Worried in Winnipeg – vapour barrier
user-968787
| Posted in General Questions on
I built in 2005 in Winnipeg (climate zone 7) with conventional 16 oc 2×6 walls and 1.5″ of exterior XPS over OSB. The tyvec housewrap is over the XPS and the exterior finish is vinyl siding with no rain screen.
Building code here required 6mil poly on the interior.
Air tightness is pretty good in this house with ACH of 1 per blower door testing. Relative humidity is kept under 35% in the winter.
I stay awake at nights worried that my OSB is quietly rotting away in this wall assembly.
Should I be worried?
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Replies
Darryl,
OK, you or your builder made two mistakes:
1. Your exterior foam sheathing should have had a minimum R-value of R-15; instead, you installed R-7.5.
2. Foam-sheathed homes need to dry to the interior, but you installed interior polyethylene.
But for goodness' sake, Darryl, have a gin & tonic and get a good night's sleep. As long as you pay attention to your indoor humidity levels -- dry is better than damp, and that means you need to run your mechanical ventilation system during the winter -- you'll probably be fine. (Your house does have a mechanical ventilation system, right?)
There are lots of things to worry about in this world, and the state of the OSB on Darryl's house is not one of them. At least you don't live anywhere near Sendai, Japan. Things could always be worse. There's no reason that "Worried in Winnipeg" can't become "Mellow in Manitoba."
Just don't do it again, OK? Next time you build a house, follow the rules.
Thanks!
Yes, i have a HRV and keep indoor humidity below 35% in the winter.
Followup question.... if i ever wanted to up the insulation on the exterior to the recommended 3" would that be a bigger mistake? Even less drying to the exterior?
Darryl,
If you increase the thickness of your foam sheathing from 1.5 inch to 3 inches, you will move your wall assembly in the direction of lower risk. But I wouldn't strip off perfectly good siding to do the work.
[Bill Rose sent me this message by e-mail and asked me to post it here:]
Picture this experiment. Take a dozen or so full sized panels of OSB and sticker them (place slats between the panels so air can circulate between the panels) and place the stack of panels flat on a good slab, outdoors, covered by a park pavilion roof so that rain and sun cannot get on the stack. Then wait. What will the OSB be like after 5 years? The product has some variation, but overall (and most of my colleagues agree) the OSB will be just fine. After 500 years? Now, most of my colleagues agree that it will be lying in a pile of disaggregated chips and resin. Next question: how about after 50 years? I don’t know and I don’t know anybody who does know. My hunch is that the good product will still be fine though it will have swelled a bit, especially at the edges, and it will have lost some of its strength.
In a real house or real building, the best that building science can do, with all its air barriers and vapor barriers and HRVs and flashing and window pans and advice to not keep things too humid indoors, is to recreate the park pavilion experiment. We can’t make it last longer than that (unless we’re willing to intentionally heat the panels during cold weather to keep them dry). OSB naturally degrades over time, though that time scale is measured in decades and the product has only been in widespread use since 1980.
I worry about OSB. Before OSB, I felt that my building science could help building owners keep their building around for whatever horizon they and their ancestors sought to keep. It takes some of the wind out of my sails, to think that the best I can do is add a few years or a few dozen to a degradation that is naturally occurring on a timescale an order of magnitude shorter than the buildings of our forebears.
But I don’t worry too much, really. And neither should you.
Bill Rose
Darryl:
Martin is correct in his advice to not worry. I live in Winnipeg, and am one of two R2000 inspectors/testers in the province. I do residential energy modelling and heating and ventilation system commissioning.
According to NBC Part 9.25.1 Winnipeg, with our ~6000 Celcius HDD, requires a minimum R7 exterior foam on your 2X6 R20 wall before you have to worry, as long as your interior RH is kept under control. This means that your ventilation system and controls need to be working (and operated) correctly and hopefully the systems have been properly commissioned, although in residential installs this is extremely rare unless it is a R2000 certified home.
You appear to know the airtightness of your house - was it tested or certified?
Cheers,
Gio
prairieHOUSE Performance
Hi Gio,
Yes the house has been blower door tested (by yourself actually).
Is it safer to go thicker on the exterior foam even with the interior poly vapour barrier?
Response to Bill: (1) "...is to recreate the park pavilion experiment." Do you think that your experiment really simulates a building? I don't quite see how it does. What about the OSB being cold and being hit with the inevitable moisture from inside the house? Etc. I'm not sure that was the best comparison. (2) How do you feel plywood compares to OSB over the long haul; 50 to 100 yrs?