How do I keep my sheathing dry?
This past summer and fall we renovated a 1950s bungalow in New Liskeard, Ontario (5700 Celcius degree days – that’s 10,260 F degree days if I’m converting correctly). We removed the interior drywall and the existing R6 batts. We also removed the siding, exposing the ½” plywood sheathing. Blueskin SA vapour barrier was applied to the sheathing from the sill plate to the top plate.
The Ontario Building Code specifies that, in our climate zone, the ratio of outboard to inboard insulation must be a minimum of 0.3 (For those who have a copy, see appendix A-9.25.5.2). So, we felt safe installing R14 Roxul batts between the studs and finishing with ½” drywall, while adding 2” polyisocyanurate board to the exterior.
We have not moved in yet, but a few weeks back we had occasion to remove a small portion of drywall and noticed the interior of the sheathing was damp. I didn’t have a moisture meter handy at the time but have now acquired one and measured the sheathing moisture content and temperature at another location and found it to be 27% and 1 degree C (interior temperature was 20 C, exterior temperature was -12C).
A few thoughts for you to refute, agree with or just chew on:
1. One learned colleague of mine suggested that this is likely a one-time wetting event due to the fact that we applied three coats of mud plus one coat of primer and two coats of latex paint to the drywall during some very cold weather in late November/early December. If we let it dry to the inside over the summer, it should be OK next winter. This makes me hopeful, but I’m still nervous.
2. My gut says that, if point 1 is not correct, this is primarily diffusion driven and not a product of air leakage. The electrician used airtight boxes and caulked his wire penetrations, all bottom and top plate penetrations were foamed. Perhaps I should pull off the quarter round and caulk the baseboards, and caulk between the drywall and the electrical boxes (in case the gasket isn’t properly compressed). I could also double check the caulking of the wire penetrations.
3. John Straub in http://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation shows how to calculate average winter sheathing temperature. Using his method, I see that my average winter sheathing temperature (1 C) is below the dew point at 35% indoor RH.
4. Also from Straub, I used TS=TI-(TI-TE)xRb/Rt rearranged to Rt=(TI-TE)xRb/(TI-TS) to see what the effective R-value of the polyisocyanurate board is at the winter average temperature (based on my good fortune of happening to measure the sheathing temperature when the outside temperature was at the winter average). Total R value of the batt plus the polyisocyanurate board is R24. So, the insulated sheathing is giving us R10 at the winter average temperature. This would be consistent with recent articles showing polyisocyanurate’s R value dropping as the temperature drops.
5. Again from Straub, using Table 1, I should need a ratio of exterior to interior insulation of about 0.52. Much higher than the OBC recommends but still less than the 0.71 I have.
6. Points 3 and 5 seem to contradict each other. Is he saying that it’s OK to have some condensation or have I misunderstood/miscalculated?
7. If I do a really good job of eliminating exfiltration (as I discussed in point 2), could reducing the vapour permeance of the drywall layer help? Currently, it could be assumed to be about 180 ng/(Pa s m2). But the article in the OBC referred to above assumes that a 60 ng/(Pa s m2). Adding vapour barrier paint would reduce the vapour permeance to about 30. So, if this is diffusion driven, we could reduce the amount of moisture that meets the sheathing; couldn’t we? But would we reduce the drying potential to the point where we are no further ahead?
8. Alternatively, I could add CertainTeed’s MemBrain plus ¼” drywall. This would, presumably reduce the winter permeance to close to 0 but still allow the wall to dry to the inside during the summer.
As you can imagine, removing the newly installed siding and adding more R-value on the exterior is not an attractive option. So, if anybody has any ideas that I haven’t already mentioned, please feel free to bring them forward.
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Replies
David,
There are several issues here. Some observations:
1. I'm not sure what climate zone you are in -- 10,000 HDD is cold, though. You are probably in Climate Zone 7. Here is a link to a map:
https://www.greenbuildingadvisor.com/climate-zone-map-including-canada
2. On the exterior side of your sheathing, you need a water-resistive barrier (WRB), but you don't need a vapor barrier. Blueskin SA is preventing outward drying; Tyvek or Typar might have been a better choice. The Blueskin SA isn't necessarily enough to make your wall risky, however, as long as you have enough exterior rigid foam -- and you may not, in this case -- and as long as your wall can dry to the interior.
3. Polyisocyanurate doesn't behave well in cold weather, so it's not a good choice for a cold climate. For more information, see In Cold Climates, R-5 Foam Beats R-6.
4. Part of your problem is construction moisture, which is not a permanent problem.
Thank you Martin.
1. We are at the southern edge of zone 7.
2. I chose to put Blueskin SA there as a vapour barrier because the foil faced polyiso board would be a vapour barrier anyway. So, I needed to locate my vapour barrier on the outside of the sheathing and, with blueskin, I could get a close to perfect air seal. The questions remain, what ratio of exterior to interior R value do I need? and would reducing the vapour permeance of the drywall layer mitigate the problem (thinking vapour barrier paint)?
3. I had heard about the issue with polyisocyanurate but thought it would still be better than XPS. Unfortunately, your recent article on the subject came months after I purchased the 2" polyisocyanurate.
In consideration of these points, would you have any further advice to offer?
Thank you for your time.
Dave
David,
It seems that the poor cold-weather performance of polyiso places your wall assembly very close to the divide that separates a safe wall assembly from a risky wall assembly.
Your decision to install peel-and-stick on the exterior side of your wall sheathing is an approach that is used by PERSIST builders. In my article on the PERSIST and REMOTE approaches, I wrote: "Instead of leaving stud bays empty of insulation, some REMOTE builders fill them with fiberglass or cellulose. This practice is potentially dangerous, however. The more insulation in the wall, the greater the possibility that the wall sheathing will get cold enough to allow condensation to form. That’s why most PERSIST builders prefer to keep framing bays empty, with all of the home’s insulation outside of the rubberized membrane."
Here's a guess: once your construction moisture dries out, your walls will probably be OK. If I were you, I would monitor my indoor relative humidity (RH) levels and make sure that they stay low, especially during the coming year. If you do that, I think you'll probably be OK.
OK . Thanks. This summer, I'll upgrade the foam on the wall between the attached garage and the house, caulk the baseboards and around the outlets, as well as monitoring the sheathing moisture content.