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Manitoba, Canada | I don’t want to discourage spray foam. It is likely the best insulation in a lot of ways. I had fully intended to use it originally.
For me though I was looking at the efficiency of the whole wall unit. I felt that if I could completely eliminate thermal bridging with continuous 2” foam sheeting on the inside of the wall I believe I can make a more efficient wall than a 3” layer of spray foam. As long as I pay attention to air infiltration both into the wall cavity and through the wall into the shop. I still have 2” of foam so basically I’m trading the extra inch of foam for 8” of fibreglass but the 2” I do have is continuous. The spray foam is interrupted by the wall components.
Because I live in a cold climate it is better to have the vapour barrier on the inside of the wall for condensation reasons within the wall. If I did spray foam the outside I would have had to spray it thick enough to have the dew point stay within the foam so no less than 3”. Any less than that and I risk having the dew point somewhere in the fibreglass and then I’d have wet or frozen fibreglass. Thus mold and lost R value. In a warmer climate a person wouldn’t need to spray as thick and in a warm enough climate, where the cold side of the wall is generally the inside, then vapour barrier is best on the outside of the wall.
Around here it is standard to put the foam boards on the exterior. I wanted a continuous layer though and doing that on the outside risked leaks around the screws. The installers would have to be extremely careful to tighten them just right. Too tight and it would dimple the metal into the styrofoam. Too lose and the screws wouldn’t be sealed. I also felt that over time any shrinkage of the styrofoam might also cause leaks. That’s just a gut feeling though. Don’t know if it would happen. By putting the styrofoam on the inside there is no risk of leaking. It really doesn’t matter if it isn’t quite tight.
Another benefit is that the styrofoam is on the inside beside the vapour barrier. Even though 2” of styrofoam is technically not a vapour barrier I believe it would take a long time for any moisture in the wall to escape if trapped between an inside vapour barrier and styrofoam on the outside.
I questioned the styrofoam company if there is a reason everybody puts it on the outside. The response was it is generally easier for the construction crew. It makes no difference in efficiency to have it inside.
I was also concerned about the sturdiness of the inside wall for attaching stuff to because there will be no wood to wood contact. I don’t think it will be a problem at all. The bottom 8’ of 3/4” plywood will be resting on the cement and secured through the styrofoam with screws to the strapping behind it. I’m putting strapping every 2’ for the first 8’ so there is twice as much strapping also. I really don’t plan on attaching anything very heavy above the plywood. I actually don’t plan on mounting much on the plywood either. Due to NAT, I like the idea of pallet racks, shelving, and keeping other things on wheels as much as possible.
Sorry for the long post. I just figured I’d explain the “why’s” because what I’m doing might not make sense for other people in other situations.
It wasn’t long enough yet so:
I had considered putting 2” of styrofoam on the ceiling also but changed my mind. It was a lot cheaper to just blow in more insulation. R60 fibergalss. I won’t have overhangs so I am getting rafters with 18” heel height to get a full 18” of insulation right to the edge.
Edited by havin'funfarming 2/11/2019 14:19
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Shop wall insulation: spray foam vs fiberglass
