Replies

1. GBA Editor
   Martin Holladay | Nov 30, 2014 08:54am | #1

   Eric,
   First, don't forget that you need an airtight ventilation baffle (thin plywood, rigid foam, or a commercial product like AccuVent) between the top of your fiberglass batts and the underside of the roof sheathing.

   Second: The main reason that people don't use XPS as an air barrier material is because of reports that XPS can shrink over time. For that reason, caulk is not a long-term way to seal the seams of XPS.

   If you want to seal XPS seams, you should use a high-quality European tape like Siga Wigluv. (Don't forget to seal the perimeter of the ceiling, where the ceiling meets the wall.) But reports of shrinking XPS might cause you to reconsider your plan. Drywall is cheap, durable, and easy to air seal.

   If you do want to use rigid foam as an air barrier, I think that you should choose foil-faced polyiso, which is easier to tape than XPS.

   For more information on XPS shrinkage, see Using Rigid Foam As a Water-Resistive Barrier. (Scroll down and read the section of the article with the heading, "Do rigid foam panels shrink?")

2. 
   fitchplate | Nov 30, 2014 10:50am | #2

   Martin … the air-sealing tape you refer to will easily stretch and firmly hold the seal of the alleged 1 to 2% rigid foam shrinkage ….

   … claimed but not verified …

   http://foursevenfive.com/foam-fails-reason-5-excessive-shrinkage/

   Be sure to read URL links at bottom of the page.

   Since the foam sheets (which should be foil faced for a no-perm air barrier) are to seal air not to insulate, taping over the potential gaps is not a risk to R-value loss of a gap.

   If you use caulk, polyether (i.e. Duralink by Chemlink) will stretch 150% and not deteriorate, shrink, or dry out like other caulks.

   This foam shrinkage issue needs to be dealt with. Take a stand, get some data, and encourage open review. BSC/JL and 475 chickened out of the issue (i.e fear of Dow's lawyers?)

3. 
   Rec1Construction | Nov 30, 2014 11:33am | #3

   Thanks for the information, much appreciated.

   If I'm understanding correctly, the recommendation is to create, essentially, a second cavity inside the top of the rafter bay by installing firring strips along the rafters and then attaching a plywood rip to the bottom of the strips - an 'airtight baffle'. (Or is a baffle simply something that restrains the insulation from reaching the roof sheathing and blocking the vent channel?)

   I could be understanding that wrongly, but it seems to me that such a system would simply move the condensation point to the bottom of the second cavity, negating the purpose of the vent system.

   Glad I asked about the XPS. Shrinkage could definitely be a possibility in a small cabin heated by a wood stove. I would have to wonder if foil-faced polyiso foam could have the same problem, though it would be simpler to tape.

4. GBA Editor
   Martin Holladay | Nov 30, 2014 05:08pm | #4

   Eric,
   You wrote, "It seems to me that such a system would simply move the condensation point to the bottom of the second cavity, negating the purpose of the vent system."

   If you are worried about condensation in your ceiling assembly -- and you should be -- then install an airtight ceiling. That's the most important step, by far, to prevent interior moisture from migrating into your ceiling assembly.

   If you make a mistake, and you have a leaky ceiling, it's possible to get some condensation on your ventilation baffles. The condensation will either diffuse outward (if the the ventilation baffle is somewhat vapor permeable) or it will dry to the interior (if your assembly is vapor-permeable on the interior).

   But, as I said, it's best to install an airtight ceiling, so you don't have to worry about condensation in your ceiling assembly.

5. 
   Rec1Construction | Dec 03, 2014 09:03pm | #5

   Thanks for taking the time to humor my questions, which I'm sure you've answered hundreds of times before on this website.

   I'm certainly sold on an airtight ceiling, and I'm not going to use any type of foam. A drywall contractor I spoke with today recommended simply using 3mil plastic as an air seal. Personally I think it sounds janky but it would save quite a bit of cost, obviously. Cheap is a relative term.

   I still do not get why it would make sense to have a second, separate cavity for the vent. My insulation guy is fairly adamant that there is no way that 8" insulation will ever block a 9 1/2" cavity, but I suppose he hasn't been out to verify that after a couple of years.

   My philosophy is that over-thinking things causes as many problems as under-thinking - but nothing causes more problems than listening to sub contractors

6. GBA Editor
   Martin Holladay | Dec 04, 2014 05:51am | #6

   Eric,
   The main reason why you want an air barrier above the fibrous insulation is to prevent wind-washing, a phenomenon which lowers the performance of the insulation layer. When cold outdoor air is allowed to flow over the top of the fibrous insulation, the moving air mixes with the air between the fibers in the top layers of the insulation, and pulls heat from your building. An air barrier prevents this mixing.

   The ideal ventilation baffle is airtight but vapor permeable. The best options include thick cardboard, thin plywood, or EPS rigid foam insulation.

   Your suggestion to use 3 mil polyethylene is not a good one, for three reasons: (a) It's hard to get an airtight seal at the perimeter of the polyethylene; (b) the 3 mil polyethylene is fragile and easy to puncture during installation, reducing its effectiveness as an air barrier; and (c) polyethylene is not vapor-permeable.

7. 
   Rec1Construction | Dec 04, 2014 06:11pm | #7

   Thank you Martin, very helpful.