Replies

1. Expert Member
   BILL WICHERS | Jun 18, 2026 04:16pm | #1

   I don't see any issue using closed cell spray foam out over the exterior wall top plate area as you describe. As long as you have vent chutes and the spray foam doesn't clog up anything (like soffit vents or the vent channels), then you still have a vented attic and the rest of the attic doesn't really change. The spray foam itself is fully adhered in the areas where it will be, so it isn't really adding any moisture risk there.

   I would skip the foam blocks and just use spray foam alone in the areas where you need it. The foam blocks aren't likely to save you any money on the spray foam installation, and they will add labor. What you MIGHT want to consider would be putting some 1/4" or 1/2" foam down over the drywall (I'm assuming there is upper floor ceiling drywall under the attic framing), then doing spray foam over that. That will make it much easier to remove the interior drywall if you ever do a renovation in the future. Just make sure that the rigid foam does NOT cover the exterior top plate in this case. I would go for a loose fit between attic floor joists. The rigid foam in this case is just providing a barrier between the spray foam and the drywall, you want the spray foam to still completely cover the exterior top plate for good air sealing.

   Note that when you get up to R60+ with blown-in insulation, it's possible to exceed the allowable weight per square foot that the drywall can support. Be sure to check on that prior to insulating.

   Bill

2. 
   user-5946022 | Jun 18, 2026 05:58pm | #2

   I read your post different than @bill wichers - I presumed you want to use the foam blocks both as a wind block for the future blown in, as well as to prevent spraying foam past the top plate and blocking the rafter vents. Since you mentioned rafter vents, I presume your attic is vented, with the rafter vents being the intake, and either a ridge vent or turtle vents as the exhaust.

   If so, your idea of using rigid foam for all of this is terrific. I did the same. I used 1/2" Polyiso, and cut a variation of this:
   https://www.finehomebuilding.com/1997/05/01/high-r-vent-chutes
   In my case, I have raised heel trusses, so I cut the "legs" on each side 2" hgh, and folded them only 90 degrees, to get plenty of venting. This leaves only the foil on one side holding it all together but that was just temporary. I measured the distance between each set of rafters (they were all within an 1" but all just slightly different) then added 3 1/8" and cut a piece of 1/2" polyiso that width. Then I scored the polyiso 2" in on each side, and folded those 90 degrees. The 2" legs were bent 90 degree to creat a 2" vent space, the foil kept the horizonal piece attached to the legs, and the extra 1/8" made sure it fit snug between the rafters. I'd install 3 -4 of them, then get out the canned spray foam and run it along the edge of the cut polyiso - this adheres the polyiso to the rafters, and reinforces the scored section that proir to the canned foam is only holding the polyiso with a bent piece of foil.

   If you have any vertical space at all, you can also score the eave end of the polyiso and fold that down as a dam, and run the canned foam on the foil side of that score and up and along the rafter sides (outbound of the top plate), and horizontal across the top plate. That will keep that part together and act as a dam.

   The neat part of all of this is that you get great R value from the polyiso. In your case, if you can get your hands on 4.5" polyiso you would not need to do all the scoring & folding. Just cut, friction fit between rafter so the bottom of the polyiso is flush with the bottom of your 2x6 rafter, used canned foam to lock it in while sealing the top plate and you will have, all in one
   - wind block
   - vent channel
   - at least R-27 directly over your top plate

3. 
   DanS355 | Jun 19, 2026 03:55pm | #3

   Yes soffit to ridge vent.
   My plan is to have a smart product like Intello or Certainteed Membrane as my primary air barrier and vapor retarder on walls and ceilings than drywall over that. That being said I guess it negates its abilities if I'm putting close cell foam over it. (partially) the foam would not be over the entire ceiling just a few feet out to achieve more r value towards the eves. Again 4/12 roof pitch very limited space. Then all blow in where I can get the depth for cost effectiveness. For that matter I could use foam board between joists and seal edges as you say as well, less spray foam. I guess multiple ways to do this.
   Does it matter since the smart product can breathe and the foam cannot. In other words, some areas of the attic and can breathe and some cannot?
   Maybe I'm over thinking this what do you think?

1. Expert Member
   BILL WICHERS | Jun 19, 2026 05:18pm | #4

   No need for it to breathe in this case. You don't need a vapor retarder in a ceiling assembly under a vented attic, but it won't hurt. I do recommend if you do that, put the membrane up, then the drywall, THEN insulate. The membrane will otherwise bow out too much and cause problems installing the drywall.

   The ceiling doesn't need to "breathe", because the attic is vented, so any moisture that gets up there can easily escape through the vents. That's not the case with things like walls, where moisture can get trapped and condense on cold surfaces if it gets in. With a vented attic, the moisture will generally be carried away by the ventilation airflow before you have any problems. You do want a good air barrier though, but the drywall alone can do that. I would install the drywall airtight though, which means seal the perimeter of the walls to the top plates, then the ceiling mud/tape work airseals the ceiling plane into the drywall on the walls.

   Bill