Fastening foil-faced polyiso under floor joists
fourforhome
| Posted in General Questions on
Martin advised, “My favorite specifications for exposed floors like yours: Install a continuous layer of rigid foam on the underside of the joists — foil-faced polyiso is the easiest type of foam to tape — and blow cellulose between the joists.” (https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/101443/better-floor#ixzz4kg9M7Udw)
Question: Is ½” poly-iso strong enough (24″ oc) to support dense pack cellulose blown into the floor joists or is 1″ needed? Assuming no drywall is needed to cover the polyiso, what is needed to fasten the polyiso to the TJIs?
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Replies
Mark,
In your first post on this question, you indicated that this is a floor assembly above a crawl space.
The first step is to consult your local building department to find out whether your local code requires rigid foam in a crawl space to be covered with drywall.
Assuming it isn't, I would advise you to install polyiso that is at least 1 inch thick. Before you attempt to fill the joist bays with dense-packed cellulose, it wouldn't hurt to reinforce the polyiso with 1x3 or 1x4 strapping, 24 inches on center, installed with long screws through the polyiso to the floor joists.
Before the strapping is installed, you would hold up the polyiso with cap nails. But I wouldn't depend on cap nails alone to hold up the polyiso if you are dense packing.