Spray Foam Under Roof Deck

The roof currently has 4 of medium density open cell spray under the deck giving us an r value of just under 22.

Spray foam under roof deck.

I ve done a lot of work in energy efficient building and design. 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. 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. The spray foam definitely has its merits.

By encapsulating the attic and leaving the old insulation up there that filth is in your conditioned space. Yeah you probably still breathed some of it but not as much as you will now with spray foam on the roof deck. No interior class i vapor retarders e g plastic sheeting are installed on the ceiling side attic floor of the. Spray foam under roof decks.

The damp foam can then transfer the moisture to rafters and decking materials leading to rot or leakage. Your indoor air quality may get worse. 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. 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 biggest weak point in my opinion is that if there is a roof leak you won t find out until the wood is rotted. Just make sure you overdo your roof when the roof is installed. 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. 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.

You may have odor problems. 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. 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. Where wood shingles or shakes.