Sheathing wetting / drying cycle
STCook
| Posted in Energy Efficiency and Durability on
Climate zone 4:
Mean outdoor T/RH for cold months (Dec/Jan/Feb) are: 36/78, 37/82 & 41/70
Proposed wall is:
Anchored brick (3.5″) veneer, 1″ air space, Tyvek, 7/16 OSB, 2×6 wall with cellulose, 1/2 gypsum, latex paint. I don’t have a lot of options here, but for reference the “standard” is 2/4 with batt.
Nothing complex about the wall but I have some questions.
- When I did a basic permanence check, I find that for the coldest 3 months, the inside OSB temperature (70F inside) will range 39-43F. Looking at inward vapor drive an indoor RH of 35-40% may condense when it greets the OSB. I don’t know what typical indoor RH is for the area, but seems like there is potential for some wetting during the coldest 3 months. Is this just part of the normal wet/dry cycle and nothing to be concerned about?
- Going further I put the wall in WUFI 2D (groan here if you must), the model indicates (north facing wall) that the OSB goes above 12%MC for 4 months peaking briefly at 15%. That was using the ASHRAE 160 defaults, changing the Air Exchange Rate from a default 0.2 to 0.25 changes those results to just 1 month above 12%, peaking at 13%. Reading ASHRAE 160, 0.2 is for Non-Designed Ventilation, vs a Designed Ventilation rate from 62.2, which is where I came up with .25-ish. So WUFI is saying the wetting is inward vapor drive dependent. If the numbers are directionally correct, at what MC and duration should one raise an eyebrow?
- I start looking at this to deciding if moving from house wrap to commercial wrap would make sense to throttle vapor drive from the brick, any opinions on doing this? My current thought is it does not really matter due to the air gap, but maybe some improvement at mortar dropping zones.
- One last thing on the wall, the OSB is not tapped and I believe I can have that done to improve the air barrier and that seems like a good thing to specify.
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Replies
Lots of issues here. For GBA readers who haven't yet started building their houses: If you are designing a house with brick veneer, there are lots of reasons to install an adequately thick layer of rigid foam on the exterior side of the OSB.
Stephen: You probably want to consider outward vapor drive as well as inward vapor drive.
When it comes to worries about the moisture content of the OSB in December, January, and Feburary, you are worried about outward vapor drive. Including a vapor retarder on the interior side of the wall assembly would be a good idea to cut down on outward vapor drive. I suggest either MemBrain or vapor-retarder paint (neither of which you mention).
Of course, another way to reduce outward vapor drive is to pay attention to airtightness. The tighter the interior air barrier, the better. If you do that, the OSB will dry out in April and May, the same way it does on most walls on most U.S. homes. All of these homes would have dryer OSB if they had exterior rigid foam -- and of course that applies to your house, too. But I already said that.
Whether this annual cycling -- damp OSB in winter, dry OSB in late spring -- will cause problems is a matter of debate. Probably not, but we'll know more in 30 or 40 years.
Next: let's talk about inward vapor drive during the summer. The 1-inch air gap will save you, but only if it is free of mortar droppings. A 2-inch gap is preferable.
Am I repeating myself yet? A layer of exterior rigid foam would stop the inward solar vapor drive it its tracks. But you don't have that.
If your air gap gets clogged by mortar droppings, inward solar vapor drive might be a problem. Wide roof overhangs lower the risk.
As I said, the best solution to all of these vapor drive problems is an adequately thick layer of exterior rigid foam.
Martin,
Ooops ... my mixup on the inward vs outward! I'm not sure why I used inward in my questions when I meant outward.
When we first started planning (and still are), I wanted R5 exterior with 2x4 walls but that style is alien where I'm building, so the compromise was the 2x6 wall. After that point, while having a bowl of oatmeal, I began to think about the OSB ;) I was mostly thinking about the brick impacts, but doing the permanence exercise showed me the outward winter wetting potential. Going to the 2X6 lowers the OSB a few degrees so makes things a bit worse.
If I can't get more than 1" of airspace behind the brick is a double layer of WRB an acceptable alternative (like another layer of Tyvek or #15 over Tyvek?)
I'll read up on MemBrain or vapor-retarder paint and see if those are options available to me.
Would a flash and batt method be something to explore here, say 2" of closed cell on the OSB then 3.5 of cellulose?
Stephen,
If it's not too late to redesign your wall, a 2x4 wall with one inch of exterior rigid foam is a better wall than a 2x6 wall with cellulose -- especially with brick veneer.
If you are worried about mortar droppings bridging your 1-inch air gap, you might want to invest in a drainage mat product like one of the products offered by Masonry Technology Incororated. This is a better approach than simply installing two WRBs -- since two WRBs won't solve the problem of mortar droppings bridging the gap.
If you can't install exterior rigid foam, you could consider the flash-and-batt approach -- although exterior rigid foam is better in all respects than flash-and-batt (because exterior rigid foam addresses thermal bridging through the studs).