Replies

1. 
   Malcolm_Taylor | Oct 22, 2024 01:03pm | #1

   As long as you have good air-sealing you should be fine. Small amounts of moisture can dry though the rafters by adsorbtion, and 1" of EPS is 2 to 6 perms.
   https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs

1. 
   MIzone5Woes | Oct 22, 2024 01:22pm | #2

   Thanks - I saw that article a while back. I want to use a more permeable material, but to do so would likely mean lower R value, as I have a 2x6 rafter (short span, plus it's a only slightly heated garage). There's no such thing as 4.5" thick batts, so I wanted the 1" EPS spaced 1" out from the roof decking, which leaves 3.5" for standard R15 batts.

   I know I could space the rafters out to jam in thicker insulation or just spray foam it, and that's still in the cards, but I guess I'm just more curious than anything at this point...

1. 
   Malcolm_Taylor | Oct 22, 2024 01:46pm | #3

   What you are proposing sounds like a good and proportionate solution to me.

2. Expert Member
   BILL WICHERS | Oct 22, 2024 03:27pm | #4

   I don't think you'll have any issues with your proposed assembly, assuming good air sealing on the interior as Malcolm mentioned.

   If you're concerned though, I have two alternate assemblies for you, both of which I've used myself:

   1- use 1/4" waferboard for your baffles. This stuff is just OSB that is too thin (1/4") to be span rated, but it works great for baffles (and for providing some support to thick layers of cellulose insulation above drywall), and it's cheap. This material is also a lot more vapor open than any of the rigid foams. The only downside for you here is that 1/4" OSB has essentially no insulating value. I'd give it an R value of maybe 0.25 at best. I love this material for random things like this, and just bought more today! :-)

   2- Put your rigid foam on the INTERIOR, under the rafters. I have often used 1/2" foil faced polyiso for this purpose, which provides a nice interior side air and vapor barrier if you tape the seams. Using this material this way will cut down on thermal bridging from the rafters, so it will perform better than if it were used between the rafters as the baffle material, and it will provide a vapor barrier to help keep moisture out of your roof assembly.

   BTW, I used 3" reclaimed polyiso on the back side of the attic kneewalls in my own home. It works great here, the only downside is getting the panels into place in the nasty little attic space you have to work with. You can significantly improve the performance of these walls by doing this. I like to replace old fiberglass batts with new mineral wool batts at the same time too.

   Bill

1. 
   MIzone5Woes | Oct 22, 2024 05:00pm | #5

   I've thought about baffles on the interior, but I do plan to eventually drywall and I've heard horror stories. It does sound like your experience with the polyiso behind your knee walls is pretty indicative of the fact that there won't be moisture issues on a wall that has a good seal directly behind the drywall

1. Expert Member
   BILL WICHERS | Oct 23, 2024 08:52am | #7

   If you use "thick" material, drywalling will be more challenging due to the need to hit the rafters with screws through the foam. 1/2" polyiso I haven't ever found to really make any difference with drywalling. Maybe up around 1" or so it would start to be more of an issue, but 1/2" is thin enough that it's easy to get the screws into the rafters without any additional effort. I've never had an issue with dimpling in the drywall either.

   Bill

2. 
   Malcolm_Taylor | Oct 22, 2024 05:03pm | #6

   One thing I should have added: If you go with Bill suggestion and use thin material like 1/4" wafer-board, you can easily stuff 5 1/2" fiberglass batts into a 4 1/4" rafter cavity.