Pole Barn Insulation - stacking foam boards, acceptable?

[challenger]

I am going to be putting up a 40x64x16 barn in the next couple months. It will be in W Michigan (Lower peninsula.) I am strategizing how I want to heat and insulate the barn.

As far as insulation, in my uncles barn we put up 1.5" foam board between the girts and then he spray foamed over the girts/foam, and lastly put up metal to finish the inside. I am trying to wrap my head around doing the same foam between the girts, but then instead of spray foam, putting full sheets of foam nailed to the girts like maybe 2" thick. It seems a lot more cost effective than spray foam. It also seems a lot easier than framing "studs" between the posts and putting in fiberglass insulation. But I have never heard of anyone putting foam boards on top of foam boards so I want to make sure my hair brained idea isn't too kooky.

Also, my brain is somehow short circuiting when it comes to vapor barriers. Is there concerns if either of those foam panels is foil backed? I basically would like to please learn:

• Why or why not this would be a good idea to stack foam sheets up like I mentioned?
• What constitutes a vapor barrier, and where I would need to put said vapor barrier in this scenario?
  • in other words, for example, if the 1.5" thick foam between the girts is foil backed, and I put foam on top of it, is this a huge no-no?

Thanks

 

[jack stand]

I did that, 1.5" foam in between the wall girts, then another 1.5" inside the girts. In my situation I built a poured 48" frost wall foundation (for insulating purposes) and simply framed 16" o/c. Tyvek on the outside.
I have enough room in the wall stud bays for R 13 batts but have found that not necessary.
I used "reclaimed" poly iso from a commercial re roofing job. This was about 18 years ago and the 4x8 sheets were around $7 each!

 

[billconner]

Ideally I think the foil face should be inside, and tape joints. It wouldn't matter if there was foil facing on the other layer as well.

If foil only on outer layer, warmer moist air might seep through seams of inner layer and condensed.

 

[dscheidt]

foam is stacked all the time to get the required thickness. you want to stagger the joints, and the innermost layer should have the joints taped or foamed. Vapor barrier properties depend on the particular kind of foam you're using. Foam itself, without a facing, is a vapor barrier once its a couple inches thick (thckness depends on the foam type).

If you're buying new foam, I'd strongly recommend EPS instead of XPS. XPS R value is overstated -- it's only true intially, while the foam bubbles are still filled with the blowing agent. That will defuse out in short order (less than a couple years), at which point it's the same as EPS. EPS is cheaper and has substantially lower global warming potential (because it uses air as the blowing agent, not a greenhouse gas). For a barn I was building myself, I'd be looking for used foam (it gets pulled off commercial flat roof assemblies), as it's much cheaper.

 

[rancherbill]

.....but then instead of spray foam, putting full sheets of foam nailed to the girts like maybe 2" thick. It seems a lot more cost effective than spray foam. ...

Might work, but you still have to fiddle to get 100% insulation coverage by the foam. Then you have the vapour barrier to do. I think your uncles strategy is easier.

 

[toyotadriver]

Nothing wrong with your plan. Putting 1.5 inch XPS between the girts is a good idea if you can. Then cover that with more foam sheets.

I did that with the roof on my shop. 2 layers of 1.5 inch XPS. 3 inches total. Been working great. I didn't want to put a ceiling in so insulated the roof instead.

Friend of mine recently found a seller on facebook marketplace who was selling 2 inch thick XPS foam sheets for $20 each. He has an upcoming project that needs a bunch of foam sheets so he bought 2 bundles (48 total sheets).



Nothing in this post should be misunderstood, interpreted, misinterpreted, or construed to be of a political nature.

 

[lazyriverrat]

I did 1 1/2 between the girts, then 2" esp over that. Great-stuff foam the seams. Then I decided to fill the void with faceless rolls of insulation. No regrets but time consuming.

 

[challenger]

I did 1 1/2 between the girts, then 2" esp over that. Great-stuff foam the seams. Then I decided to fill the void with faceless rolls of insulation. No regrets but time consuming.

Did you nail the 2" esp to the girts to hold it up? This seems like a great option. I am planning to scour marketplace and find foam boards when the time comes. I am wondering if you start to get a couple layers, how to fasten them to the wall.