Certainteed Membrain: Reviews and opinions, please
New build, climate zone 6a. High altitude snow country. My insulation contractor is really pushing me away from doing a ccSPF on the underside of my sheathing and very strongly suggesting I use this certainteed membrain. Here are my options for roof design for my un-vented 2.5/12 pitch cathedral ceiling shed roof:
A) 14″ TJI rafters with 5.5″ of ccSPF and 8.5″ of netted and blown in insulation
B) 16″ TJI rafters with 16″ of netted and blown insulation but using Certainteed Membrain – a nylon smart vapor retarder.
The Membrain is stapled to the underside of the TJI’s and has a 6″ overlap between sheets. It doesn’t sound like the seams of the membrain (not membrane :)) are taped or sealed.
What experience do people have with this product?
Is method B a good design? It is more than $10,000 cheaper than option A.
My walls are exterior foam and netted and blown fiberglass stud cavity with a latex paint vapor retarder. How do I connect this membrain to my latex paint so I have one continuous vapor retarding control layer?
I really appreciate any insight into this product.
Cheers
Steve
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Replies
Steve,
When you talk about "netted and blown-in insulation," what is it? Fiberglass? Cellulose? Something else?
Briefly, Method B (as described in your question) violates the building code. The colder the climate, the riskier the approach is.
For more information, see these two articles:
"Can Unvented Roof Assemblies Be Insulated With Fiberglass?"
"Smart Vapor Retarders for Walls and Roofs"
EDIT: note removed in light of Martin's comment.
Martin,
Netted and blown is fiberglass. According to our county building inspectors office they require either a minimum of 4" of ccSPF or we can do blown in fiberglass with the certainteed membrain. Apparently our county has a carve out allowing the membrain approach. I'm 90% certain that is in an unvented cathedral ceiling but I'll double check. My contractor and insulator swear by this method but I'm skeptical hence my posting here.
Steve
Steve,
The two articles I linked to in my first response address the building science issues and code issues fairly thoroughly. Especially in Climate Zone 6, I think the approach is risky.
I suggest that you read my two articles and make your own decision.
To add to my previous comment what is the reason that method B is a risky approach for an unvented roof?
I understand that due to stack effect air will rise into the rafter bays. If my sheetrock air barrier and my sheathing air barrier is robust this shouldn't be a lot of air. The smart membrane is supposed to prevent vapor diffusion into/out of the rafter cavity until the rafter cavity becomes humid enough. Granted air leaks will likely permit warm air to hit my cold sheathing and then condense on the inside of the sheathing. At that point the membrane should then become vapor permeable and allow the rafter cavity to dry to the inside of the house, thus preventing a moisture problem.
That is how I understand the system to work but obviously I'm not understanding the system right. Insight is much appreciated.
Steve
Steve,
Experience shows that achieving an airtight roof assembly is extremely difficult. You're battling the stack effect, which is a powerful engine, and that engine is eager to exploit small cracks. That's why home performance contractors (and roofers) often see failed cathedral ceilings with rotten roof sheathing.
Because of the regular reports we receive here at GBA about disastrous ceilings, I've become conservative in my recommendations on cathedral ceiling details.
Steve, it often takes ten years or so for moisture-related building failures to show up. Membrain has not been around that long, and there is a chance your contractors have not been in business that long either. I have seen rotted ridges myself, and a lot of presentations showing dramatic failures. I agree with Martin and would be cautious. I put more faith in the depth of building science research this site and others report on than I do in the experience of the average insulation contractor. I know a few top-notch insulation contractors who I regularly learn from, but most are not building science experts--they are experts at selling and installing insulation.
The analogy I use for how tight the vapor retarder would have to be sealed is, if you flipped the house upside down and filled it with water? That represents more pressure than the stack effect, but it illustrates how perfect the membrane would have to be to avoid problems. Even then, although diffusion passes much less vapor than air leaks, it still allows vapor through--enough to accumulate and cause problems. I'd play it safe if I were you.
Michael and Martin,
Thank you. My gut tells me not to trust the membrane. The science points me in the direction of how to do it well. My bank balance reminds me to be mindful on how I spend my money but I have to remind myself that an extra $15,000 spent now to do the job right might make a huge saving in 10 years time when I have to re-do my roof because of rot.
Steve
Steve,
You wrote, "My gut tells me not to trust the membrane." I think you are making the right decision, but it's not really about "trusting the membrane." It's about trusting the assembly -- which lacks the code-required ventilation channel between the top of the insulation and the underside of the roof sheathing.
MemBrain works. It's a smart vapor retarder. But it isn't a particularly good air barrier, and even if it were a perfect air barrier, it wouldn't guarantee that the installers wouldn't leave rips, holes, or defects. MemBrain isn't magical. It can't be expected to change a flawed roof assembly (missing the required ventilation channel) into a roof assembly that works.
Martin you are right. Earlier design decisions have led me down the un-vented roof assembly. We are doing a 2.5/12 shed roof and I'm concerned that trying to vent a roof of that shallow a pitch over such a long distance (44ft). Hence the reason I've ended up at an un-vented roof design.
I try to avoid foam more than most designers and builders, but there are times where it is warranted. This is one situation where I regularly spec one of the HFO-blown foams in a flash-and-fill assembly. $10K is a lot of money but it won't look like so much if you have to address a rotting roof in the not-distant future.