What is the best way to insulate a Multi-material wall?

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Steve J
Updated: Oct. 1, 2020

I would like to know if there are any issues or preparation work required when placing 2" EPS insulation against poured concrete walls+concrete blocks+bricks, all part of the same wall assembly?

As the bricks that sit atop the concrete walls are uneven (along the grout and brick face), would it be best to apply a skim coat of mortar, like Type S to parge and even the surfaces, then add my EPS?

I am planning on insulating my Cold Room (area below my porch), similarly to the rest of my basement (2" EPS+stud/mineral wool batts) and condition it with the interior space. I plan on having a door and pass through vent. I have removed all the wood forms and also plan to add at least 2" of EPS to the concrete ceiling and mineral wool. My cold room is a small 6' x 6' space.

Responses (7)

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Steve J 3 years ago
Mike Reynolds 3 years ago

Hey Steve, thanks for the clarification. The good thing about exterior insulation is that it keeps the walls warm but can risk trapping moisture. The bad thing about interior insulation is that it keeps your foundation wall cold. This is not a big problem with poured concrete, but it can be a problem with block foundations as they are only held together by the mortar. So they are more at risk of cracking when exposed to the seasonal freeze / thaw cycle. 

It's tough to really visualize what you have with the three material types, however it's still easy enough to give you tips. If the block wall is an exterior supporting foundation wall then there is the risk of cracking when it freezes. So before doing anything I would inspect it pretty carefully for cracks and see it it heaved or moved all. If so, I'd have a professional come and assess the safety of adding insulation. 

As for materials, there is no need to switch materials at all. EPS rigid foam insulation becomes a vapour barrier at about 2 inches thick, and that is fine to put directly against the cold foundation wall. The most important thing in a basement is not to sandwich wood studs between a concrete wall and a poly vapour barrier. That's where you'll get mold. 

The portions of your wall that are above grade will experience more heat loss for sure, but that isn't something to concern yourself with too much and no need to switch methods. 

You could look over this page about How to insulate basements so they don't develop mold which should give you a pretty clear idea of what the concerns and solutions are. I'm hoping that helps guide you a bit more, let me know if not. 

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Steve J 3 years ago
Mike Reynolds 3 years ago

The way you explain it I don't really see any risk Steve. The risks with insulating basements are exterior walls, and mostly involve vapor barriers trapping moisture in with susceptible materials like typical stud walls and fiberglass insulation.  You can see in the article linked above how to do basements safely with EPS on exterior cold walls, there is even less concern with walls that aren't cold or wet from exposure to the ground. 

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Steve J 3 years ago

Thank you again Mike! I will reread the article and proceed with my reno.

Mike Reynolds 3 years ago

Happy to help Steve!