Attic floor insulation
tsloss
| Posted in General Questions on
I was considering laying polyiso foam board on top of the bottom cord of my attic truss. I could seal it, and then put cellulose on top of that. This would allow a space to run wires, plumbing, etc. between the foam board and the drywall ceiling. Is there any reason not to have that gap, other than the expense of the foam board? Thanks.
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Replies
Tom,
I'm not sure how you would ever get an adequate seal with all the webs to work around. Why not just seal the electrical and plumbing as they penetrate the top plates?
Why not just bury the plumbing and electrical in the cellulose in the usual way, and seal the penetrations as Malcolm suggested? Persoanlly, I’d run all the electrical across and up from below, not up, across the attic, and down as is sometimes done. If you keep the wiring out of the attic, you’ll have a LOT less holes to seal!
For plumbing, you should just have a few vent stacks. Supply piping should not be run in the attic.
Bill
Tom,
Installing and sealing the polyiso as you describe is more difficult than you think.
1. You can't step on the polyiso.
2. You have to cut the polyiso to fit around all of the webs of the roof trusses, and then seal all of the seams of each piece of polyiso.
Just do it the regular way -- use your drywall ceiling as your air barrier, and blow cellulose on top of the drywall. Here is a link to a relevant article: "How to Insulate an Attic Floor."
You aren't the only one who sees the benefit of having a service cavity below the air barrier to run wires and whatnot without worrying about air sealing. A way to do that without the problems others have pointed out is to put the service cavity below the trusses.
Unless you’re building on a slab, you already have a service cavity under the floor. In many cases, the floor won’t be insulated, either.
I don’t know why so many want to run mechanicals in the attic space, there are usually better options that have much less impact on proper air sealing and insulation.
Bill
Hi Tom -
For all the reasons given already, I would not attempt this approach to getting continuous insulation and air seal. If you need another reason not to attempt this: trusses are designed to move quite a bit and that will make air sealing even more difficult and risky over time.
Peter