Replies

1. Expert Member
   Dana Dorsett | Feb 22, 2019 09:42pm | #1

   What means "...0pen spray foam..." and "...vapor setting..." in 'merican dialect?

2. GBA Editor
   Martin Holladay | Feb 23, 2019 05:51am | #2

   Dana,
   I think "open spray foam" probably means "open-cell spray foam."

   I think "vapor setting on the OSB sheathing" probably means something like "moisture condensing or accumulating on the OSB sheathing."

1. Expert Member
   Dana Dorsett | Feb 23, 2019 07:49pm | #5

   Those are good guesses, but limiting the guesswork to just one of the significant terms would make it easier.

   It's not even 100% clear that this necessarily an unvented roof.

   A vented open cell insulated cathedralized ceiling is extremely moisture safe at any thickness in zone 4B.

   An unvented open cell insulated cathedralized ceiling in zone 4B has some moisture risk, but not an extreme moisture risk unless the air is being intentionally humidified. That risk can be mitigated with an interior side Class-II vapor retarder (vapor barrier latex on gypsum board) or a smart vapor retarder. The foam itself is not sufficiently vapor retardent at any reasonable thickness to sufficiently protect an OSB roof deck from peak moisture levels that are risky. (At R200 it's probably enough, maybe even R150.)

2. 
   jackofalltrades777 | Feb 23, 2019 11:26pm | #6

   Martin,

   Sorry, I was posting this quickly and should have proof-read my question. I revised my question but you are correct, I was asking about open-cell spray foam and moisture condensing on the OSB sheathing.

3. GBA Editor
   Martin Holladay | Feb 23, 2019 06:14am | #3

   Peter,
   If you are contemplating using the "flash-and-batt" approach, you want closed-cell spray foam, not open-cell spray foam. For more information on the flash-and-batt approach, see this article: "Flash-and-Batt Insulation."

   If you want to insulate your roof assembly with open-cell spray foam, you want to aim for the minimum code requirements for your climate zone -- that is, R-49, which means about 13.5 inches of open-cell spray foam. You should know, however, that the use of open-cell spray foam on the underside of roof sheathing is associated with moisture problems, so this approach requires caution. For more information, see these two articles:

   "Open-Cell Spray Foam and Damp Roof Sheathing"

   "High Humidity in Unvented Conditioned Attics"

1. 
   jackofalltrades777 | Feb 23, 2019 11:31pm | #7

   Martin,

   So the better route if using spray foam on the underside of the roof sheathing is to use closed-cell spray foam instead of open-cell.

   If I understand the issue correctly. Using open-cell spray foam on the underside of roof sheathing in a Zone 4B climate, and then installing a vapor impermeable roof (asphalt shingles, metal roof attached to OSB sheathing). Would be a risky build-up that could cause the roof sheathing to absorb moisture without the ability to ever dry out completely. Is that correct?

1. GBA Editor
   Martin Holladay | Feb 24, 2019 05:16am | #10

   Peter,
   If you are creating an unvented roof assembly using closed-cell spray foam, the roof sheathing won't be able to dry in either direction (unless you switch to concrete tile roofing or a similar type of roofing that is vapor-permeable).

   That's normal. Assuming that the OSB or plywood roof sheathing is dry on the day that the spray foam is installed, the assembly will work.

   Of course, there are some builders who prefer a roof assembly that can dry in one direction. But if you want that type of roof assembly, you have to plan ahead.

4. 
   Peter Yost | Feb 23, 2019 12:46pm | #4

   Hi Peter -

   Couple of weeks ago I was asked to take a look at an open-cell spray foam installation in an attic, about 12 - 15 inches deep (called because of icicles and staining at the roof eaves after really cold spells), Climate Zone 5.

   Turns out that vapor had made its way through the open-cell spray foam to wet the interior face of the roof sheathing. The rest is a long story BUT one thing of note is how high the homeowners were running their interior wintertime relative huidity (to protect a grand piano).

   So, the answer to your question about how thick the open-cell spray foam needs to be is: NO thickness will prevent any moisture getting to the sheathing. It all depends on interior relative humidity and the vapor permeance of the exposed open-cell foam.

   Control interior relative humidity (maintain at or below 35% in the winter), mix the air in any attic space to keep moisture from accumulating in the attic, and install a vapor retarder to the exposed surface of the open-cell spray foam.

   Peter

1. 
   jackofalltrades777 | Feb 23, 2019 11:34pm | #8

   Peter,

   Would 6.5 inches of closed-cell spray foam be thick enough to prevent moisture from getting to the sheathing?

1. GBA Editor
   Martin Holladay | Feb 24, 2019 05:12am | #9

   Peter L,
   If you are considering a flash-and-batt job (or a flash-and-fill job), the building code requires that roofs in Zone 4 include a closed-cell spray foam layer having a minimum R-value of R-15. That's about 2.5 inches of closed-cell spray foam. (For more information on this requirement, see "Flash-and-Batt Insulation.")

   Even 2 inches of closed-cell spray foam would be enough to prevent moisture from getting to the sheathing. But it has to be a little thicker to make sure that the interior surface of the cured spray foam stays warm enough in January to avoid condensation.