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Rigid foam: Shrinkage? Creep? Taping and foaming seams?

Brad Hardie | Posted in General Questions on

So I was prompted to post this as a separate thread….

I’m in the middle of building a zero-energy home and barn. I’m using LOTS of EPS reclaimed foam on the exterior. To be exact….
The barn gets:
2 layers of 2.75″ on the walls & 5 layers of 2.75″ on the roof
and on the house:
3 layers of 2.75″ on the walls & 6 layers of 2.75″ on the roof…
Like I said, “LOTS”!

I’ve hemmed & hawed for literally hundreds of hours about insulation for my project, considering nearly every material – well except pink fiberglass batt insulation (I was beaten with it as a kid!), and researched and dug up material on virtually every way insulation strategy. I settled on PERSIST and or the REMOTE technique – check out CCHRC.org for more info – it is a fantastic solution.

Ultimately I wanted to be foam free, as my project is fossil fuel free too, but the price for the reclaimed EPS insulation was simply just too good to pass up. Plus, I wanted to build a REMOTE house too, and they used EPS, so it was a perfect fit. I found the reclaimed EPS I’m using from what was InsulationDepot.com and is now Nationwide Insulation. It is in really remarkable condition.

Since deciding on my insulating technique, my neighbors have thought I was going into the styrofoam insulating business because we have five tractor trailer loads of EPS on the jobsite. I’ve had several many discussions with folks and have read as much as humanly possible it seems on the best building practices. I even took classes from renowned folks on insulation, several in fact. There was one specific class, with one set of instructors whom I had some very passionate conversations about this strategy I had chosen, and the strategies they liked (cellulose primarily). Great folks…I REALLY love a passionate type.

The root of problem was (in their eyes) that I had not decided to tape and foam every seam, on both the walls and roof. Now I’ve tried to read nearly every study about exterior insulation strategies and many weren’t advocating for any/just one/ or both tape and foam at the seams. These guys were, and it kinda threw me for a loop…it was a lively conversation. The conversation was about how no matter how many layers you put on, you’re going to have creep (not from gravity, but from shrinkage), whether the shrinkage was caused from aging, or temperature. There would be creep and shrinkage on every layer, and I would be doomed if I don’t tape and foam.

I had spent a good bit of money on this mutli-day class and they were telling me what so many others weren’t. I was again frustrated. Even though the manufacturers and a few scientists stated that foams do shrink (but most of that was initial aging, and if temperature based – not multi-layered), they (the scientists) were recommending it for general project use.

So after this long, drawn out post – what is the word on the street? I was going to both tape and foam my seams – is it excessive (I already know that the amount of exterior foam is “excessive”)? Do we really need to do foaming at the seams, everywhere (cracks, holes, broken corners – sure)? Do we really need to tape every seam on every layer? Should it be done on just one thinner layer or two, or even when you are putting it on multi-layered and thick?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    Q. "I was going to both tape and foam my seams - is it excessive? Do we really need to do foaming at the seams, everywhere (cracks, holes, broken corners - sure)? Do we really need to tape every seam on every layer?"

    A. To my mind, you need, at a minimum, to tape the seams of the outermost layer of rigid foam with a high-quality tape like Zip System tape or Siga Wigluv tape. If I were you, I would repair big holes and broken corners with canned spray foam (trimmed after curing) before taping the seams.

    Of course, if you have multiple layers of rigid foam, the foam seams should be staggered.

    Whether or not it makes sense to tape the seams of the lower layers of rigid foam is a judgment call. When you are installing three layers of rigid foam, as you are on your roof, taping the seams of every layer is probably overkill. But your decision depends on your budget, your energy performance goal, and whether or not you have an obsessive personality.

    Good luck with your project.

  2. Brad Hardie | | #2

    Martin,

    I was definitely planning to spray foam any cracks, hole and broken corners like you suggest, that to me is and always has been a given for me at least. I'm looking for any data on the; what, when, how much, and where - in regards to the foam shrinking, and the relation to (canned) spray foam and tape has on the outcome.

    I'm not certain the budget supports taping at every seam - unless there is data to support doing it any other way would be ridiculous and a serious mistake. I'd say my energy performance goals are also based on the last sentence.......I think taping the outer layer as you say or taping both layers on a two layer setup is wise and prudent (but I don't necessarily know of any hard evidence to back that belief up). Lastly, I think anyone REALLY interested in this stuff....MUST have a slightly obsessive personality (guilty as charged)!

  3. Rob Myers | | #3

    Martin,
    I don't want to muddy the water here and I welcome a critique from a more knowledgeable person.
    I am all for redundant design but surely the use of tape depends on the structure of the wall and the intended function of the foam. Brad describes this as a PERSIST/REMOTE type wall which implies that the air barrier and water resistive barrier are at the sheathing. If this is the case, then what is the point of taping the foam (i.e. what exactly are you trying to protect).
    In regards to foam shrinkage: If the foam is fitting poorly then there is obvious value in filling voids. However, I don't see how taping and/or foam filling will have any effect on thermal performance if the foam shrinks.
    Looking only at the effect of shrinkage on thermal performance, I did a few quick calculations - and hopefully I got them right (as you know I don't do this for a living!).
    For walls with more than one layer of foam, the only factor that I could see that affects the loss of performance due to shrinkage is the number of layers of foam (assuming staggered edges). With no inner wall insulation and a fixed overall thickness of foam, then for 2 layers of foam, the % loss in R value is roughly the same as the % shrinkage of the foam, at 3 layers the % loss in R value is about half the % shrinkage.
    If there is interior wall insulation (as in a REMOTE structure), the effect becomes even smaller.
    As an example to show the magnitude of the effect - for a 2x4 wall (R12) with 2 exterior layers of 3" foam (total R24) with offset seams and a foam shrinkage of 1.5% the R value of the assembly changes from R36 to R35.73 (0.74%). Assuming my numbers are correct (and hopefully someone will check them), then I don't feel that this is a big concern.

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