What is roof deck insulation?

Above-deck insulation

Rigid boards, often polyiso, are laid over the deck, followed by a nailable layer of sheathing (nailbase panels combine the two) and then underlayment and shingles. This is standard on low-slope roofs and used on residential cathedral ceilings, additions and high performance homes. Fastener lengths and shingle warranty requirements change with this approach.

Below-deck spray foam

Open cell or closed cell spray foam is applied to the underside of the deck and the gable walls, sealing the attic. It is common in Georgia when the ducts and air handler live in the attic. The residential code's unvented attic section governs the type and amount of insulation required.

Pros

• HVAC equipment in the attic sits inside conditioned space, cutting duct losses.
• Fewer air leaks between the attic and the house.
• No attic vents needed.

Cons and cautions

• Roof leaks are harder to detect because foam hides the deck.
• Some shingle manufacturers set conditions for warranties over unvented decks, so check before reroofing.
• Installed cost is higher than insulating the attic floor.

Not to be confused with

Radiant barrier roof sheathing, which has a foil face to reflect heat but adds little R-value. Read OSB vs plywood decking and attic heat and shingles.