Sub Slab Polyiso
Tim_O
| Posted in General Questions on
Hello all, I’d like to bring up a discussion point. I can obtain Polyiso boards at a similar price per inch to EPS (factory seconds).
I have come across this page that seems to say that below grade usage is fine. I always thought that was not possible with Polyiso, so this is news to me:
https://www.polyiso.org/page/Below-Grade
And the article on moisture testing:
https://www.polyiso.org/page/TB602
Granted – this is all from Polyiso.org, which is an organization created to promote polyiso. So, have others here used polyiso below grade? I need to do some digging to see what the manufacturers claim as well.
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Replies
Tim-O,
Interesting as up until now I thought it as precluded from use under slabs for both compressive strength and moisture reasons. Here's what Martin said about it:
"Polyiso is not rated for ground contact or burial, so polyiso should never be used to insulate the exterior side of a basement wall; nor should polyiso ever be installed under a concrete slab on grade. It’s perfectly OK to use polyiso to insulate the interior of a basement wall."
Where did his info come from I wonder? Is there another report out there stating it, or is it just something that’s been said for a long time?
freyr-design,
I don't know. It's something I didn't question because that was what everyone said when it came up in discussions here.
That's what I thought as well. Compressive strength is right around 15psi I believe, so it can be used under the slab, but not under footings, load bearing, etc where you might use 25psi foam.
But outside of strength, I thought the main concern was that polyiso absorbed water.
That is my understanding as well: polyiso will absorb and hold water, and you don't want it getting wet. Not sure if that's because it'll degrade more quickly over time, or because it suffers a performance hit, or both?
I know Kohta Ueno weighs in here sometimes, so maybe this will catch his eye and he can offer insight. He consulted recently for us on an "inverted flat roof" assembly (https://buildingscience.com/documents/insights/bsi-052-seeing-red-over-green-roofs check out figure 3) with all the insulation on the exterior, and advised that XPS was the only insulation option for that application.
I know it's not a direct comparison, but the concern was polyiso getting waterlogged, which I'd imagine would be true in a subslab application as well.
Polyiso is available with compression resistance higher than the typical 15 psi but I'm not sure how easy it is to get. It is porous enough to absorb water but its tight pores and typical foil facing means that the water is likely to remain in the foam, reducing its R-value. I once had a pile of polyiso stored outside and it became waterlogged and very heavy. I don't know of any official declaration that it can't be used below grade, I just don't think it makes sense to use a product below grade that has those qualities. Perhaps if the soil were extremely well-drained it would be safe.
Michael,
I wish I understood this better. Closed Cell is supposed to describe a foam that can not take on water, yet I've had EPS hot-tub lids take on enough moisture to easily triple their weight. Are we maybe talking about two different things, one being the cells themselves taking on water, the other water making its way into the spaces between them?
One of the points Charlie Sullivan made in a earlier discussion about this was that EPS being cheaper than Polyiso, the only incentive to use the latter would be the better R-per inch, but when is the depth of sub-slab insulation an issue?
Malcolm,
Great example re: EPS hot tub lids. I stretch mine to 4 years, and by then it's heavy enough to rip the lift-assist arms off the hot tub when raising/lowering. I have never understood why it retains so much water over time (seemingly without ever losing any).
Paul,
For years I did work for a nearby resort with 24 hot tubs. They had spare foam inserts so they could cycle them from the lids to a heated storage area where they dried out. It took several weeks to a month.
I brought this up here on GBA about five years ago and no one had a good explanation.
Malcolm, I would say that "closed cell" is a relative term. It's more accurately called 2-lb foam, being an average of 2 lbs per cubic foot, vs. an average of 0.5 lbs per cubic foot for open-cell foam. Most of the cells in 2-lb foam are sealed but not all of them. Some of the cells in 1/2-lb foam are open but some are probably closed.
EPS is a different beast; it's styrene beads, which themselves are water-tight, pressed together tightly with steam to melt them together. The result is small pathways between beads. The higher the PSI rating, the more tightly the beads are pressed together, leaving less and less room for liquid water and even water vapor to move through.
Thanks Michael.
I think we need a backyard foam test to go along with the backyard tape test. Someone leave a sheet of EPS/Poly/XPS in their yard and report back next year with the weight delta.
This study seems to sort of match the hot tub info from Paul and Malcolm. According to this, in the same ASTM C1512 test that was performed on Polyiso, EPS actually picks up more moisture content.
https://www.airfoam.com/EPS-vs-XPS-Drying-Potential.pdf
Further digging led me to this study. Showing that EPS absorbs quite a bit of water, and actually reduces it's R value ~30% over 20+ years.
https://scholarworks.alaska.edu/bitstream/handle/11122/14916/ASTM%20C1512%20Evaluation%20of%20%20EPS%20and%20XPS%20Insulation%20for%20Embankment%20Applications%20.pdf?sequence=1&isAllowed=y
Tim_O,
Using EPS under slabs was the topic where the discussion about hot tub absorption took place. I had (and I guess still have) three questions:
- Is there something unique to the situation of the lids in hot tubs, like maybe the unremitting exposure to steam, that makes the EPS so readily take up moisture, and precludes any possible drying? In other words, maybe it isn't a good comparator.
- Does EPS under slabs take on much moisture, or does it reach an equilibrium with the gravel below it, which should be pretty dry?
- What would a second vapour-barrier below the foam do?
The University of Alaska Fairbanks project was looking at EPS used under roadways, but it seems to show that EPS continues to accumulate moisture overtime when it is underground as well. I agree, there must be some equilibrium eventually.
Tim_O,
Thanks, that Alaska study is interesting. Since the primary source of uptake is vapour-diffusion, I wonder what the effects of either facers on the foam, or a layer of poly underneath would have?
I agree, it seems like poly below the foam would protect you pretty well. It seems like it would be beneficial no matter what type of foam used as well.
I could go the Jake Bruton/Matt Risinger route and put the foam over my slab.
Malcolm, I have also wondered about hot tub insulation. Vapor drive at a hot tub is always to the exterior, and the lid is impervious vinyl, so moisture is trapped. I think that over time, the heat and pressure just force vapor deep into the EPS pores. Once it condenses, it has too much surface tension to drain out. We need a high-tech, waterproof, vapor-permeable cover over the EPS!
Michael,
"Vapor drive at a hot tub is always to the exterior"
Which makes me wonder how wet sub-slab foam - especially at or slightly above grade - really gets.
As usual it is raining here today here on Vancouver Island, but the soil next to the house protected by the overhangs is powder dry. I suspect the fill under my slab on grade foundation is too. Unlike the foam used to build up the substrate on highways, the foam is protected by stem-walls on all sides, and has a capillary break below it. If the foam reached an equilibrium with the surrounding fill, I suspect it wouldn't be very wet.
I think it depends on the situation. Damp foundations are the norm here on older homes, and not uncommon on new homes. A few years ago I had a foundation contractor scoff at my foundation details for a walkout basement. Then he asked why I didn't have foundation windows on the uphill side. When I asked why we would want those, when the rest of the windows were triple-glazed European models, he said "for cross-ventilation so it doesn't get too moldy inside."
https://www.constructioncanada.net/sink-or-swim-a-comparison-of-below-grade-rigid-foam-insulation-moisture-performance/3/
One more study. But it doesn't talk much about polyiso. More about XPS and EPS and their different performances when exposed to water.
My conclusion is that polyiso probably is fine, depending on the facer. If I were to use it, I'd opt for foil faced and put it above the vapor barrier, below the slab.