ccSP reducing moisture issues - if you were not using the correct amount of exterior foam. If you do use the right amount of exterior foam, then the sheathing will never get below the dew point and you won't have moisture issues.
    5) Have you asked your installers for a copy of what they are referring to at the latest building science? It would be interesting to see what they come back with.

  2. charlie_sullivan | | #2

    Daniel, I agree with Jonathan. On this last question 5), my guess is that Latest building science = studies show that mentioning the words "building science" is now an effective way to sell expensive spray foam without having to actually learn any subtle concepts about moisture transport.

  3. dfvellone | | #3

    To clarify, my engineer specified from the exterior (not including siding) 2) 1" layers xps, seams staggered and taped, full dimension 2x5 studs dense packed cellulose. Though it was specced for sheetrocked walls I'll be using blueboard and plaster. Continuous Intello-plus vapor retarder will probably be used instead of insulweb. There is no sheathing beyond the exterior foam. That gets furred out with 1x material for an air space and sided directly. I can certainly bump up the foam to 3" if that's what I'd need to prevent moisture issues.

  4. charlie_sullivan | | #4

    Yes, if you bump the insulation up to 3", you'll solve the potential moisture problem. And you can use EPS and save money and save the planet compared to XPS. With the Intello, you've have some extra protection against moisture problems so it will be an extra-robust wall. You can also use MemBrain, which is cheaper than Intello.

  5. dfvellone | | #5

    My apologies for not just doing an Internet search, but can you explain the xps vs eps argument. I understand the environmental impact of xps, but from a wall assembly standpoint how do each compare in a situation like mine? Will they perform similarly?
    Thanks very much for the help. Much appreciation.

  6. Reid Baldwin | | #6

    From a performance standpoint, they key property is R value. EPS has an R value of about 4 per inch which stays the same over time. XPS, when new, has an R value of about 5 per inch. Over a period of time, the length of which is subject to debate, the blowing agent leaks out and is replaced by air. Once it has leaked out, the R value is about the same as EPS. Graphite infused EPS has an R value of about 5 per inch which stays the same over time.

    When I looked up prices in my area, XPS was the most expensive, then graphite infused EPS, then regular EPS. Based on that, I could not find any argument in favor of XPS over graphite infused EPS, even ignoring the climate change impact. As between regular EPS and graphite infused EPS, if you are able to increase thickness to get the same total R value, regular EPS will probably be cheaper.

    The other rigid foam option is Polyiso. Polyiso has the highest rated R value per inch. However, the R value decreases at cold temperature by an amount that is not fully understood. EPS and XPS, on the other hand, increase slightly as temperature drops.

    There are some differences in vapor permeance which can matter for thin layers. However, at over 2", common practice is to assume all drying will be to the interior for all foam options.

  7. Dana1 | | #7

    The notion that "the latest building science indicates that in my climate foam is superior" is BS, especially when you're talking about putting a premium priced high R/inch product where it's performance is being severly undercut by the thermal bridging of full-dimension framing. Put the cheap stuff between the studs, save the foam budget for the exterior where A: You get the full benefit of it's R value and B: It can provide dew-point margin at the sheathing.

    The only benefit of foam cavity fill is ease of air sealing, and 5" of open cell foam is at least as air-tight as 5" of closed cell. The difference in envirnomental impact is huge, since half-pound foam is 1/4 the polymer content of 2lb foam, and open cell foam is blown with water instead of HFC245fa. The R-value of o.c. foam is comparable to cellulose.

    At 5.0" cellulose or half-pound open cell foam would run between R18-R19. When the warm side of 1.5lb density "Type-II" EPS is 40F (the condition below which the sheathing would be taking on moisture) it's performance @ 2" is more than R9, and it would be (just barely) sufficient for dew point control on it's own. In combination with an interior side smart vapor retarder it is quite safe, but of course, more is always better. At 2" the vapor permeance of Type-II EPS is still in the 1.2-1.5 perms range, which means there is some drying capacity toward the interior. At 3" that would drop to 0.75-1 perm, which still isn't terrible, not moisture-trap territory.

    At 2" the XPS would be performing at about R11 under the same 40F sheathing conditions, but would over the course of a handful of decades drop to that of EPS of similar density. But the vapor permenance of 2" XPS is 0.5-0.6 perms, which is getting to be pretty vapor tight, which means the assembly MUST be able to dry toward the interior to have much resilience. With a cavity-fill of closed cell foam, even 4" of 2lb polyurethane is already under 0.3 perms- tighter than the XPS. With open cell foam the drying rate toward the interior would be controlled by either the interior paint (standard latex is 3-5 perms), or the smart vapor retarder (variable).

    Polyiso is still pretty good when the COLD side is 40F, it's when the AVERAGE temp through the foam is below 40F that it's performance really begins to suffer. When the warm side of the foam is 40F it's true performance will be somewhere around R8, which means somewhat more moisture accumulates during the coldest hours. But since it's performance rebounds quickly at warmer outdoor temps the drying season begins earlier than with 2" EPS or XPS. As long as you have at least 2", and decent drying capacity toward the interior (== no closed cell foam) it will be fine.

  8. GBA Editor
    Martin Holladay | | #8

    You've gotten some good advice so far. I'd like to provide links to articles that I think you should read:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Choosing Rigid Foam

    How to Install Rigid Foam Sheathing

    Designing a Good Ventilation System

    Here is my advice:

    1. Make sure that your layers of exterior rigid foam are thick enough.

    2. Green builders prefer EPS to XPS.

    3. Dana is right: exterior rigid foam is better than spray foam between the studs.

    4. The need for a mechanical ventilation system (which may or may not be an exhaust system) has nothing to do with the decision on spray foam. No matter what type of insulation you choose, you need a good ventilation system.

    5. I hope you have figured out how to create an interior air barrier when you are installing 2x5 studs on the exterior side of your timber frame. Getting a tight barrier on the exterior side of the timbers is tricky. Buildability is important.

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