Replies

1. GBA Editor
   Peter Yost|Mar 17, 2019 11:49am|#1

   Hi Ronald -

   Yes, that "sixth-side" air control layer is critical; I have had a number of building investigations in cold climates in homes where the ceiling was T&G decking with air permeable cavity insulation that led to moisture problems.

   It doe not matter what the air control layer is so long as it is continuous; most of the time the ceiling air control layer is gypsum wallboard because it's easy to get continuity at the exterior walls with the ceiling with GWB but so long as the ceiling air control layer is well connected to the exterior wall control layer, the material just needs to be airtight itself.

   It may seem "silly" to then deck a perfectly good GWB ceiling with T&G wood, but you are going for the aesthetic AND the performance and you can't get both with just the T&G.

   Peter

2. Expert Member
   Akos|Mar 17, 2019 01:02pm|#2

   Ronald,

   It is possible to detail 6 mil poly to be an air barrier, it is something that is easy to mess up though, I wouldn't recommend it.

   Since you are looking for a high R value ceiling, one option would be to put 1" of rigid foam on the underside of the rafters with taped seams as your air barrier. You can then nail up your T&G directly over this with 2 1/2" nails. This would be about the same cost as drywall plus mud/tape but gives you a bit of an R value boost.

   I don't think spending too much time on air sealing the vent baffle is worth the effort. It is mostly there to keep the ventilation channel open. Taping the seals would be good as any leaks would drain to your eaves.

3. Expert Member
   Zephyr7|Mar 17, 2019 01:30pm|#3

   >”I have had a number of building investigations in cold climates in homes where the ceiling was T&G decking with air permeable cavity insulation that led to moisture problems.”

   This is what caused a MASSIVE mold problem in my own house. I found the problem after I bought the place. Luckily the remediation got picked up my my roofing contractors insurance (they missed some vent details that made things worse when they reroofed the place for me). It looked like a picture from a book of how not to do a roof. Tongue and groove very shallow pitch vaulted ceiling, terminating into a wall, insulated with cut-and-cobbled 2” XPS that wasn’t ceiled and was laying on top of the tongue and groove. The roofers sealed off the improvised “vent” (loose flashing) too.

   What I did after the mold remediation people left was to apply closed cell spray foam to the underside of the roof sheathing, and drywall the ceiling with care to make things air tight. That one project for one (admittedly large) room saved about 20-25% on the homes heating bill the next year.

   The lesson: tongue and groove ceiling material isn’t any kind of air barrier. You might as well use Swiss cheese for your ceiling.

   Yes, poly can be an air barrier but it’s tricky. You’re better off using a layer of drywall and install the T&G over the drywall. You could probably use the thin 1/4” or 3/8” drywall for this since the T&G would provide structural support to prevent sagging, but 1/2” drywall seems to cost about the same.

   Bill

4. 
   Ronald Proctor|Mar 18, 2019 02:20pm|#4

   Thanks All for your replies.
   I went back and reviewed the GBA articles: Insulating low-slope residential roofs, How to build an insulated cathedral ceiling, and Venting a low-slope shed roof, all of which add and lead to further questions.

   To add to my previous description, my design has a 47' long 2:12 roof. And it my intent to vent the roof from end to end (upslope) terminating under wind and rain-protective 4' overhangs.

   So one question is whether this configuration can successfully vent without mid span cupolas or "doghouses". In my earlier post I was imagining that a continuous vent the depth of the TJI upper flange (1-3/8") might satisfy the venting needs. But one of the above articles recommends at least a 6" ventilated space above the permeable insulation. Now I wonder if that 6" continuous would be enough venting without additional cupola/doghouses. Or maybe 4"? Why is the 6" depth suggested?
   I also like the idea of installing 1" foam board to the bottom of the TJI to create the air barrier before attaching the pine T&G ceiling. More thoughts again appreciated....
   Ron

1. GBA Editor
   Martin Holladay|Mar 18, 2019 02:47pm|#5

   Ron,
   You've got a low-slope roof, so you have to follow the rules for low-slope roofs. You've already read the relevant article, but here's the link in case you want to refresh your memory:"Insulating Low-Slope Residential Roofs."

   There are lots of failures (moisture problems, mold, and rot) with low-slope vented roofs, so disregard the rules at your peril. You need the 6 inches of air space, and you need to vented cupolas, because there isn't enough of a height difference from soffit to ridge for air flow unless you follow those rules.

   Most builders conclude that an unvented assembly (with an adequately thick layer of rigid foam on the exterior side of the roof sheathing) is the easiest way to proceed.

2. Expert Member
   Akos|Mar 18, 2019 05:55pm|#6

   Ronald,

   With your long roof, you still get a pretty good height difference, adding cupolas is not going to increase that by much.

   I think if you stay away from light color roofing and do a decent job of sealing the ceiling, you should be able to vent with standard soffit/ridge setup.

   One option might be to put 2x4 purlins on flat across your TJI instead of plywood (most metal roofs can go directly over purlins). This way you can also add in some vents on the sides, so no matter which way the wind is blowing you will get some flow.