Do I need another air barrier?
user-901114
| Posted in General Questions on
I’ve extended my rafters with luan gussets and 2x4s behind a future knee wall in my raised heel, attic trussed house. Zone 6. I’ve also built 1 inch vent chutes out of 1inch tuff r polyiso. Now I can fit two 5.5 inch roxul batts in the rafters, under the poly ISO chutes. Question. Do I need an air barrier under the roxul? If so, does it also have to be a fire rated? If not fire barrier rated, what are my options? I’d like to avoid sheetrock and use wood up in this part of the house. The ceiling below is 1/2 blueboard and plaster.
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Replies
Stephen,
Q. "Do I need an air barrier under the Roxul?"
A. I think so. Mineral wool batts are air-permeable.
Q. "If so, does it also have to be a fire rated?"
A. That's a question for your local code officials -- codes and code interpretations vary widely. From a building science perspective, however, I advise installing a rigid, durable air barrier material like drywall, ThermoPly, OSB, or plywood.
Martin
Assuming the iso was foamed to the rafters, wouldnt that create his air barrier?
Jesse,
It's possible to create an air barrier with polyisocyanurate rectangles and canned foam, but it's fussy work. Moreover, rafters change size due to changes in moisture and temperature, and these changes eventually may compromise the seal at the perimeter of each polyiso rectangle.
I like to have two air barriers whenever air-permeable insulation is used: one above the batts and one below the batts.
I already did the pesky work of making 1in vent baffles out of 1in polyiso with can foam edges. The framing has been dried in for a year so i hope it won't move too much.. It was actually quite easy but required patience.. Two barriers will better than one, I just hate the thought of crawling in behind that kneewall again after the roxul is in. Makes me itch just thinking about it.
Jesse and all, placing rigid foam between framing to me is almost always going to have air leaks and almost always going to need backside venting. And almost always IMO a bad idea, a bad build.
Even two layers of continuous rigid foam taped perfectly needs to have permeable layers adjoining IMO. Time and time again I have seen horrendous failures when trapped moisture has rotted or molded the next layers when moisture had no where to go.
Albert Rooks and those advocating vapor open builds are understanding of this. Yes it is possible to build multi-layer low perm structures that do not fail, but I think that type of home will be best built at a factory with engineers in charge of the design, and assembly and QC of all. Guys with hammers and all of us,just trying some idea we have, is not going to be successful, often, again IMO.
(no apology for all the commas and parenthesis I use... I like to and will for now...LOL)
To be clear. There is a one inch space between my roof sheething and my foam board, creating a vented roof. Then r46 of roxul in the rafters.
AJ
explain how the proposed assembly is any different from that of a rainscreen over foam?
To me moisture obviously dries out in the vent space or in...I am not seeing where it is getting trapped.
I am curious because, on a current design I have a small section of roof I am using ijoists where I plan on nailing 3/8" plywood to the underside of the top wood flange to create my vent and air tight layer and then blowing the rest of the 14" ijoist full of fiberglass/cellulose. Obviously perm rating is much higher with plywood then polyiso.
Jesse, there is nothing more to explain. Pieces of building materials leak air. The leak can be a problem. Some of these build ideas are prone to failure. The assembly has to be perfect everywhere for the life of the building. Not possible. Venting and permeable layers help with moisture but then heat loss is the issue. Jesse, build in a proven way or be prepared to deal with your concoction and how it performs. There are others worth copying and or hiring. My previous post is mostly to do with rigid foam use and people not understanding what happens when it is actually used.
AJ, I think what you're saying is that you hate foam. Especially when it doesn't have a perm layer on either side of it. Is the design I'm using risky? How? Room Air-air barrier-r46 roxul- sealed 1 inch foamboard chute-1 inch airspace-plywood-triflex-steel roof.
I should add that my vent chutes run from soffet to attic / ridge vent.
Stephen, I love rigid foam. In your build your foam is creating your vent chute which may eliminate problems in the chute. It may also leak as the build is not forgiving. In cathedral ceilings and tall walls any moisture on the non vented side of the foam is trapped and may cause damage. You may ask yourself, how did I get here?" And you may ask..... http://m.youtube.com/#/watch?desktop_uri=%2Fwatch%3Fv%3DI1wg1DNHbNU&v=I1wg1DNHbNU&gl=US
Replacing the top foam board with rigid Roxul is the solution to untrap moist air at the top of the insulation side of the rafter spaces. To me some sort of permeable valve is needed at that upper point. It compromises the insulation but makes the build safer as to rot and mold.
AJ. Is drying to the interior a myth?
Stephen,
Great idea adding all of that insulation.
This is a little bit of guesswork, but it's worth talking about. A drawing showing the layers and barriers would help. There is a concern about potential vapor on this roof.
The vent chutes are great to help your roofing and sheathing to stay as healthy as possible, but as you know, they won't be able to vent any accumulated moisture under the PI. For that to vent into your soffit to ridge channels, you'd have to have some undesirable air leaks.
The MW batts that im guessing that you will use are probably pretty low density if they are squishy. Like 2lb AFB batts? If so, that makes them really air permeable. The air will carry moisture and in your cold climate the 1" PI could get cold enought to condense and make water in the "swing months" of fall and spring.
There is such a thing as drying to the inside. It looks like you would have to rely on it. I like Martins two air barriers on this. If you stop the air from entering at the MW face, you stop the moisture with it.
At the risk of being a broken record, a permeable membrane would help with this. It can be applied with enough planned flex joints that it will "move" with the assembally. There are a lot of good membranes on the market that can help you with this.
Hopefully this is a help and that I understand your layers.
Btw... I think AJ has seen enough carnage and caution is a good idea. A WUFI exam is a way to check how it may "wet" and "dry". There is a free version and you may enjoy the task.
Thanks for the input Albert and AJ. I really like the SP workshop Albert. I have time to make adjustments since this is all DIY. I am in no rush and would rather get it right. I could easily cut strips into my vent chutes and stuff them with permeable rigid board insulation. One thing I should mention is that the vent chutes terminate in my attic just shy of my ridge vent. There will be r70 of cellulose up there. So another option could lower the roxul comfort batts, leaving a space under the foam board chutes promoting the ability to dry in the event that moisture gets through the membrane. In this scenario, there would be a one inch airspace on either side of the foam board. I could continue the theme by venting the underside of the foam board chutes with accuvents in the attic.
Stephen,
It finally occurred to me that there is no "mold food" under the PI. MW won't support mold growth. Soo, maybe it's not so bad to fill it with the comfort batts. As you know, the issues comes when the impermeable foam boards are facing wood in a way that the wood is wetted and unable to dry, and in the right temp ranges that will grow mold. That doesn't sound like the case here.
I still like the membrane since it will make the batts work the way they are supposed to: A truly dead air space is a lively insulator.
Thanks for the compliment, I'm glad you have time on this. I'm by no means the WUFI guy in our outfit, Graham Wright is the one to make the "educated judgements"...
Albert, Is there a reasonably priced membrane you could recommend?
Stephen,
I should know the market pricing better... but the day goes by too fast...
Of course I like our SIGA Majpell: http://www.smallplanetworkshopstore.com/products/SIGA-Majpell-5-%252d-4.9-ft-roll.html
I also like the intello plus that the guy's at 475 have: http://www.foursevenfive.com/index.php?main_page=product_info&cPath=70_76&products_id=219
Martin Holladay likes to recommend Membrain, but I don't know it's cost or where it would be near you, but I'm sure that you could find it and price it. http://www.certainteed.com/products/insulation/mold-prevention/317391
How and what you hang your membrane with is equally important in my mind. I like to use more tape than staples, and have done attic retro-fits with the SIGA Twinet in a similar way that is shown on the Majpell product page installation video in the above link. The SIGA Twinet will work with any of the other membranes that you'll find.
Tyvek, done.
Thanks guys. Great feedback. I appreciate your time. Steve