Insulating rubble foundation

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Ottawahome
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Insulating rubble foundation

Post by Ottawahome » Mon Sep 30, 2019 9:28 pm

Hi everyone and thanks for reading / helping out - I really appreciate it:

I'm finishing an old 1913 basement in Ottawa (very cold) with a rubble foundation and am looking for opinions on how to insulate.

When I demoed the previous owners work I noticed that they had PL-bonded a Styrofoam right against the foundation. They then placed foam batts right on top of the styrofoam. They then used a very thin bendable plywood type product that was placed on top of the foam batts. Lastly, they drywalled over everything. When I pulled it all down there was mold everywhere.

Does anyone have experience in insulating these old rubble foundations? They're very permeable and I've heard different opinions that range from:

1) Framing 4 inches off the wall and only placing 'floating' insulation batts between the studs, allowing the wall to breathe a ton. No vapor barrier and nothing touching the foundation or the insulation.

2) Spray foaming the entire foundation but leaving the bottom 2 feet untouched for breathability.

3) Combination of Styrofoam + foam batts + spray foam in the rim joists.

Any help is greatly appreciated!
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Shannon
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Re: Insulating rubble foundation

Post by Shannon » Mon Sep 30, 2019 10:01 pm

I’m afraid I do not have any experience with finishing a rubble basement. I can see that these could be very tricky and maybe one way will not fit all. IMO the biggest mistake in the original was the lack of a proper air vapour barrier and also likely compressing the batt insulation with the plywood. My gut says that in most cases Sprayfoam applied directly to the rubble wall would work best creating both a insulation layer and air/vapour barrier all in one. The wall itself should be able to still dry to the exterior.
I can’t say this with 100 percent certainty however. Hopefully someone else here has some actual experience.
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Ottawahome
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Joined: Mon Sep 30, 2019 8:55 pm

Re: Insulating rubble foundation

Post by Ottawahome » Tue Oct 01, 2019 10:55 am

Hi Shannon and thanks so much for the quick reply -

Spray foam does seem likely to be the best option.

I'll keep asking around or maybe someone else here has finished a rubble foundation in a cold Canadian climate.

Thanks again.

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Aaron
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Re: Insulating rubble foundation

Post by Aaron » Tue Oct 01, 2019 11:27 pm

I have limestone foundation in my 1890 house here in Minnesota. I don't think ANYTHING can be done with it. It just needs to breathe, shed its efflorescence, and just be left alone. I have to "sweep the walls" with a broom and all this sand and grit just falls to the floor. It's annoying but I dare not try and cover it.

Ottawahome
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Re: Insulating rubble foundation

Post by Ottawahome » Thu Oct 03, 2019 9:20 am

Thanks Aaron I appreciate that info. I did just find an interesting article released by the National Research Council of Canada. If you scroll to the bottom of this link it talks about rubble and stone foundations. Seems that EPS rigid board insulation (Not XPS - it will hold moisture) and roxul batts is a good combo, allowing the rubble to breathe well and not interfering too much with the freeze / thaw cycle. Will likely go with this option on my home. Thanks again.

https://www.nrcan.gc.ca/energy-efficien ... tion/15639

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Shannon
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Re: Insulating rubble foundation

Post by Shannon » Thu Oct 03, 2019 9:46 am

This is a good article However I see it was last updated in 2016 so check with your local building code department to be sure what you are doing meets code for your area.
Also I see no where in the rubble wall section that it talks about using any kind of foam except for sprayfoam. They speak of Roxul or other batt type insulations.
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