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I'm converting a carport into heated space and decided to install a subfloor to raise the room to the same level as the rest of the house. I have roughly 1"-3" of clearance below my floor joists, so it isn't a crawlspace. I'm not planning on making it a vented space, and I've read about different approaches to seal spaces like this for insulation purposes.

Here is a picture of the most difficult part of the insulation project before the rim joist is installed:

Edge of subfloor

I'm trying to figure out which insulation solution is the best for my scenario:

  1. It's technically an 'unvented crawlspace'. Can I use air-sealed rigid foam insulation along the rim and above the bricks and be done with it?
  2. Should I use R19 kraft-backed batts all the way along?
  3. Should I use R30 kraft-backed batts all the way along?
  4. Combination of two of the above?

Note: I'm in US climate zone 3 and care more about cooling my house than heating it.

I've read about heat soaking through the floor. Would this be an issue with approach #1? Would the difference between R19 / R30 actually be noticeable?

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  • If you'd expect the air and soil below the room to always be cooler than the indoor air temperature you won't lose much energy that way. Whether there's a cost benefit to insulating depends on the temperature differential. – isherwood Apr 13 '20 at 19:07
  • Considering it's a concrete carport slab, I would think it would be cooler than the indoor air temp usually. This area gets up to 114 F or so in the worst of summer. – Dragonsdoom Apr 13 '20 at 19:44
  • Is there any chance of water getting into what used to be the car port? Like a sloped yard, or a normally quiet stream? – SteveSh Apr 17 '20 at 20:45
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On a similar project, I put down 2" (?) foam board insulating panels, then filled the rest of the cavity with Rockwool bats. Inspector was happy. Here's what it looked like from the outside, before the wall was finished (opening used to be a doorway)

enter image description here

Edit 1: Added picture of filled joist spaces

No, I filled the entire cavity

EDIT 2

Original plan called for (from the existing slab upwards) crushed stone, 6 mil vapor barrier, 4" concrete slab, up to near the top of the foundation wall. Then the sill plate with the floor joists on top of that. Final step was filling the rest of the void with closed cell foam insulation. We deviated a bit because of the added cost of the stone & concrete.

enter image description here

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  • When you say you filled the rest of the cavity, based on the image it looks like you don't have the space between the joists filled, just the space under the joists. Am I reading that right? – Dragonsdoom Apr 13 '20 at 19:40
  • thanks for adding the additional information for clarification – Dragonsdoom Apr 15 '20 at 11:48
  • I'm planning on marking this the accepted answer soon, I just want to see if someone is willing to offer a second perspective. – Dragonsdoom Apr 15 '20 at 11:50

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