Sub-slab installation advice - slab edge insulation?
Last Post 31 Jan 2022 07:46 PM by kalcium. 10 Replies.
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kalciumUser is Offline
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26 Jan 2022 03:40 AM
Hi, I'm renovating a house in New York, we are building an addition with a finished basement. We are going to put down 2" EPS foam underneath the slab but I am wondering if we should also insulate the slab edges and if so how? For now we have a footing and then concrete walls poured on top. The basement walls will have spray foam insulation, but I'm wondering if we should add an edge of foam between the slab and the foundation wall to create a thermal gap, and what the pros and cons of this might be.


newbostonconstUser is Offline
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26 Jan 2022 01:06 PM
You want to use XPS not EPS.....If you do 2 inches around the edge the contractor could screed off it when laying the concrete. You might be able to glue it to the wall with a foam gun. I would send it through a table saw to cut it to make a nice flat level cut.


"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
kalciumUser is Offline
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26 Jan 2022 03:08 PM
I've ended up leaning towards EPS based on the fact that XPS can slowly lose R-value value and obviously can't be replaced and is somewhat less 'green'. It was also informed by this article, which also leans towards EPS (https://www.greenbuildingadvisor.com/article/whats-the-best-way-to-insulate-a-basement-slab). I've also heard from my contractor and architect concern that the foam around the edge of the slab would provide a conduit for water from below to come up - does anyone have any thoughts on that and whether concrete over the top would be more protective of under-slab water pressure vs 2" of foam at the edge of the slab?


newbostonconstUser is Offline
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26 Jan 2022 04:54 PM
There should be drain tile at the footing level....your slab is above that...there shouldn't be any water to come up or to degrade the XPS.


"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
kalciumUser is Offline
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26 Jan 2022 05:12 PM
Not sure what you mean by drain tile. As of now it's just a footing with concrete wall poured on top, there will be gravel under the slab then foam then poly then slab. How would a drain tile fit into this? Or are you talking about outside? We do have waterproofing and drainage board on the outside walls.


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27 Jan 2022 02:36 PM
EPS is what you want to use. Greener and ultimately same R-value.

We would pour our concrete slabs within ICF walls. So the 2.5" thick ICF interior EPS provided this thermal break from the ICF concrete core. Depending on how you plan to finish your floor, you could just place 1-2" thick EPS between concrete wall and concrete slab edge. You could also cut the EPS in a triangular shape such that it is full thickness at bottom of slab but tapers to zero thickness at slab surface.

If you have any water pressure near your footings or slab, you have a bigger problem to solve... Any water will eventually find its way through a slab or wall. You need to ensure that you have good drainage around your building...either by properly selecting the building site or using other drainage and water proofing construction techniques to mitigate issue.


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newbostonconstUser is Offline
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28 Jan 2022 12:00 PM
ICF EPS is good....Home Depot EPS not so good....


"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
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28 Jan 2022 03:54 PM
Have never used HomeDepot EPS. One really needs to get correct grade of EPS (i.e., density, strength) for the specific application. Low density/strength EPS can be used for this slab/wall interface thermal break application, but you likely wouldn't want to use it as ICF EPS or sub-slab insulation. We like using Nudura Hydrofoam for our hydronic radiant floor heating sub-slab applications, but it can be pricey.


Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
kalciumUser is Offline
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28 Jan 2022 04:49 PM
Thanks for the responses. We are now leaning towards PolarGuard GPS from Polar https://polarcentral.com/. It uses Neopor from BASF - graphite infused, which apparently helps with R-value. The 2 1/8" board is rated at R-10 and 13psi. I've been told this will behave similarly to EPS w.r.t. moisture and is more green than XPS.
As for the outside, we have waterproofing membrane, drainage board, leading to a drain pipe at the footing that runs to a sump pump to pump it to Cultecs (we couldn't drain with gravity due to a lot of rock on site).
I drew the attached picture to show some of the options. Higher res link here:
Option 0 (not drawn) is do nothing. Just put the slab on gravel. This was the original design.
Option 1 just runs the insulation under the slab, butting up against the footing. Since we hadn't planned for insulation at the time the foundation was dug, this approach has the advantage of not losing ceiling height.
Option 2 runs the insulation over the top of the footing and up the wall, then the slab floats on the foam.
Option 3 (not drawn) would be to run the foam on top of the footing, but no insulation at the edge (so the slab would touch the foundation wall).
I'm wondering how much insulation value we'd be losing by going with option 1 vs 2, and whether that is worth keeping the additional 2" ceiling height (we're at ~8ft now, without the insulation)
SailawayRB and newbostonconst, what do you think of the assertion that putting the foam between the foundation wall and concrete (per option 2) would provide a vector for moisture to come up? This is what the contractor and architect are saying, but I'm not convinced - if there is water under the slab, it is just as likely to migrate through the concrete as through a 2" foam at the edge IMO.

Attachment: foundation.png

sailawayrbUser is Offline
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28 Jan 2022 09:48 PM
"SailawayRB and newbostonconst, what do you think of the assertion that putting the foam between the foundation wall and concrete (per option 2) would provide a vector for moisture to come up? This is what the contractor and architect are saying, but I'm not convinced - if there is water under the slab, it is just as likely to migrate through the concrete as through a 2" foam at the edge IMO."

Well yeah...that was the point I tried to make in my previous response... Presumably you will adequately address any foundation drainage issue and you will put 10 mil or thicker PV sheet under the slab and up the concrete walls a couple feet. Contractors often take shortcuts but just enough measures to not have water issues for a year or so...

Ceiling height is a personal thing. I would never want less than 9'. We have 10' flat ceilings everywhere in our place. I am not a fan of cathedral or vaulted ceilings. I like simple one level rectangle shape designs that can be easily made super energy efficient, constructed for relatively low cost and are easy to maintain. I am not a fan of houses that make an architectural statement. As I said, this is a personal thing...often acquired and refined from constructing and living in many places.


Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
kalciumUser is Offline
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31 Jan 2022 07:46 PM
We decided to go with option 1, with insulation under the slab between the footings only. This seemed the best tradeoff.


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