Economical but effective airtight and vapor-impermeable sealant for OSB?
I’m in northern CZ-4- Mixed Humid. I plan to have a 2×6 advanced framed wall with OSB on the outside and unfaced fiberglass batt cavity insulation. There will also be a layer of Continuous External Insulation, a 1″ Rain Screen and Brick Cladding. The type of CEI is as yet undetermined.
I want the assembly to be Vapor Impermeable to the potential vapor drive from wet brick and Air-Tight. The OSB would dry to the interior.
My question pertains to a possible coating for use on the OSB to accomplish this.
I see many very pricy coatings (usually STPEs) from several manufacturers that are available with varying degrees of vapor permeability. It would seem that a simpler and much less expensive solution would be to use a sealer of some sort, concrete sealer comes to mind but there must be others as well. Would Concrete Sealer applied with a roller to the outside of the OSB provide a PERMANENT, AIR-TIGHT, and VAPOR IMPERMEABLE barrier? There is concrete sealer that is available for use on “green” concrete so that presumably the minimal moisture content of the OSB would not be a problem. I obviously don’t need to be concerned about discoloration of the OSB. The seams of the sealed OSB would be taped.
If not concrete sealer do you have an alternative recommendation?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Ted,
First of all, your use of the abbreviation "CEI" confused me. Then I figured it out -- I guess it means "continuous exterior insulation."
To summarize: You are building a wood-framed wall in Climate Zone 4. Your cladding will be brick veneer, and you are apparently worried about inward solar vapor drive.
You don't have to reinvent the wheel here. The standard detail is to include a layer of rigid foam on the exterior side of your wall sheathing. Any type of rigid foam would work, but foil-faced polyiso is by far the easiest type of foam to tape. With normal attention to airtightness, you'll be fine.
The best way to create an air barrier with these details is to tape the OSB seams. You could use Zip sheathing sealed with Zip tape, or ordinary OSB sealed with a good tape like Siga Wigluv. Some builders skip the OSB tape, and just trust the taped polyiso to be the air barrier. Depending on your airtightness goal, this can work fine, too.
There is no reason to try to use a concrete sealer on OSB. The concrete sealer manufacturer isn't going to support this installation, so why experiment?
I'm not sure why you sound so worried. If your worries have to do with choosing a WRB, perhaps these articles will reassure you:
All About Water-Resistive Barriers
Using Rigid Foam As a Water-Resistive Barrier
Initially, in my plan, I had intended to use a layer (an inch) of PolyIso as the Continuous External Insulation, but after reading numerous cautions about it's decreased efficacy at lower temperatures (In northern Zone 4, we're heating dominant), I've concluded that a different insulator might be better. EPS is less expensive, with lower R-value, but has a higher permeability. XPS has an R-value that is in the middle, and a lowish permeability, but has environmental problems. I don't have enough space in my plan to cover a layer of PolyIso with something additional to "warm" it effectively. Looking at the economy of the assembly in both materials and labor, it seemed that a coating for the OSB that is both vapor impermeable and air tight would solve both issues with a lower cost solution. [I am looking for VERY Air-Tight - verified by pressurized fog testing before external or cavity insulation]. For example: EPS or XPS over a well, but inexpensively, coated OSB would seem to provide that. There are Liquid Applied Air Barriers, those Silyl-Terminated-Poly-Eithers, that are vapor impermeable; and there are a variety of proprietary Membranes that would work. But, they seem to be higher cost solutions in terms of both equipment and/or labor.
In an article, "Painting On Your Vapour Barrier", Dr. Lstiburek suggests the use of Vapor Retarder Primers with a permeability of about 0.5 for use on the interior walls to prevent vapor intrusion into the cavity. I am hoping to find a similar solution to be applied to the outside of the OSB, which would make the OSB air-tight and function as a vapor barrier. And one which when sandwiched between the Continuous External Insulation and the OSB would be permanently effective as well. Inexpensive, easy to apply, and effective.
Ted,
A 1-inch layer of EPS (in conjunction with a ventilated rainscreen gap) will provide a perfectly adequate layer to prevent problems with inward solar vapor drive.
The least expensive way to make your OSB airtight is to tape the seams.
EPS can be sourced with a plastic facer too, and this facer can be taped just like polyiso. The facer will make it quite vapor-tight, and if it's shiny, it'll give you a free radiant barrier in conjunction with the air gap behind the brick. It may not be worth much in a primarily heating climate, but it might be all but free, so no reason not to take advantage of that.
EPS also comes with foil-facers, commonly used (often inappropriately, without air gaps) in roofs in the southern US. With shiny foil facing the brick cladding it'll give a modest thermal performance boost, and it's easy to air-seal using high quality temperature rated foil tapes.