Double-stud walls and air barriers
I am working on the design of a double-stud wall residence (approx. 12″ thick) in climate zone 6b and am currently detailing the air barrier & estimating costs on a variety of different products/approaches.
The insulation will consist of dense pack cellulose and the exterior cladding will be lap siding on a ventilated rainscreen in some areas and vertical board & batt (basically a ventilated rainscreen on its own) in other locations.
https://www.greenbuildingadvisor.com/blogs/dept/musings/one-air-barrier-or-two
I have previously reviewed your article on 1 vs. 2 air barriers (above) and am not 100% sure which side of the fence I sit on for that discussion. It seems that for double-stud walls there is a stronger case for 2 air barriers. The interior air barrier to reduce the chance of warm interior air hitting the obviously cold sheathing. (maybe most importantly for the double stud wall) The exterior to form a good WRB. (Second most important?) If the project budget doesn’t have room for both a high quality interior & exterior air barrier where is it most effective to spend the money?
On the interior with an airtight drywall approach for best practices I would gasket the drywall & use airfoil boxes (which are quite pricey). For the exterior I have the increased cost of either a tape for all the sheathing joints & openings in the walls or a liquid applied flashing for all the joints & openings so that the air barrier is the wall sheathing. On top of that I would still be applying a WRB over the entire assembly.
Any suggestions/discussion on the above is welcome.
On a side note, and probably worthy of a separate question, I am very interested in the Prosoco fast-flash system but wonder if it is possible to use just the fast-flash for the sheathing joints, penetrations & openings (for the air barrier) but not use their entire-wall WRB product and instead use either tyvek or building paper to save cost? I haven’t found anything in my research combining the 2 approaches.
Thanks.
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Replies
James,
If your wall will only get one air barrier, I vote that you should put it on the interior.
I disagree with your statement that "a good WRB" needs to be airtight. There is no reason that a water-resistive barrier needs to be airtight. All a WRB needs to do is shed water.
That's why asphalt felt makes a perfectly good WRB. It certainly isn't airtight, but it will do a good job keeping your wall dry -- especially if you include a rainscreen gap between your siding and your WRB.