Last week I wrote about estimating whether insulation was worth it for our shed if we're going to use it as a home office. While I'm waiting for the carpenter to put a new roof on it, I'm thinking through what finishing the walls would look like.

The Facebook side of the discussion on the insulation post was very helpful, and got me thinking about putting closed cell foam panels directly against the concrete walls. Because it's both a vapor barrier and efficient insulation it reduces the risk of condensation happening between the concrete and the insulation.

I'm less sure about what comes next. Whatever I'm doing is not structural, and there are a lot of trade-offs. All of the options involve the layer of closed-cell foam directly against the concrete, but then they vary on how they're finished. Standard options:

  • Build a 2x3 wall, then drywall. This gives room to run the electrical, and wood to attach the drywall to. The wall would be attached to the subfloor below and roof joists above. Nothing needs to go through the foam to the concrete, and the wood framing makes later work easier.

  • Build a 2x4 wall. Same advantages as a 2 x 3 wall, but now there's enough space in the cavity that it could be insulated. I could either use this for a larger amount of insulation total, or reduce the thickness of the foam.(video example)

  • Furring strips. Run strips of wood horizontally along the wall, and attach them through the foam into the concrete. Then run a second set vertically, 16 inches on center, to attach the drywall to. There's room for electrical as long as you dig out a bit of the foam for the boxes. (video example).

Less conventional:
  • Verticals inside: Instead of using furring strips, put vertical 2x3s from floor to ceiling, 16 inches on center. Attach them a the top and bottom (roof joists and subfloor). Attach the drywall to the wood. Run the electrical through the ceiling, and then down into the appropriate bay so you don't need to drill through any verticals. Essentially the same as the furring strip solution, but without needing to put fasteners through the foam into the concrete. This seems better than the furring strip approach?

  • Verticals outside: Attach the drywall to the foam with adhesive. Then instead of standard drywall joints, run 2x3s vertically 48 inches on center. Electrical in conduit, attached to the verticals. Mildly funny looking, doesn't feel quite as nice, maximizes space if you ignore the verticals.

foam finish overall R-value cost
1" verticals outside 1.5" R-6.5 $372
2" verticals outside 2.5" R-13 $580
1" verticals inside 3" R-6.5 $468
2" verticals inside 4" R-13 $676
3" verticals inside 5" R-19.5 $988
1" 2x3 4" R-6.5 $536
2" 2x3 5" R-13 $744
1" 2x4 insulated 5" R-21.5 $930
2" 2x4 insulated 6" R-28 $1138

This doesn't include the cost of the fasteners or adhesive, but seems about right. I'm leaning towards the approach of vertical 2x3s sixteen inches on center under the drywall with 2" foam, since it seems to offer a good balance of cost, insulation, and minimizing wasted space.

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Are there any tradeoffs you should consider aside from what you've mentioned (cost, insulation value, appearance)? Access to wiring in the future (for upgrades/changes)? Ability to mount things on the wall, or bring additional utilities (water, sewer, networking) into the space?

To my eye, the cost difference, for something that'll last many years, is minimal - it may be 3x difference, but still only $766 absolute. I don't know the value of insulation - what's the climate like where you're building? Are you worried only about heating, or will you have cooling needs as well (insulation is critical for both, but planning airflow and mechanisms is a lot more complicated for cooling)?

For me, in the northwest United States (temperate, wet, rarely very hot nor cold), I'd optimize for appearance and access, over insulation. That would be _VERY_ different if I lived in a desert or tundra, or the midwest US which alternates between desert and tundra.

Ability to mount things on the wall

Nice to have, but not critical?

Bring additional utilities (water, sewer, networking) into the space?

Not planning on water/sewer. I should decide about ethernet and do it at this stage if I'm going to. Probably yes ethernet?

what's the climate like where you're building?

Northeast US. Primarily heating. Climate details: Not planning for AC, though the window will be able to take a window unit if need be.