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Insulation recommendations for a brick & block addition?

Hello GBA,

My project for the next few weekends will be to insulate and frame a room built from brick and concrete block (formerly our garage so the floor is a slab). We've built a detached garage and the plan is to convert the old garage to living space. This is permitted work so beyond a personal desire to do it correctly, I also need our township to inspect and approve the work. (We have no GC, this is all weekend warrior work). Because of what we're saving on labor I can afford to splurge where it makes sense. However I don't think I want to consider spray foam so my request is for best practice based on readily available big box store materials.

We are in Zone 5 (Chicago suburbs).

Summary of the area and what I think is the correct approach. I've also included a photo of the room.
1) Floors: The floor is an old garage slab that will be raised to the height of rest of the house using sleepers.
My plan: 6mm plastic sheeting base (taped at seams), a continuous 2" layer of XPS (also taped at seems), 2x4 pressure treated lumber laid on its side (for better weight displacement so as to not compress the rigid foam), then 2x8 laid on edge, then subfloor.

2) Walls: Walls are all two layers: Brick exterior, concrete block interior.
My plan: First install a continuous veneer of 1-2" of rigid foamboard over all of the concrete block (taped at seams). My thought was this would be a thermal break, an air barrier, and added R value. Behind the foamboard I would then build conventional 16 on center 2x4 framing. Eventually this would be filled with rock wool batts. So in other words, the wall would consist from outside to inside: brick, concrete block, rigid foam, 2x4 framing with batt insulation, drywall. If this is a good plan then does the type and thickness of rigid the foam matter? I was thinking of using a faced EPS (one side metallic, the other plastic) that I believe is not air permeable.

3) Ceiling/Attic: traditional rafter construction. No trusses.
My plan: The roof has a low pitch so my thought is to install vent baffles while I can do so from the main floor via a ladder (better than crawling around in the attic). My plan is to cut to fit rigid foam boards for DIY baffles using an approach detailed in a 2011 FHB article. The vertical portion (adjacent to soffitt) would be tied into and air-sealed against the rigid foam installed on the wall. We'll eventually insulate the bottom of the attic to R40 or greater.

Lastly, we will be installing a multisplit so there will be not ductwork to consider.

Would anyone recommend a different approach to any of this?

Many thanks,

photo (4).JPG102.45 KB
Asked by Brian Gray
Posted Jul 18, 2014 1:44 PM ET


2 Answers

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I don't see any major red flags -- except for the fact that you are choosing low R-values. For the record, the 2009 IRC requires the following minimum R-values for homes in Climate Zone 5: Floors, R-30; Walls, R-20; and ceilings, R-38.

Answered by Martin Holladay, GBA Advisor
Posted Jul 18, 2014 2:54 PM ET


Thank you Martin. Pardon the pun but I'm floored at how high the R-value minimum is for the floor. I had no idea. Our village is not 2009 IRC but I suspect they'll approve the extra insulation provided nothing I do conflicts with 2006 IRC (which I will sort out on my own).

Out of curiosity, do you know whether there is a best practice for meeting R30 on a slab? My thoughts are either stack 6" of XPS or combine 2" of XPS with 5 1/2 of Roxul batts between the sleepers.

Thank you,

Answered by Brian Gray
Posted Jul 18, 2014 4:11 PM ET

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