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Old 05-07-2019, 10:42 PM   #1
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Under slab insulation

Can someone shed some light or share their experience on under the slab insulation? I am planning to put put 2 inch EPS on top of the gravel and then top ip with 6 mil poly however there are few things I are not clear to me. Do I need to put EPS on the footing as well or do I terminate it right before the footing? What kind of EPS should I be using? How do I terminate the poly?
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Old 05-07-2019, 11:08 PM   #2
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Re: Under slab insulation

We only see it in the walk out area of the basement where the footing is lower and not in the way. Unless you are doing in floor heat there is not much benefit for the rest of the basement. We only have an 18" frost depth so we can put the foam for 2 ft from the wall under the slab, or against the wall inside or outside the foundation down to the footing or 2ft.
If you are insulating to where the footing is exposed I would just stop at the footing because when you finish the basement you will likely build an insulated wall above the footing.
We do a chalk line in the walls for the top of the concrete then we glue sill gasket just up to the line with acoustic sealer and then bring the poly up to the same line and glue that with acoustic sealer.
I am told that the sealer we use is not available in the US and quad caulk is used instead.
Do not use flat bottom gables with scissor trusses.
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Old 05-08-2019, 01:04 AM   #3
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Re: Under slab insulation

Without an in-floor heat source, the insulation really doesn't accomplish anything.
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Old 05-08-2019, 06:43 AM   #4
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Re: Under slab insulation

A couple of issues.
Recent thread discussed rigid foam becoming water logged over time which I didn't think happened. More reading on that issue would be good as once soaked with water that foam would be virtually useless.

I'll add a link on insulating basements that talks about footings, except I don't recall it mentioning the previous issue above.

Moisture, both water and damp soil need a lot of attention to keep the surrounding area as dry as possible.

There are also different grades of rigid foam, some are stronger and specifically made for below grade applications (buried in the soil). The typical blue or pink are not.

And as mentioned, your intended use affects how much effort and money you should sink into that hole. Making a basement into quality living space is both difficult and expensive.

I volunteer my help and opinions, but you are responsible for what YOU choose to do with that information.
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