Replies

1. GBA Editor
   Martin Holladay | Nov 10, 2018 07:55am | #1

   Sarah,
   There is no building science reason you can't do it, but there may be economic and environmental reasons to choose a better (cheaper and more environmentally friendly) approach.

   Experienced builders know that it isn't that hard to focus air sealing efforts on leaky areas, and then blow cellulose on the attic floor. That's cheaper than spraying an inch or two of closed-cell spray foam. If your builder has never done this before, one possible approach is the blower-door-directed air sealing approach.

   Here is a link to an article that describes the usual ways to air seal an attic: "Air Sealing an Attic."

   You may also want to read this article: "How to Insulate an Attic Floor."

1. 
   jamestimmerberg | Dec 05, 2018 04:54pm | #2

   Martin,

   Is it easier for a contractor (from the perspective of the employee who is actually crawling around the attic) to air seal doing flash-and-fill?

   i suspect that a lot of homeowners are in a situation where the insulation contractor effectively has a local monopoly; the kid that the contractor is sending into the attic is working for beer money (not out of a love for building science); and, the homeowner is not able to crawl around in the attic to check the air sealing (without potentially putting a hole in the ceiling). In that scenario, would flash-and-fill potentially be worth the extra money? If so, is there any minimum thickness of spray foam that is required?

   Jim

2. GBA Editor
   Martin Holladay | Dec 05, 2018 05:04pm | #3

   James,
   Worth the extra money? Only if the results are good. If your workers are "kids working for beer money (not out of a love for building science)," then there is a good chance that a high price doesn't guarantee good results.

   You might want to require verification of leak sealing with a blower-door test -- regardless of which approach you take to sealing the leaks.

   Q. "Is there any minimum thickness of spray foam that is required? "

   A. For the answer to that question, see "Air Leakage Through Spray Polyurethane Foam."

3. 
   brp_nh | Dec 06, 2018 07:43am | #4

   "Does it make sense to spray the Entire vented attic floor with closed cell foam to ensure a good air seal?"

   My only building experience is with my own very well air sealed house (zone 6), but based on that, the answer is no. You can create a good air barrier between your upper floor and the attic with painted finished drywall, taped OSB/plywood/etc, or a taped membrane from companies like Siga and Pro Clima.

   In our case, we had the 2nd floor ceiling drywall put up before the 2nd floor interior framing, so it's a continuous unbroken air barrier (except for one plumbing vent and the solar conduit). Then there's more than 20" of blown cellulose on the attic floor (on top of the drywall).

   With any method, just try to minimize the number of holes (that you'll have to seal) in your air barrier. In particular, minimize or eliminate things like ceiling can lights.

4. Expert Member
   Dana Dorsett | Dec 06, 2018 11:30am | #5

   For less money and more than twice the R per dollar you could seal the attic floor with 3" of open cell foam, but that's still a very expensive method of air-sealing and insulating the attic floor.