I've got a 'pole barn' (shop) that I'm looking to insulate. Originally I planned to use R-21 Owens Korning faced fiberglass batts. They are the slightly pricier high density ones at 5.5" thick.

However, on further inspection, I now notice that the insulation 'cavity' formed by the horizontal studs are actually ~ 7" deep, because of a second 2x6 installed 'vertically' along the siding-side of the horizontal 2x6. (Please see pic -- i'm not great at phrasing).

So - this means I'm going to have a significant void somewhere in the wall. I could push the insulation all the way back, touching the back side of the metal siding, or I could leave it flush with the face o the interior 2x6, leaving the void between the insulation and the exterior-facing metal siding.

I'm not insulation expert, but I sure don't like either option. It seems I am guaranteeing a significant uninterrupted air cell where surely convection currents will wreak their havoc. View attachment 756472

(for the 'big picture' - my plan was to just install the bats (walls and ceiling), cover with either sheetrock or OSB down low, and likely leave the batts just friction-fit in the upper bays, or cover with FSK stapled to the studs. I'm not trying to live in the space, just make it more comfortable, and I'll install a mini-split to moderate temperatures. The previous owner partially insulated/drywalled/OSB'd parts of the shop and I'm hoping to finish the job).

one thought I had was to scrap using the 5.5" thick R21's and go with the less expensive "R19") insulation that is a bit thicker. My understanding is that the actual R value of R19 is worse once compressed in a true 2x6 wall. Since my wall is more like a "2 x 7", the insulation wouldn't be compressed and I'd probably get closer to R19.

I'm wondering if -- given the inevitability of an air gap and convection -- the "R21" would perform worse than the R19 would!

Cost (and labor costs) are definitely a concern, so any solution that requires a ton of custom-cutting is probably not an option. I would imagine 1.5" thick foam-board or similar foam up against the siding would probably be a great solution, but that would probably double my costs once labor is considered.

I would just do the 5 1/2" but with friction fit, I might be worried about it sagging but the paper backed stapled to the outside of the 2x6 might hold it in place.

 

but the paper backed stapled to the outside of the 2x6 might hold it in place.

Neal - when you say 'outside of the 2x6', do you mean the face of the interior-facing side of the 2x6?

its a good point - I hadn't considered the sagging factor.

 

No, stapling it to the face of the studs on the inside of the structure will help keep it from sagging.

 

A couple of comments. Your concerns about r-19 vs r-21 are unnecessary. The difference would be measured in energy costs (hard to estimate) and comfort. The comfort will be the same.
Personally I would like the 1.5” rigid foam board bet the ribbed profile of the metal siding will still allow some convection. The metal siding will also complicate air infiltration from the outside which will affect the performance of the fiberglass insulation. Air seal as best you can.
Are those bays a standard 16” space?
Bud

 

Bays are 24"

 

I like the idea of 2" XPS against the metal with R19 in the bays, stapled to the stud faces. OR, even 24" Rockwool in 5 1/2" thickness.

 

I have a steel building that in Southern Arizona. Certain times of the year I had moisture on the inside of the steel sheets. Condensation. I solved the problem with closed cell foam. I could not get my head around have moisture form and run down the sheets inside my walls.

 

Along the lines of what SWD is suggesting, this might be a good application of the old “flash and batt” here a 1.5” later of spray foam could fill the gap, eliminating any air infiltration or convection, and then filled with the batt of your choice.

Bud

 

Bud, would you stop at 1 1/2" of foam with the foam guy already there. Would you not go ahead and fill the entire cavity with foam, or at least to 3 or 4"?? Curious.

 

Filling the entire cavity could involve two applications, added cost. Filling just the 1.5” should be less xpensive yet yield most ofthe foam advantages. Quote from the foam company would sort It out.

Bud

 

I had them fill the cavity all 9.25". Energy is only going to get more expensive.

Depending on the region 3" of sprayed might be enough for the use.

 

One factor might be, this is not intended to be living space so anythig above the original 5.5” maynot be ncessary. Up to OP.

Bud

 

Spray foam is only good if filling the whole void.
They cannot lay down a smooth 1-1/2".

1-1/2" SM board would be nice in there.

Or, just leave an air gap at the siding. If it sweats on the inside of the siding, there is something wrong with the insulation installation.

 

Spray foam is only good if filling the whole void.
They cannot lay down a smooth 1-1/2".

1-1/2" SM board would be nice in there.

Or, just leave an air gap at the siding. If it sweats on the inside of the siding, there is something wrong with the insulation installation.

Two questions:
1.) what is "SM board"?
2.) Not sure exactly how to prevent condensation on the siding interior surface. If moisture is being produced inside the shop (whatever source...it's got a variety of uses), then any of that moist air that gets past the sheathing could certainly condense on the cold interior. I suppose a moisture barrier (poly sheeting?) would probably be wise?

 

Yeah as much as I'd like to have spray foam throughout....it's "just a shop" and that'd probably be $15k+ to install, PLUS the batting....I can buy a lot of electricity for 15k (especially considering what an R-21 batt will provide).

 

If I need to use poly sheeting - does that mean I can/should use unfaced batts?

 

I believe they say to never double up on VB.

 

Right - I think I've heard that before. Believe the issue is that vapor/liquid could get stuck between the two VB's. Though I wonder if that'd be a significant concern if the two VB's are literaly pressed up against eachother.

The bummer is that the previous owner already insulated about 20% of the shop and just used paper-faced batts, then drywalled over top.

I suppose the next question is whether "well-stapled" paper-faced batts would be a sufficient vapor barrier? Or would unfaced fiberglass + polyethylene be superior (I would imagine the latter would be a lot less 'leaky').

 

Some reading on the origin of vb and need:

https://buildingscience.com/sites/default/files/document/bsi-073_macbeth_does_vapor_barriers_c.pdf

Bud