Replies

1. 
   Jon_R | Sep 27, 2019 01:24pm | #1

   You should also ask "if the roof has a leak, what will allow it to dry fast enough to avoid severe sheathing rot?" And "which is more repairable?"

1. 
   Blevinsamy | Sep 27, 2019 10:53pm | #3

   Great advice. I'm open to answers. Thank you.

2. Expert Member
   Dana Dorsett | Sep 27, 2019 02:17pm | #2

   >"Multiple hurricanes in Savannah GA have elevated the controversy over closed or open cell insulations in attics."

   At 2" or more closed cell foam adds significant structure to the assembly making it more hurricane resilient. But strictly as an insulating material it's one of the least-green products out there, especially if it's the cheaper HFC245fa blown stuff.

   It's a lot greener to give it 1" of closed cell which is sufficient for dew point control in your climate, with the rest of the R being fiber insulation, or 2" (for the structural enhancement) + fiber. For attics in cooling dominated climates, using a higher density organic fiber such as blown cellulose or denim batt gives it a lower thermal diffusivity and somewhat higher perormance during the cooling season than any all-foam or foam + fiberglass solution.

   >"If an asphalt roof ever has a leak can it be detected in closed cell installations?"

   That's really the wrong question. All leaks are eventually detected, but with spray foam insulation (any type) the path of the water makes it harder to detect WHERE the leak is.

   At 2" or less closed cell foam still has a reasonable drying path toward the interior via vapor diffusion if not blocked by aluminum facers or unperforated radiant barrier, etc. At R30+ of all closed cell foam it's getting close to true vapor barrier territory. Even at R38 half pound open cell foam has more drying capacity than just 2" of closed cell foam (R12-R14).

   >"In the crawl space under the house is closed cell or fiberglass insulation with foil back?"

   Though Savannah is technically in climate zone 2, where crawlspace wall insulation is not required by the IRC, a ground vapor barrier sealed to the foundation walls, just 1" of closed cell foam on the foundation walls would outperform a code-minimum R13 (fiberglass or closed cell foam) between the joists, and would keep the structural wood and everything else in the crawlspace drier by not exposing it to ground moisture or outdoor air. In most houses that would even be less foam than R13 closed cell foam (2") on the subfloor.

   At 1" most closed cell foam is just slightly more vapor open than a Class-II vapor retarder, and will limit the rate of both ground moisture and outdoor air humidity coming through the wall to fairly low levels. At 2" it's well into Class-II territory, but it's not necessarily "worth it" to go that thick for either moisture control or energy use reasons.

   A code-min R13 of open cell foam on the subfloor is WAY too vapor-open for vented crawlspaces. In an air conditioned house the temperature of the subfloor would be well below the summertime outdoor air dew point and take on quite a bit of moisture. During the summer the outdoor dew points in Savannah are over 70F more than half the time:

   https://weatherspark.com/m/17828/7/Average-Weather-in-July-in-Savannah-Georgia-United-States#Sections-Humidity

1. 
   Blevinsamy | Sep 27, 2019 10:55pm | #4

   Thank you for sharing your knowledge and detailed advise! Anything else I should be thinking of? Anyone have a trusted vendor in my area?

3. 
   Blevinsamy | Sep 27, 2019 10:56pm | #5

   Great advice. I'm open to answers. Thank you.