basement insulation XPS with studs already done.

Old 10-14-17, 09:02 AM
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basement insulation XPS with studs already done.

Hello - I have a couple questions regarding insulating my basement foundation wall.

A good portion of my basement has walls that have wood paneling on them. These walls have fiberglass batts between the studs. I'm not a fan of wood paneling, and have read that batts are not good up against below grade foundation walls. therefore when i do this i plan on removing the wood paneling and the batts, but keeping the studs.

I also have a laundry room/mechanical room that has open walls (no studs or insulation). I would like to insulate but the thought of finishing this room with studs and drywall isn't a high priority.

I have a couple questions:

1. For the walls with studs, to insulate with XPS, could I simply cut the XPS to fit between the studs (gluing them to the wall) and then use expanding foam around the edges of the XPS? Almost everything i read online about using XPS on foundation walls starts with a blank wall and the XPS going up right against the wall, and then building the studs up inside the XPS. Here I already have the studs.The technique i'm proposing is similar to the technique used for rim joists below:

2. For the mechanical room, if I use XPS, is drywall required for proper fire rating?

I've attached a picture of the open basement walls in the laundry room to let you know what i'm dealing with.

Thank you.
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Old 10-16-17, 06:58 AM
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No problem on the cutting to fit and then foaming.

Generally if it's in the interior of the building then it needs to be covered with drywall for the fire rating. But of course, in a mechanical room (I assume unconditioned) drywall is prone to issues. I'd check with local code but Dow Thermax doesn't require being covered and there may be others now as well.
Old 10-16-17, 08:56 AM
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As far as I know Thermax (Dow) is the only one that has invested the time and money necessary to get the approval to be left exposed in some installations. Local code is always the final authority. But that doesn't mean that other foil faced foam boards don't have some protection value. I've used some Dow Tuff-R where it will eventually be covered by drywall utilizing a gap to benefit from the radiant barrier, but that foil covering is a lot thicker than aluminum foil, I have the cute to prove it. My point is, local code authorities might accept something other than Thermax.

In all cases the objective with the rigid foam is to ensure that warm humid air cannot reach a cool condensing surface.

Old 10-16-17, 08:57 AM
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I'm not a fan of wood paneling, and have read that batts are not good up against below grade foundation walls.
Fiberglass insulation in a basement is not an issue, I have installed in every house I have finished the basement, it's a problem if there is water/moisture and it gets and stays wet.

If you have opened up the walls and do not see any issues then I would not rush into replacing if everything looks ok.

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