Replies

1. 
   jekyll975 | Feb 04, 2021 09:34pm | #1

   This is the result of the simulation.

   
2. Expert Member
   Akos | Feb 04, 2021 11:12pm | #2

   Your interior relative humidity is WAY too high. Sound be around 30% in the winter time.

   Even with the recommended ratio of rigid insulation on the exterior, there will be some minor condensation during cold spells but nothing that will cause any damage. This is partly why you need to make sure the assembly can still dry a bit towards the interior. You don't want condensation at the average temperature of the coldest 3 months at a reasonable indoor humidity.

   You can read more about it here:

   https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation

   P.S. Depending on the facing of the PU foam you are using, it can be still somewhat vapor permeable. Make sure your air barrier or your foam facer is a true vapor barrier.

   A low of -12C doesn't sound like Zone 6. I would check your heating degree days (zone 6 is 4000 to 5000 HDD18C), extra rigid never hurts but it does add more cost.

1. 
   jekyll975 | Feb 05, 2021 05:35pm | #3

   Akos,

   Thank you very much for your response. You're totally right, my indoor RH was too high.
   I read the great article you mentioned, and now understand better what happens with humidity.

   You suggest to create a true vapor barrier on the top of the second roof deck (by a membrane or the facing of the PU, like foil).

   I understand that it will reduce the condensation risk between the two roof decks, but what happens to the water eventually caught between bitumen sheet membrane (vapor impermeable) on the surface, and the vapor barrier under the PU ? It seem like there is no way to dry.

   I live in zone 6 (south of Newfoundland, Canada), the average temperature of the coldest 3 months is around -2°C, and 4500 HDD18C. The outdoor humidity is relatively high yearlong since it's a small island.

   Thanks again.

1. 
   creativedestruction | Feb 06, 2021 07:33am | #4

   The foam insulation has to be dry when installed, because you're correct there is no substantial way for it to dry. WUFI might assume it starts wet.

   Its crucial the interior air barrier is as tight as possible. Low or zero perm helps if interior humidity is high but airtight matters more. The roof surface (SBS in your case) is always perfectly tight, so some incidental condensation can occur where moisture migrates up to it. It's not a problem as long as drying capability exceeds wetting, so the sun hitting the roof drives that back down in spring. What little gets in gets back out the same way.

1. 
   jekyll975 | Feb 07, 2021 04:23pm | #6

   Jason,

   Thank you for your comment.

   "Its crucial the interior air barrier is as tight as possible."
   I will be very picky on that point.

   "What little gets in gets back out the same way."
   I think that is a very important point. If it came in it should be able to come out.

   Theoretically, the layers of foil on both sides of the PU are true vapor barrier. How can condensation occur in those layers, through the joints ? Wouldn't it be difficult to dry inward through rigid foams ?

1. 
   creativedestruction | Feb 07, 2021 06:16pm | #9

   In my experience there is no such thing as a "true barrier" to anything. Joints, corners and penetrations = tiny imperfections even for the best installers. This is why no one's hit zero CFM on a blower door. If the fastidious air barrier is there though, we're talking an inconsequential amount, again assuming drying capability exceeds wetting. The sun as an awfully powerful dryer on a black colored roof.

3. Expert Member
   Akos | Feb 06, 2021 06:26pm | #5

   I would check your roofing manufacturers installation recommendation. Some ask for a vapor barrier at the roof deck in zone 6, some ask for a vapor retarder, some are fine with just an air barrier. As Jason said, the most important part is a solid air barrier, don't skimp on that.

   Since you have no way to dry through the SBS, it makes sense to have some permeability to allow for a bit of drying towards the interior, so a vapor retarder ( craft paper) makes the most sense over the first roof deck.

   I would also check about the 2nd roof deck. Usually SBS is installed directly over a protector board on the foam. Also 3/4 plywood seems a bit overkill for roof deck unless you have very wide truss spacing.

4. 
   jekyll975 | Feb 07, 2021 05:23pm | #7

   Akos,

   Thanks again for your recommendations.

   "Since you have no way to dry through the SBS, it makes sense to have some permeability to allow for a bit of drying towards the interior, so a vapor retarder ( craft paper) makes the most sense over the first roof deck."
   --> So I will put a vapor retarder on the first roof deck (that will also be my air barrier). But if the PU (furthermore foil faced) and the PSE are already vapor barriers, what's the point of putting a vapor retarder underneath ? It seems to me that the eventual condensation will be blocked by the foil or the PU itself ...

   "I would also check about the 2nd roof deck. Usually SBS is installed directly over a protector board on the foam. Also 3/4 plywood seems a bit overkill for roof deck unless you have very wide truss spacing."
   -->You're right about the SBS, there's usualy a first layer, sticked on, or nailed through the insulation, and a second heat-welded membrane on the top. But I prefer to keep the SBS independent from the insulation for future renovation. You're right, OSB 3/4 may be overkill, i will look further to optimise this point.

   So, I can now precise my conception that looks like that :

   – Drywall 1/2″ (first air barrier)
   – Around 12″ of static air (any risk of convection ?)
   – Trusses about 20″ (to be defined by engineer)
   – Fiberglass insulation R30 / 8″ (raw or kraft faced ?)
   – 1st roof deck - OSB (2 perms - vapor semi permeable)
   – Vapor retarder Kraft paper membrane (0,8 perm)
   – 4″ of polyurethane foil faced insulation (vapor barrier)
   – 1″ of PSE insulation (> 1 perm, vapor semi-permeable)
   – 2nd roof deck - OSB (2 perms - vapor semi permeable)
   – Roofing underlayment (if necessary, depending on the surface membrane system)
   – Heat-welded base sheet membrane SBS, or a peel and stick system

   Another option is to put Ondura panels on the top of the roof instead of SBS. Even if it is a low-slope roof, some venting would happen between the panels and the 2nd roof deck. It would allow more drying possibilities for the assembly. In that case, i should put a vapor permeable underlayment on the 2nd roof deck.

5. Expert Member
   Akos | Feb 07, 2021 06:05pm | #8

   Most roofing insulation that I have seen is either fiber faced polyiso or EPS. These are all either vapor permeable or semi-permeable. I have never worked with PU panels, so can't comment too much about it.

   With torch down the roof is torched to the protecto board (thin glass faced ashaplt coated board) not to the polyiso directly. During roof replacement, the protecto board is replaced but the polyiso stays if in good shape. Usually the problem is that the iso will have water in it from leaks thus it must be replaced as well.

   You'll have to figure out how to hold the batt insulation against the first deck, this generally does mean craft faced or insulation wires. Kraft facer is a smart vapor retarder, just not as vapor open when at as some of the engineered products.

   Not familiar with PSE insulation, but the stackup still seems a bit complicated.

   -1/2 drywall
   -trusses with batts
   -1st deck, seams taped as barrier
   -kraft paper
   -5" plyiso, two layers with seams staggered
   -protecto board
   -SBS

   Very few roof panels work on a 12% slope, most are rated for 25%, a small handful for 16% unless mechanically seamed. I would raise the slope if you want to go metal roof. With a metal roof, you would definitely need the 2nd roof deck or strapping over the rigid. Most metal roof panels can be detailed to allow for a bit of venting.