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Community and Q&A

Slab to foundation wall: Air Barrier and capillary break detail with interior rigid foam

user-6356169 | Posted in Green Building Techniques on

We’re planning on insulating our basement walls with 2 inches of Thermax (foil-faced polyiso) rigid foam, installed with adhesive on the interior of the wall. We’ll also have 2 inches of XPS beneath the 4 inch slab, and would have these different types of foam connect right on top of the footer, similar to the detail shown at:

So between the slab and the foundation wall there will be the 2 inches of Thermax. Question is … how should we handle the air barrier at this line where the slab butts up against the foil-faced Thermax on the wall. Would any particular type of sealant be recommended for this application (foil to concrete)? Or should we maybe just seal between the XPS and the Thermax, before the slab is poured? Or is this all unnecessary – will the concrete against the two types of foam be sufficient as a seal? Note: We will not be furring or framing up the walls or finishing the basement for now (maybe later.)

Also, in reference to above linked detail, if anyone would like to explain the implied installation of the capillary break, that’s also confusing to me. We will have some type of capillary break, yet to be determined, but were thinking it would just be installed (or liquid applied) between the footing and the foundation wall. This detail seems to show it lapping up above the sub slab foam. I guess part of this is to prevent capillary movement of moisture up from footing to the foam on the wall? But why would it need to lap over the poly under the slab? And wouldn’t this make any sort of sealing between the sub slab foam and the wall foam a lot trickier? Could we just extend the capillary break flat on the footing so that it would also be under the end of the Thermax? Wouldn’t that be good enough to prevent any wicking?

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  1. GBA Editor
    Martin Holladay | | #1

    The usual way to seal the crack at the perimeter of a basement slab is with high-quality caulk. You caulk this crack after the slab has cured.

    There are lots of different materials that can be used as a capillary break above a footing. Some are liquid-applied products; others are sheet membranes. While the detail that you linked to shows a membrane lapped in a specific way, you don't have to use a membrane and lap it as shown in the detail drawing.

    For more information on this topic, see Capillary Breaks Above Footings.

    -- Martin Holladay

  2. user-6356169 | | #2

    OK, understood that the normal way seal this crack is with caulk, but since we won't have a concrete to concrete seam, but a concrete to foil face seam, would that change anything in terms of the type of sealant recommended? Would polyurethane caulk still be best?

    Also, I'm wondering about air leakage up the back side of the polyiso. That will be stuck to the wall with adhesive, per manufacturer's instructions. But I can imagine that there would be a decent chance for air channels unless installer (won't be me) is very careful to create a continuous and substantial horizontal bead of adhesive. Do people ever worry about air leakage along the back side of interior foam board?

  3. Expert Member
    Dana Dorsett | | #3

    "Self-leveling" polyurethane caulk is the right stuff for sealing the joint between slab & foundation wall, or other cracks in the slab. It's also the right stuff to use sealing the slab to wall foam that extends below the top of the slab.

    DO NOT put Thermax between the slab & foundation wall!!

    Use EPS or XPS in that location, extending from the footing to slab-top level or higher. Polyisocyanurate (Thermax) will wick & retain moisture, whereas polystyrene (EPS, XPS) won't. Even if you installed a capillary break at the bottom edge of the foam it's unlikely that it would be rugged enough to seal perfectly every during the slab pour & cure. ( It's common to keep slabs saturated-wet for days after the pour to limit cracking as it cures. Good luck on keeping the polyiso dry enough everywhere in that scenario!)

    At 2" the XPS has a labeled LTTR of R10, but over the lifecycle of a house it will decline to ~R8.4 as it's climate damaging HFC blowing agents leak out. If you use EPS of similar density and compression ratings (1.5lb density Type-II EPS is good enough) it would start out at R8.4, and would remain R8.4, but it's significantly cheaper than XPS, and for 3" you'd have R12.6 now, R12.6 in 50-100 years. You could also opt for 2.5" EPS (a fairly standard thickness) for R10.5, which is usually ~15% cheaper than 2"/R10 XPS.

    EPS is blown with pentane which has but a small fraction of the greenhouse gas potential of the HFC used for blowing XPS. Most of it leaves the foam in the first day after manufacture, and most is recovered at the factory (presumably to meet air pollution regs), often burned for process heat rendering it no more harmful than CO2 (compared to 1000x CO2 or higher for most HFC blowing agents.) Even the fraction of pentane that escapes or diffuses out of the material in the following weeks is only ~7x CO2 for greenhouse gas potential.

  4. user-6356169 | | #4

    Many thanks Dana, for catching us before we made a big mistake! I'm realizing now that I've overlooked a very important characteristic of rigid foam: it's water absorbing potential. I think I was confused because polyiso is considered fairly vapor impermeable (perhaps just due to facing??) when used as part of as a wall assembly. But I see now that vapor permeability and water absorptiveness have to be considered separately. So we definitely won't use the Thermax until we get above the level of the slab. Our builder might have caught us on this one, but I'm not sure ...

    And we've been trying to use EPS instead of XPS wherever possible, mainly because of the kinder manufacturing process, but I didn't realize that XPS lost more of it's thermal resistance over time. We decided on XPS under the slab, because I've seen information suggesting that EPS might not be strong enough under a 4 inch slab. But sounds like you think that as long as we get 1.5lb density Type-II EPS or better, we should be OK. Am I getting that part right?

    In terms of tendency to absorb water (what's the term d'art for this property??), are you saying that EPS and XPS are about equivalent, that they basically are non-absorptive??

  5. Expert Member
    Dana Dorsett | | #5

    Polyiso without facers varies in vapor permeance with density, but it semi-permeable. But it's also hygroscopic (unlike polystyrene).

    Type II EPS (~1.5lb nominal density) is typically on the order of 15psi @ 10% deformation, just like 1.5lb XPS (Foamular 150, etc) Type-IX (~2lb density) EPS is typically 25psi, but not really necessary under a reinforced 4" slab. Foam under footings has to be specified by an engineer to ensure compression & creepage issues are well bounded for the specific loads the will carry, but under-slab foam isn't very critical in residential basements or slab on grade (except under grade-beams.)

  6. GBA Editor
    Martin Holladay | | #6

    You might want to read this article: Choosing Rigid Foam.

    -- Martin Holladay

  7. user-6356169 | | #7

    Thanks, Martin. All laid out very clearly there. Don't know how I missed that article.

  8. onslow | | #8

    I would like to add some thoughts and observations to the thread as I am currently finishing the detailing of my own basement slab. I my case I opted for external insulation on house and foundation so my slab edge meets the wall -concrete to concrete. As others noted, self leveling polyurethane is the best long term sealant for this location and material. Since you plan to use foam at the edge of the slab, the urethane might work or might attack the foam. Best to check. You might find that the foam will stick to the slab pour enough to cause the gap to appear behind the foam which will hamper sealing the edge if radon is the concern.

    The gap will appear as the concrete shrinks and the edges will pull away from the walls. I planned for this and made sure to prevent random bonding points by attaching sill plate foam to the walls at the finished floor height. The final gaps vary from 1/2" to 1/8". I am using a grinder to prep the slab edges for best adhesion and stuffing backing rod into the gap to create the appropriate sealant thickness. It might be a good idea to google slab to slab sealant procedures.

    If you have soil moisture issues, perimeter drains might not be enough to keep the soil inside the foundation perimeter as dry as desired. I think it is a worthwhile cost to place washed stone 2-4" deep below the foam to eliminate damp soil staying in contact with the foam, particularly EPS. 4" of stone will also allow pre-installing radon collection piping, something that gets crazy expensive after the fact.

    I used recycled XPS to be green (and cheap) and found 3" 25 psi material. I was very grateful for the extra density after seeing all the stomping around that goes on while setting the plastic barrier and metal mesh. In other contexts I have not been impressed with the durability of EPS sheet. As a side issue. the plastic sheeting I chose came from Home Depot. It is called Americover and is string reinforced 6 mil, 12' x 100'. Worked like gangbusters and only ~100 bucks a roll. (PSR612)

    Regarding capillary breaks between footings and walls. My foundation has multiple step-downs which creates two problems. The idea of a film on the footings is trickier to execute and when the perimeter drain is placed, the potential wet line ends up looking more like the trim board along side the steps in a staircase (if that image works). Some of the liquid applied barriers I checked on called for the footing to cure for too long before application. They were not cheap either.

    Ultimately, I went with a fairly expensive product called Xypex ,generally used commercially. I left behind a miserable damp moldy mess of a basement, so the prospect of a waterproof one seemed heavenly. The Xypex additive forms crystals that fill in the micropores and cracks up to (?) 1/32 which is supposed to stop bulk water transmission and most if not all of the wicking upward. Transpiration of moisture is still possible, but after a year and a half of experience I can safely say that my basement is heavenly and dry as any I have seen. I have had no problem putting cardboard boxes directly on the floor for extended periods. Nothing behind the pegboard or sheet goods leaning on the walls to concern me either.

    Last thing to warn of....don't let the crews put the perimeter drain on top of the footing against the wall. It is all too common an error which bring the wet line up closer to slab level in most foundation/slab layouts. It also places the wet line above the capillary break. Water is patient and persistent so having standing water right where the wall sits demands absolutely perfect wall coating and sealant at the wall/footing junction.

    Hope this helps.

  9. user-6356169 | | #9

    Thanks, Robert, for taking the time to detail your approach. You offer lots to think about. And you seem to be as cautious as I am about relying on a seal between foam and concrete.

    I’m also similarly concerned about a gap forming between the back side of the vertical foam (XPS or EPS) coming up from the footer and the foundation wall. Robert suggests that the foam might stick to the slab and be pulled away from the wall as the slab cures. Since there would be no hurry to apply the polyiso sheets to the walls above the slab level, couldn’t we, after the initial curing/shrinkage of the slab but before installing the polyiso foam above the slab level, apply some of the same sealant to the back of the top side of the vertical foam coming up from footer, where it meets the wall? So in effect we’d be sealing the foam on its front side to the slab concrete and on its back side to the wall concrete. I suppose that if the bead were too large along the top back side, this could cause problems getting a tight butt between the two sheets of foam (XPS or EPS on walls up from footer to slab and polyiso on walls above slab.) Any thoughts on this approach?

    Our builder’s idea is this: bring the vertical foam up from the footer, but only to a level that would be about 2 inches below the slab surface. So the 4 inch slab would be reduced to 2 inches at the edges. I’m hesitant to have a discontinuity in the thermal insulation at this point – seems like the cold concrete along the edges might lead to condensation and mold problems. But this approach would at least allow for a concrete to concrete seal at the wall/slab intersection, and would eliminate our concerns about air leakage along the back side of the foam. (And radon is a big concern in our area.) Any thoughts on whether this would be a reasonable tradeoff?

    Finally, can anyone offer assurance that self leveling polyurethane sealant would not attack the foam? Are there specific brands that are spec’d as foam compatible?

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