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.
Spray foam under roof deck.
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.
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.
The roof currently has 4 of medium density open cell spray under the deck giving us an r value of just under 22.
The spray foam definitely has its merits.
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.
Yeah you probably still breathed some of it but not as much as you will now with spray foam on the roof deck.
You may have odor problems.
Spray foam under roof decks.
I ve done a lot of work in energy efficient building and design.
Your indoor air quality may get worse.
Where wood shingles or shakes.
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.
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.
No interior class i vapor retarders e g plastic sheeting are installed on the ceiling side attic floor of the.
Just make sure you overdo your roof when the roof is installed.
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 damp foam can then transfer the moisture to rafters and decking materials leading to rot or leakage.
Instead of installing a couple layers of polyiso with staggered joints and then decking he is proposing using a product like the gaf thermacal insulated nailing base.
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.
It would come with 3 1 2 or 4 of polyiso adhered to a 7 16 deck.