Smart Membrane for Wood Interior
joenorm
| Posted in General Questions on
Do all interior wall coverings other than drywall need a smart membrane?
I am building a small shop building and am considering doing the interior in either wood paneling or plywood. Do I need to apply some sort of air barrier to the studs first if I go this route?
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Replies
joenorm,
All walls need air-barriers. If that is done on the interior, and you are using a finished surface that lets air through, then you need something underneath to limit air movement. That can be a smart membrane, or poly. Plywood could also be detailed as an air-barrier using gaskets at the seams and perimeter.
Another alternative is to move the primary air-barrier to the exterior, and detail either the sheathing or WRB to be airtight.
Joenorm doesn't say what climate he's in. In a heating dominant climate you want to block vapor at the interior wall and allow it to dry to the exterior. In a cooling dominant climate you want to block it at the exterior and allow the wall to dry to the interior. You also want to allow drying to the interior in a heating-dominant environment if the wall is vapor closed to the exterior. In all cases you want to limit air movement.
So poly on the interior is only a good choice in climates that are strongly heating-dominant.
I am in 4c climate. Would taped foil faced polyiso be appropriate for the the air barrier on the interior ceiling?
If the roof is vented and the ceiling can dry to the exterior this is an excellent choice. If the roof is unvented then the ceiling needs to be able to dry to the interior.
If I understand correctly you're suggesting applying polyiso to the bottom chord of your roof trusses or rafters. My only concern is fire risk. You'll want some kind of ignition barrier, i.e. gypsum board or plywood covering it.
I'm not 100% certain about ignition with polyiso but I know it's an issue with other foams. So hopefully somebody else reading the board will correct me if I'm wrong. If I were you I'd certainly look into the fire issue, especially since you plan to use it as a workshop.
I’ve actually wondered this myself. I understand that EPS an XPS have really undesirable characteristics when it comes to fires, but if I’m reading things correctly, polyiso is a type 1 fire barrier and generic plywood is a type 3. I’m sure there’s some thing I’m missing, but would it truly be better to put plywood on the outside of polyiso in terms of ignition barriers?
Yes, it's a vented roof. What about the walls? Same?
The siding may end up being metal if that makes a difference.
joenorm,
For a hobby shop in the PNW I would focus on giving the walls and roof good drying ability to the outside (so a rain-screen behind the metal siding, and a deep roof vent channel). The chances that you will ever air-c0ndition the space is pretty low, so poly is fine. In our temperate climate it's also hard to make any case for interior foam board.
I asked Joe Lstiburek this question when he was our guest on the BS+Beer Show a couple of days ago. He said that if the wall is designed and built properly then a variable permeance membrane (they aren't particularly "smart") should not be necessary, but that they don't cause problems and can make up for mistakes of design or construction. He said a properly designed wall is one proven to work; if you need to run a WUFI model to know whether your assembly is safe, it's not a properly designed wall.
This is just a hobby shop in a mild climate. So nothing too special about the wall, 2x6 construction with fluffy insulation in the cavity. The only thing different is using interior paneling vs drywall(which I have considered an air barrier in the past)
You definitely need to limit indoor air from reaching the cold sheathing. Drywall's air permeance is used as the definition of an air barrier. In the past I have sometimes used drywall under wood paneling, but now I use a variable permeance membrane. I strongly recommend it in your situation.
Thanks, I will use one. Are there other choices besides Intello?
Yes, I often spec Siga Majrex, which costs about the same as Intello and can be ordered online here: https://performancebuildingtapes.com/product/siga-majrex-200-siga-tapes/. I recently used Rothoblaas Clima Control and the builder and I were happy with the results. They are both similarly durable to Intello but Rothoblaas is targeting distribution thorough lumberyards.
For reduced durability and somewhat higher perm ratings, for half the price of Intello or Majrex you can get Certainteed Membrain through your lumberyard.
I'd think faced batts would be fine. You can tape the seams in the facing with Tyvek tape if you're being fussy.
Would that be paper faced or plastic?
Either.
Michael,
"He said a properly designed wall is one proven to work; if you need to run a WUFI model to know whether your assembly is safe, it's not a properly designed wall."
I wish that advice were more frequently followed.
I see that as circular logic.
DC,
How many first time home builders here on GBA think up their own (novel) building assemblies, rather than choose between the well performing ones we know work? (hence the popularity of articles like Five Cathedral Ceilings That Work when we answer questions).
How many assemblies do we see that rely on maintaining very good air-sealing or low indoor humidity for their safety over time, with little margin for error?
I think that's what he is getting at.
I see your point. I guess it depends on who the "we" is when we say "the ones we know work."
DC,
Me and you :)
I’m building a 1300 sq ft shop in Ashland, also 4c, with similar questions. I am planning on 1/2” plywood for the first 8’ of height with just membrane of some kind above that (my peak is about 15’). I used the zip system for my exterior air barrier with a rain screen behind wood and metal siding. My insulation estimates both recommended a WMP-VR flame retardant vapor barrier. I feel like variable permeance membrane would be a better solution but I don’t know whether any of them are flame retardant yet.
I spoke with a local insulation company and they said I don't need ANY vapor control layer at the interior wall.
Another example of opinions from local contractors varying greatly from advice I get on GBA. On the one hand I believe in the building science but on the other I have have never heard of moldy walls in my area.
Joe,
They are correct, you don't need a vapor barrier. What you do need is an air barrier. In case of wood panels, the panels themselves are a good air barrier as long as the edges fall on studs. You can install them as standup sheets, or if you install them horizontally, install blocking where the seams will land. If you want a bit better, a bead of adhesive under each panel edge to really seal it up.
joenorn,
Your original question confuses air and vapour-barriers, which has lead to the subsequent discussion being a bit confusing too. Smart, or variable perm membranes, are primarily for vapour control - and yes your insulator is right, I don't think the IRC requires a vapor-retarder in zone 4.
The primary concern with buildings that have wood as the finished inside surface is stopping moist air making its way through the wall an wetting the sheathing (although the risk is much lower in a shop than a house). That's why an interior air-barrier is useful. Using a variable perm membrane for that would just add vapour control as a bonus.
Thank you for the clarification. So the craft facing on batts is a vapor control layer, and we are saying I don't need that.
What I do need is an air barrier and the purpose is to inhibit warmer air from meandering through interior and hitting the backside of the exterior wall. Is this correct?
joenorm,
The craft paper does no harm, but no it isn't necessary.
Air barriers really have two functions:
They slow air movement through the walls which limits the air changes that occur between the building and its surrounding. That improves energy efficiency and comfort - and for that function they can be anywhere in the wall. In Canada they are still usually at the interior. Most projects on GBA use the exterior sheathing instead.
They also, as you say, slow the cycle of warm moist air into the walls which can end up in the sheathing by adsorbtion. For that you need something close enough to the interior that it doesn't become a condensing surface. So typically it is whatever is the inside surface the walls, or a membrane of some type behind them.