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Insulation in Walkout (mixed-walls)

Posted by engineer_ryan (My Page) on
Tue, Jun 24, 08 at 14:59

So I've thoroughly digested the Building Science piece on basement insulation, and I'm almost ready to go. I'd like some feedback on one area, though.

We have a walkout basement - so the entire back wall of our basement is normal framing. The front wall in completely concrete. The builder has insulated everything in the basement with unfaced batting (where there's framing) and foil-faced rolled-fiberglass insulation that is attached to concreted and framing.

So I intend to remove the foil-faced insulation everywhere, and on concrete: glue 2" EPS to the walls, frame, insulate further with unfaced batting, then 1/2" drywall.

On the framed wall I will leave the existing batting and apply drywall to the framing.

It's the mixed walls on the sides that I'd like advice on - the foundation stair-steps down on both sides, such that there is no framing on the front side, and it steps down so that there is 50/50 framing and concrete at the midpoint, and then a completely framed wall towards the back.

Should I apply EPS only to the concrete portion of the wall, and then build the framing on top out with furring strips so it sits flush with the EPS, and then frame normally? Or use EPS top to bottom, again with furring strips to make the framed portions flush? My instinct is that it doesn't matter - the latter will just give me more insulation. I'm not sure if there's a moisture issue I'm not thinking of, though.

Sorry for the long (first) post, just trying to get my head around this one.


Follow-Up Postings:

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RE: Insulation in Walkout (mixed-walls)

EPS on the concrete.

I prefer XPS (extruded polystyrene) as it provides greater R value per inch, but EPS provides greater R value per $ spent.


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RE: Insulation in Walkout (mixed-walls)

Thanks worthy. I'll look more closely at XPS. Any brands that you prefer for either product?


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RE: Insulation in Walkout (mixed-walls)

I've used only Dow Styrofoam brand XPS tongue and groove for sheathing and basement insulation. Dow invented XPS, but I don't know of any differences with other brands.

As you can see, I'm a fan of XPS for sheathing too.

51 Chel. ext. XPS i

Here, extruded polystyrene (XPS) tongue and groove insulation is used instead of OSB or other
sheathing. The XPS insulates and acts as a backup drainage plane. (The 15lb felt the masons un-
necessarily added is the primary plane.) It's also an air and vapour barrier.

Here is a link that might be useful: What is XPS


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RE: Insulation in Walkout (mixed-walls)

Looks like nice work, worthy.

Looking at Dow's installation instructions, they don't mention taping the seams. I suppose the mechanical joint (t&g) obviates this. Do you concur?


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RE: Insulation in Walkout (mixed-walls)

It's not necessary to tape. But for my own basement I would consider it if the boards weren't really secured against the concrete wall. The less moisture laden air getting behind the foam, the better.

In the house above, I taped the butt joints on the inside.


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