Basement framing + insulation

Old 03-04-22, 03:53 PM
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Basement framing + insulation

Hi all! I will be starting the basement finishing at my in-laws soon. It's a new home build and some of the basement has poured concrete walls. Here is my plan and I'm hoping it sounds/looks good. I will be using rigid foam on the entire exterior concrete wall (thinking 1"). This will be adhered and seam taped. I will then frame out my 2x4 wall and finish it with batt insulation for added insulation. I have a few questions surrounding this though...
1) Is there a reason to insulate the void between the upstairs sub floor and the framed wall top joist?
2) Do I need to leave a 1/2" gap or so between the rigid foam and the concrete floor? Maybe sealed with foam?
3) Although I will attempt to have the framed wall directly up against the rigid foam, will the batt insulation serve as fire blocking or do I need to use expanding foam if there's a crack between the top joist and the foam board?

I have an email in to the city inspector to get some clarification on these questions, but it's a very small town and I'm not keeping my fingers crossed.

Old 03-04-22, 06:03 PM
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You need to insulate the exterior rim joists around the perimeter of the exterior walls first.

Typically you want to use 2" rigid insulation, not 1"... to ensure the interior surface of the foam never gets cold enough to sweat. Thermax is the only fire rated foam I know of, so anywhere it's going to be exposed (unfinished rooms) you need to use that along with foil tape. If you are framing over it and drywalling over it, it can be any sort of rigid foam.

It's usually a good idea to shim the foam up so that you can better seal the gap at the floor with spray foam, yes.

Your framing will only be tight to the foam if the foam is perfectly plumb. The main reason you leave a gap is so that you can make the wall plumb even if the foundation wall is not. Foundations sometimes bow in as well, and the space gives you a little room for "what if".

You should fireblock the top plate before you frame, so you typically want to 1, insulate and seal the rim joists with 2" rigid and spray foam the edges. 2, put a band of 5/8" drywall on the ceiling everywhere you top plate will be (as fireblocking). 3, using fire rated foam, seal that fireblocking where it meets the foundation or sill plate. 4, insulate your walls with your rigid foam, taping all seams. 5, frame your walls leaving a gap between the framing and the foam. 6, Fireblock vertically between the studs and wall every 10 feet or less.

And of course, your local inspector trumps anything I said above. Some guys prefer encapsulated pillow/blankets on perimeter walls. You see that in a lot of new houses, mainly because it's fast and can be done without a lot of seams.

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