Basement XPS Insulation Installation

darrencarterNovember 9, 2011

Hi all,

I've seen a lot of postings on this topic, but like others, I still have a few questions. I live in the metro area in Minnesota, and plan on installing 2" XPS insulation directly against the cinder block walls in my basement (circa 1964), followed be 2x4 walls with drywall. My question has to deal with the nature of a block wall having lots of cavities with the mortar joints.

1. If I use an adhesive to glue the XPS to the walls, will there be an issue with the small gaps at the mortar joints condensing water?

2. Do I need to seal the top and bottom edges with a caulk?

3. Do I butt the XPS all the way to the floor, or do I leave a gap? I've heard about capillaries and rising damp, but do not fully understand what they mean.

Thanks in advance!

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Installed properly, the XPS ensures that warm inside air does not hit the cold foundation wall and condense out moisture. That includes the mortar joints. For best results, either tape the XPS joints or use ship lap/tongue and groove XPS.

If the wall is plumb, there should be no foam board edges that stand out from the wall. Polyurethane foams (Great Stuff, eg.,) can be used for any gaps or around irregular projections or obstructions.

Butt the XPS all away to the floor. Indeed, one of the ways to inhibit "rising damp" on basement floors is to use XPS, so don't worry about using the XPS to the floor.

    Bookmark   November 9, 2011 at 12:44PM
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Thanks for the quick response. So to get an air-tight seal, should I run a bead of caulking along the top and bottom of the XPS between it and the block, or is it good enough just having it adhered tightly against the wall?

    Bookmark   November 9, 2011 at 2:05PM
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According to Building Science Corp., building codes require XPS to be mechanically attached to the wall. (p.2) That plus the foam board adhesive should hold it tight. No caulk required. For the mechanical attachment, just run a couple of 1"x2"s or 1"x3"s across the boards from wall-to-wall using concrete screws or powder charge actuators (e.g., Ramset) to secure them.

    Bookmark   November 9, 2011 at 8:30PM
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I'm about to do the same thing, and would highly reccomend using a metal framing system such as Armstrong quikstix instead of lumber, and mounting the sill track on a strip of XPS. I've never found lumber or fibreglass batts to be anything but a disaster in basement finishing.

    Bookmark   November 10, 2011 at 8:17PM
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Thanks again for all of your help. I have one last question. If I use XPS to insulate the rim joists, what if any thermal barrier do I need? I plan on hanging a ceiling in the basement, so is this enough? I understand the walls will have the gypsum, but not sure what the ceiling tiles will be yet.

    Bookmark   November 11, 2011 at 1:56PM
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According to Building Science Corp., fibre glass is perfectly acceptable as a supplemental insulation between wood studs. I have used it in several homes I have lived in.

I've also used light metal non-structural framing in basements in conjunction with XPS/spray foam walls. In that case, I always place the base plate on a 1" strip of XPS to inhibit moisture from capillary action or minor flooding.

Where ceiling tiles are being used, it's easiest to provide an ignition barrier for the foam with sections of friction fitted gypsum board, i.e., just snap the board to the rough size and push it in.

    Bookmark   November 11, 2011 at 2:46PM
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