Closed-cell foam packs roughly twice the R-value into each inch and is a vapor retarder. Open-cell foam costs less, is not a vapor retarder, and needs to be much thicker for the same R-value. In a Department of Energy (DOE) Building America guide, closed-cell, medium-density foam is listed at R-7 to R-9 per inch and open-cell, low-density foam at R-3.5 to R-3.8. Both make a good air barrier. Your climate zone decides which is safe under a roof deck, and in cold climates the answer is closed-cell.
| Closed-cell (medium density) | Open-cell (low density) | |
|---|---|---|
| R-value per inch (DOE guide) | R-7 to R-9 | R-3.5 to R-3.8 |
| Vapor | A vapor retarder | Not a vapor retarder; more permeable |
| Air sealing | Excellent air barrier | Air barrier |
| Cost | Higher | Lower, but must be applied much thicker for the same R-value |
| Other | DOE notes it adds structural strength and resists moisture | Dries more readily, per the Building America roof study below |
| Both | DOE: expensive and difficult to install; interior foam must be covered by a fire-rated material | Same |
Under the roof deck: follow your climate zone
For filling the whole rafter cavity with foam in an unvented attic, the DOE guide gives these Building America recommendations by IECC climate zone:
- Zones 1 to 3 (hot): open-cell foam can be used, provided indoor humidity is not high.
- Zones 4 and 5: closed-cell foam, or open-cell with a spray-applied vapor-retarding paint on the interior side.
- Zones 6 and colder: closed-cell foam.
To cut cost in colder climates, a contractor can put a layer of closed-cell foam against the deck and add cheaper fiber insulation below it. The guide’s recommendation is 3 inches of closed-cell in Zones 1 to 4 and 4 inches in Zones 5 and 6. Zones 5 to 8 and Marine Zone 4 also need a Class II vapor retarder, such as latex-painted drywall or kraft-faced batts, against the attic side of the fiber.
In every zone, the guide says interior foam must be separated from living space by a fire-rated material, such as 1/2-inch gypsum board.
Roof leaks and drying
Building America researchers modeled small roof leaks against both foams in Miami, Seattle and Minneapolis, and sampled 11 roofs already in service. All the roofs sampled had sheathing moisture well within the safe range. In the models, open-cell roofs dried out “much more readily” than closed-cell roofs. In Seattle, though, open-cell roofs on OSB showed elevated moisture once leakage passed 0.6% of rainfall, while closed-cell roofs handled up to 1%. For cold-climate (Zone 6) homes, the researchers recommend keeping winter indoor humidity below 40%.
Foam is not a roof repair. Fix leaks, flashing and damaged sheathing first, and photograph the framing before it disappears behind foam.
Crawlspace and basement walls
For crawlspace walls, the guide gives two equivalent options: 3 inches of closed-cell, medium-density foam, or 10 inches of open-cell, low-density foam. For basement walls, it says closed-cell foam provides “the best moisture control of all interior foundation insulations.” Open-cell foam takes up much more room for the same R-value.
What to get in writing from the installer
- the exact product and its evaluation report;
- target thickness and R-value for each area;
- your climate zone and the assembly layers, including any vapor-retarder paint or fiber layer;
- the fire barrier or ignition barrier the code requires;
- how a vented attic will change to unvented, and how that affects ducts, combustion appliances and ventilation;
- re-entry time after spraying and the fix for foam that is off-ratio or does not adhere.
The DOE guide dates from 2012 and cites the 2009 IRC. Your local code edition and the product’s evaluation report govern.