Spray Foam Under Roof Sheathing

We have an insulation contractor in the area that does the expanding spray in place foam insulation.

Spray foam under roof sheathing.

On houses with the roof system forming the ceiling in the upstairs such as in a 1 1 2 story house he recommends spraying the foam directly to the bottom of the roof sheathing. Increasingly building scientists are investigating why osb roof sheathing on many spray foam insulated roofs stays damp for months at a time. The anecdotes and data are enough to provide a few rules of thumb for designers and builders who want to install spray foam on the underside of roof sheathing. Take your pick as long as your building inspector approves.

If anyone has please contact me with evidence that being said open cell is a great product as well however in your case you. Spray foam on underside of roof sheathing. To comply with the building code the spray foam layer should have an r value of r 15. Polyurethane or polystyrene spray foams can provide thorough coverage to roof rafters and attic walls in much less time than is needed for other materials.

Open cell spray polyurethane foam ocspf and closed cell spray polyurethane foam ccspf insulation sprayed to the underside of roof sheathing is a popular strategy for increasing roof insulation levels in all climate zones. Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. Unvented roof strategies with spray polyurethane foam spf have been used since the mid 1990s to provide durable and efficient building enclosures. Moisture generated indoors or that infiltrates into the home is responsible for the bulk of the moisture in an attic insulated with spray foam on the underside of the roof sheathing.

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. Install spray foam insulation along the underside of the roof deck to create an insulated unvented attic which can provide a conditioned space for hvac equipment that is located in the attic. The formula in the building code is somewhat more conservative than dana s recommendation. It s not coming from above the roof and it s not some new moisture source resulting from the spray foam.

Unlike solid insulation products that require careful measurement cutting and fitting spray foam can cover even the most uneven surface instantly. Foam insulation blown on bottom of roof sheathing. That s about 2 5 inches of closed cell spray foam assuming r 6 per inch.