Optimizing insulation methods in PEMB building.

[Bradley5]

We are working on designs of a couple buildings, mostly used for manufacturing so mostly no interior finishes other than the insulation facing. What is common is the "super saver" method, which is decent, but I am questioning the need and extra expense of the poly liner as opposed to just laying in the faced batts. I know we also want and need a product between the steel frame and steel sheathing to provide a thermal break. What is common is 2-4" of unfaced batts. This is where I am having most issue because in other buildings, we have identified a concern with all the overlap seams of all the steel panels that leak horribly. This leads to 'wind washing' of the batts right under it, effectively short circuiting the insulation.

For this reason, I have some consideration to having that outside layer of batts between the sheathing and frame to have a facing, but actually install it backwards so the facing is in contact with the sheathing. I believe it can help channel any condensate that could occur under the panels, and provide additional air seal.

Now, from a building science perspective, this may not make sense having effectively two vapor barriers, BUT that would assume something that is probably less true in these buildings, which is they leak by design. Ideally you want an air seal on both sides of glass batts, but that seems to rarely ever happen.

I'd be curious of any thoughts or alternative methods? Keeping in mind the level of complexity being up on a building, working in the elements.

 

[JAE]

Won't two layers of seals possibly trap moisture forever? A bad detail I think.

 

[Enable]

Seems like you're mixing up air barriers and vapor barriers (which is no doubt common, see here), and haven't mentioned the exterior climate (at least in your post) while making the decision about insulation requirements / vapor barrier locations (which is a big no-no especially since you seem to be talking about a kraft face batt, which is a "smart" vapor retarder that changes characteristics based on ambient conditions). I think you should probably consult a building science specialist. This stuff is terribly easy to mess up but can have devastating consequences; just ask FEMA how easy it is - and they are supposed to know things.

 

[Bradley5]

Seems like you're mixing up air barriers and vapor barriers (which is no doubt common, see here), and haven't mentioned the exterior climate (at least in your post) while making the decision about insulation requirements / vapor barrier locations (which is a big no-no especially since you seem to be talking about a kraft face batt, which is a "smart" vapor retarder that changes characteristics based on ambient conditions). I think you should probably consult a building science specialist. This stuff is terribly easy to mess up but can have devastating consequences; just ask FEMA how easy it is - and they are supposed to know things.

LOL, you nailed it with FEMA! Regarding building science, yes, I have honestly wanted to have that conversation with someone that truly understands this challenge.

Correct about the whole vapor/air barrier deal. I really do NOT want a vapor seal on the top side, I just want a better air seal because wind washing of fiberglass batts is a real thing, largely reducing effective R value. No interest in using kraft face for any of this.

I did talk with one company that does use a type of butyl rubber tape for the steel panel overlaps, which honestly would solve a lot of this, but he said they usually only use it on the roof! I think they are trying to seal out rain because the walls will otherwise leak horribly with pressure differentials. Just steel on steel.

As to the weather where these buildings go, that is a wide range. Midwest. 0-100% humidity, -10-110F temps. But as a function of the buildings, being machine shops with 100s of gallons of water based coolants, indoor humidity will always be a fight, so an interior vapor barrier with a good seal is best IMO. This is probably another proponent for not using any sort of vapor barrier on the top side.

 

[XR250]

Spray foam it. Seen that a bunch.

 

[Harbringer80]

This belongs in a different forum.