Spray Foam Under Roof Deck

The spray foam definitely has its merits.

Spray foam under roof deck.

The damp foam can then transfer the moisture to rafters and decking materials leading to rot or leakage. But spray foam insulation used at the underside of the roof deck would need to be covered by a thermal barrier and intumescent coatings are commonly used. By encapsulating the attic and leaving the old insulation up there that filth is in your conditioned space. With enough closed cell spray foam under the deck or enough rigid above the deck interior moist air can t condense if the there is no condensation you don t need drying thus no need to vent.

Going back to the moisture which develops under the underlayment if this moisture has nothing to dry it out air movement under the roof deck you have a problem and having an insulation directly under it exacerbates the problem because any moisture will expand in high heat conditions. How spray foam should be installed on the underside of the roof decking the unvented attic space is completely within the building s thermal envelope. The biggest weak point in my opinion is that if there is a roof leak you won t find out until the wood is rotted. The roof currently has 4 of medium density open cell spray under the deck giving us an r value of just under 22.

You may have odor problems. Where wood shingles or shakes. If you use open cell spray foam on your roof rafters you should cover the finished application with a paint or sealant that retards water vapor. I ve done a lot of work in energy efficient building and design.

Yeah you probably still breathed some of it but not as much as you will now with spray foam on the roof deck. Just make sure you overdo your roof when the roof is installed. Your indoor air quality may get worse. The amount of closed cell spray foam or rigid insulation is somewhere between 20 to 50 of the r value of the overall cavity insulation depending on your climate zone.

The movement of air into indirect conditioning of an unvented attic is a slow gradual process that will not be noticeable by occupants and typically will not put any strain on the hvac system. Closed cell foam has been applied underside of roof decks since the late 60s early 70s and to my knowledge i have not seen or heard of a rotted substrate under and conditions closed cell foam was applied. It would come with 3 1 2 or 4 of polyiso adhered to a 7 16 deck. If spray foam is used in these climates zones vapor retarder requirements apply drying to the interior is limited and a vented space may still be needed directly under the roof deck.