Hi.

I recently posted a question concerning the installation of 1" sheets of paper-faced polyiso applied directly to my concrete block basement walls.  That thread may be found here: http://www.greenbuildingtalk.com/Fo...fault.aspx

Because the material was priced so well, I went ahead and bought 48 sheets of the insulation.  Unfortunately, my subsequent research on polyiso has revealed that it is a poor solution for below grade applications.  As such, I am stuck with the insulation (can't return it because it was factory direct).  I am now trying to determine if I can make use of it in some other way.  Which leads me to the following proposition.

I have previously described the construction of my 1968 house on this site.  Briefly, the attic consists of site-built 2x4 trusses placed 24" apart.  There is approximately 4" of blown fiberglass insulation sitting on top of the drywall ceiling.  I had intended on air sealing the attic floor and blowing an additional 12" of cellulose over top of the fiberglass, but Dana1 advised against that due to structural concerns.  He advised that I modify the site-built trusses to create Larsen trusses, spray one inch of closed cell foam on the roof deck, and then wet-blow in blanket cellulose.  I really like this idea and have started the modification process.

I am now approaching a point in which nearly all of the trusses have been converted into Larsen trusses and will need to apply the spray foam.  I am wondering if I could actually cut down the 1" polyiso into 24"X96" sections, place that between the rafters, and then apply a thin layer of closed cell foam over that to acquire a good air seal.  I could then install the mesh fabric and blow in the cellulose. 

I realize that this approach may be unconventional.  My research indicates that polyiso is typically used on the exterior of the roof deck, however, it seems like it will conceptually accomplish the same goal if it was applied on the interior surface.  Please correct me if I am wrong.

If that won't work, I have 48 sheets of 1" paper-faced polyiso that I will let go at a slightly discounted price (less than I paid) in the upstate SC.

To protect the roof deck the permeance rating of the deck foam matters. With some unknown facer rating, it's hard to tell if it'll be too vapor open or tight, but if it's foil-faced it's EXTREMELY vapor tight, and will block drying toward the interior. Roofing materials are usually semi-impermeable and often wet- so drying toward the exterior is only guaranteed with a vented roof.

Leaving a 1" gap between the roof deck and the iso and sealing the edges would work as long as you have at least a minimimal amount of soffit-to-ridge venting of that mini-cavity. The IRC calls out R5 min of air-impermeable foam between the roof deck and fiber in a SC climate to be able to skip interior vapor retarders. By the time is said & done, a buck a square foot for 1' of closed cell foam directly on the roof deck might start to look pretty good.

Thanks, Dana.

As I understand it, the permeance of the product I have is rated at <1.5 perm.  I am not sure if that is high enough to allow for interior drying.  Moreover, since I am unfamiliar with the permeance rating system I have no idea if that <1.5 rating indicates that on average the product allows less than 1.5 perms to pass or if the actual mean permeance is something substantially lower and the manufacturer is 95% confident that 1.5 perm. is the highest permeance rate one would see in any sheet of material.

It sounds like the best approach is to try to recoup the money spent on the iso, apply it toward XPS for the basement, and continue with my attic sealing project.

If it's rated <1.5 perms it means it might be half that, but probably not lower.

Anything more than 0.5 perms (and maybe even a bit lower) is going to be fine. I doubt that any FIBER facer on iso is under 0.5 perms ( but foil is under 0.1). In that case you'd be fine snugging it up to the roof deck and sealing the edges with 1-part foam to make it air-tight, and at ~R6 it would comply with the IRC minimum for your climate zone.

As long as it's not truly paper, and would support mold you can use it below grade too, just be sure to put a sill-gasket or some other material between the bottom edge and the slab, or stop the insulation a few inches above the slab, to keep it from wicking in anything should it ever actually get wet. Iso can soak up a lot of water if you let it, but it's a slow process. With semi-permeable facers it can still dry through the facers, but you don't want the cool-damp side to be made of a material that would feed mold.

If you swap it out for something else, EPS is usually cheaper per square foot per R, and more vapor-open than XPS, which would be good for use in the basement. You can go up to 4" thick or ~ R15-R16 with EPS, but with XPS you're limited to 2"/R10 before it's too vapor tight, raising the risk of rot at the foundation sill.