A Question for Michael Maines
MALCOLM TAYLOR
| Posted in General Questions on
Mike,
A couple of weeks ago you made a comment which (and I may have got this wrong) was about the desirability of having an interior vapour-retarder in hybrid roof assemblies. Perhaps something about how you learned they accumulated some moisture even if the ratio was sufficient?
I’m sorry I can’t find the thread, and didn’t give it enough attention at the time. Do you remember it, and can you flesh out what you learned?
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Replies
Malcolm, that's one way to get my attention! After my article on flash-and-batt came out in 2010 (https://www.greenbuildingadvisor.com/app/uploads/sites/default/files/Why%20Flash%20and%20Batt%20Makes%20Sense.pdf), I got many calls and emails from people who were convinced to use that system and found condensation on the interior face of the foam, sometimes running down the wall in quantity. That's when I realized how important it is to keep indoor air from easily circulating through the fluffy insulation.
An air barrier (such as drywall) is much more important than a vapor retarder, but because many things can go wrong, I prefer to make sure there is a vapor retarder as well. The IRC calls for a class 3 vapor retarder when you have a proper ratio, i.e. painted drywall, so that's a minimum.
I may have stated in that thread that I prefer a separate membrane for vapor control, which is true, but less important when there is foam involved and/or the exterior sheathing is airtight than when there is no other air control layer.
Edit to add: to be clear, I don't recall any complaints about condensation from people who had installed drywall, painted or not, only from those who hadn't installed drywall, or who installed wood boards on ceilings with no air barrier or vapor retarder.
Mike,
Thanks for the reply - and sorry for the method of attracting your attention. I had a sense of urgency as I had recently given advice in a thread here I suspected might be wrong about whether it mattered that there was an interior air-barrier or vapour retarder in those assemblies
Next time I'll just project the image of a bat on a cloud.
Haha, that might work, or maybe project the BS+Beer logo. I check my email fairly obsessively so you can always reach me there too. Or by text; my number is on my emails and website. Most days I check in here several times as well.
Mike,
On a related note:
When I look at the illustrations and descriptions in Joe L's guide to Vapour-Diffusion Ports I don't see any warm side air-barrier either.
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
I don't know if this is an omission, perhaps because the illustrations are schematic, or there is something inherent to the assembly which means they aren't necessary (although I can't think of what that would be).
Vapor diffusion ports are limited to hot/humid climate zones, which should not have an interior vapor retarder. My comments before this were for cold or mixed climate zones.
In general, the building science cognoscenti have been saying for a while that vapor movement is much less important than air movement in an assembly. But some moisture will diffuse through an air barrier, so I like the belt-and-suspenders approach.
Geez Malcolm. I'm old enough that when I get a message with just someone's name in the subject line it's always an obituary. No need to scare me like that.
I had the same fear!
Oh God, sorry - I never thought...
I've edited the title
I'll have to admit thinking something along the lines of "what happened to Michael Maines" when I first read that title. It's good to see that he's OK and will be able to keep building green structures though! ;-)
Bill
Oh geez, that's terrible! And hilarious at the same time.