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Few topics in building science have been as thoroughly haggled over as vapor retarders, the materials designed to keep water vapor from migrating into wall and roof assemblies. In a paper published nearly 20 years ago, Joseph Lstiburek, a principal at Building Science Corp., summed up the problem this way: Building professionals may have come to understand the challenges of water in liquid and frozen form, but water vapor is “in a class of craziness all by itself.”
The problem is that water vapor doesn’t always stay in vapor form. When it comes into contact with a surface that is below the dew point, vapor becomes a liquid. This can cause a host of problems inside wall and roof cavities. Vapor retarders are intended to stop water vapor from moving into a wall or roof assembly in the first place so this can’t happen.
“The fundamental principle of control of water in the vapor form is to keep it out and to let it out if it gets in,” Lstiburek wrote. “Simple, right? No chance. It gets complicated because sometimes the best strategies to keep water vapor out also trap water vapor in.”
That was the problem with polyethylene sheeting, once a widely used vapor retarder in cold climates. In technical terms, poly is a Class I vapor retarder with a perm rating of no more than 0.1, on a par with sheet metal and glass (a “perm” is a measure of how much water vapor can pass through a material). Poly was typically applied on the warm side of a framed wall. In a heating climate, and when detailed carefully, the poly prevented warm, moist air from traveling into wall cavities in the winter where it could condense on the back side of cold sheathing. But if water…
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19 Comments
Thanks Scott!
I'm moving to use more Larsen truss exterior insulation. Am I finally tiered of all that EPS stuck to me with my 6-8" layers? With a 2x4 or 2x6 load bearing wall (with fiber insulation) and air sealed sheathing, do we need to worry at all even with T&G interior. At that point, it will always dry to the interior and the sheathing will always be safe with all that cellulose on the exterior. Any thoughts mentors?
Ah Jeez, on a sq ft basis a lot of these products really do cost as much as sheets of drywall for the materials! However, the labor to put these smart membranes up vs drywall is just a fraction.
This is a great overview.
One thing Kohta said confused me a bit:
“If you need a vapor retarder, you may want to step back and [consider if the wall was designed properly],” Ueno said. “If you’ve designed yourself into a corner where you need a vapor retarder, I always recommend one of these variable perm products.”
I get that in the context of having used inadequately thick exterior foam, but what about the most common walls we build with all fibrous insulation? Was Kohta saying they were a bad design choice, or that they didn't need a vapour-retarder?
Hi Malcolm! Great question--I guess I should have put a few caveats on that of (a) I'm a bit Zone 5-chauvinist, given that's where a lot of my work is, and (b) I'm referring specifically to Class I and II vapor retarders, not Class III/latex paint.
So that being said--if you're in CZ 5, and you have a vented cladding with plywood or OSB (or other materials), you are allowed to use a Class III vapor retarder. Which covers a lot of construction in a lot of climate zones. Of course, the requirements get more stringent as you get colder, into CZ 6 etc., and Class III is a less viable of an option. The table below is pulled from the link here.
GM-0903: IRC FAQ: Insulating Sheathing Vapor Retarder Requirements
https://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-insulating-sheathing-vapor-retarder-requirements
For reference, we've looked specifically at the problem of 'how necessary is vapor control in walls in CZ 6'--research at U Waterloo; key stuff is on slides 20 through 30. Summary: running a 2x6 wall in CZ 6 with 50% RH flatline all winter long (i.e., scary high interior RHs), using Class III/latex paint doesn't work. But same conditions, adding Class II vapor retarder paint worked great in the following winter.
NESEA: Moisture Safe? The Writing is on the Wall.pdf
https://www.buildingscience.com/sites/default/files/2017-03-08_nesea_be17_ueno_for_pdf.pdf
Thanks Kohta, that makes a lot of sense.
Will smart membranes eventually become "dumb" ?
This is still a fairly recent technology. What will these membranes look like in 50 or 100 years, when the building will still be standing ? Isn't it a bit risky to assume they will work over such a long time? Kraft paper in my 1970's house is turning to dust when handled.
Moisture management in cold climate is fairly critical. How confident can we be that they will last a long time and work as intended?
Thierry,
Maybe that's where Eric Whetzel's approach makes sense: Use them as a belt an suspenders approach, as opposed to something that the wall fundamentally relies on for its performance.
And maybe that's what Kohta was saying too?
The ProClima INTELLO Plus as we at 475.supply and distribute in North America since 2010, was introduced in Germany in 2006. It performance has been verified with real life confirmation of it's vapor permeance (so 15+ years now) as well as rapid age tested to similate longer usage periods. The INTELLO 100x vapor variable permeance (form 0,13 dry to over 13 in humid conditions to facilitate inward drying actually slightly improves with time. We also have several projects that are monitored with moisture sensors and continue perform as designed.
That's good to hear. Which parameter(s) is it that the improves with time?
I have used Membrain on quite a few projects and have been happy with the product. This product fits the bill when you are on a budget build but still trying to use modern building techniques and products. I ordered online right from Home Depot.
There was a common theme in the article that MemBrain was difficult to work with or tears easily. Did you experience that?
yes, very flimsy- intello much better-worth the cost-differential for sure
Would you still use a product like this internally if you're applying a product like Blueshield to the external? Zone 5, no more than 1 inch of exterior insulation.
What about housewrap in place of something like Siga/Proclima? The higher vapour permeability may introduce increased winter time movement through the wall to the sheathing in climate zone 6. But would also increase the inward drying during the warm months?
Also, wouldn't the total permeability be affected by the continuous drywall +paint that most homes are built with currently as Chris mentions in the article? If the Majrex has a max perm of 3.8, but there is drywall in front of it with paint, isn't the total perm of the assembly much lower? At what point is it equivalent to just poly all over again?
The point of vapor retarders is to slow vapor movement. "Housewrap," more precisely called WRBs, are vapor-open and do little to nothing to slow vapor movement.
Majrex's perm rating is lower than 3.8 when dry; it only opens up if there is enough moisture present to require drying to the interior. Latex paint is around 5-8 perms so it drops the inward drying ability slightly but not by much.
I thought 300 perms is considered high?
https://www.greenbuildingadvisor.com/article/filling-rafter-bays-with-fluffy-insulation
Is 40 perms in house wrap + drywall + paint = high? Maybe to Jon Riley's point in the article that it is sufficiently low to save a few hundred to a few thousand dollars?
300 perms is very high. Less than 0.1 perms is considered a class 1 vapor retarder, 0.1-1.0 perms is considered class 2, 1.0-10.0 perms is class 3, and over 10 perms is considered vapor-open.
I'm not sure how that's relevant to the discussion. As stated in the article and comments, the IRC model code provides guidance on what makes a safe assembly. Jon Riley insulates many of my projects and I continue to spec Majrex or Intello on most of them.
Thanks for taking my question. I guess to formulate it better. Shouldn't the drywall assembly function as the vapour barrier, but then use a high perm house wrap as an air barrier?
Perhaps the other question is, what is the impact of multiple materials with different permeability placed next to each other? What if you have from outside in: majrex, 1in unfaced EPS, drywall, paint. What is the perms of that assembly? Is it simply the lowest material (in this case the majrex?)
You should have one really good air control layer in your assembly. Having multiple air control layers is usually a good thing, but the important thing is to have at least one. Where in the assembly it goes is less important. In most cases I like placing it at the sheathing, either by making the sheathing itself airtight or with a separate product, but in some cases it makes more sense to put it elsewhere in the assembly. Drywall can be made airtight--in fact, drywall's airtightness is used as the definition of airtight for code compliance--but it's usually easier to locate it elsewhere on new construction.
Painted drywall is a vapor retarder (not barrier); regular latex paint provides around 5-8 perms, while oil-based or shellac-based paints have lower perms. It can be used as the main vapor control layer but with some assemblies it's not as safe as having a separate, dedicated vapor control membrane.
When you have multiple vapor-retarding materials in an assembly, the total vapor permeance isn't just that of the highest-perm material, and you don't add them together. I've seen the math on how to calculate it but it's complicated. One rule of thumb I've seen and like is that in a heating-dominated climate, the highest-perm materials should be on the exterior, with at least 5 times the perm rating of the least permeable material, which should be located near the interior.
This is a good overview article: https://www.greenbuildingadvisor.com/article/an-intro-to-the-control-layers-of-a-wall
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