Stopping air leaks is the single most important part of making a house more energy efficient. You can stop air on the outside with plywood, housewrap, and tape, but the best air barrier is a system that incorporates the whole wall or roof assembly.
As it turns out, drywall is excellent at stopping air. If you doubt that, try to blow through it. The weak spots are the seams between sheets and the holes that you have to cut for windows, doors, electrical boxes, and can lights. The process for installing drywall as an air barrier is called the airtight-drywall approach (see “Energy-Smart Details” in FHB #214 and online at FineHomebuilding.com), and it relies on caulks, sealants, canned foam, and gaskets to seal the weak spots.
The first step to airtight drywall is to identify what building scientists call the thermal boundary—insulation to us regular folks. The air barrier needs to be continuous along the thermal boundary. This is especially important where interior walls join exterior walls at rim joists or in places where chases are run for plumbing or electrical work.
We decided to hang drywall in the garage shop in FHB’s Project House using the airtight approach, partly to show you how to do it and partly so that the editors could make me do their dirty work. The outside of the house eventually will be covered with housewrap and rigid-foam sheathing. The drywall, therefore, is not the primary air barrier but is the interior part of an air-barrier system.
Materials you’ll need
To seal up this garage shop, I used flexible caulk, construction adhesive, and cans of expanding foam. Various types of gaskets are also often used in airtight-drywall jobs. The most basic material, of course, is drywall. To…
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"One of the great things about drywall is that the seams don’t have to be perfect. Gaps can be filled with foam and mud. For gaps larger than 1⁄4 in., use expanding foam".
One of the great things about good drywall boarders is that they get their seams tight. And if they end up with a 1/4" gap they use filling compound not foam.
Owners that think their drywall crew is going to monkey about with a dozen different methods of sealing while hanging the board are going to be very disappointed. Rely on gaskets and gasketted boxes as much as possible. Have the sills and penetrations through the framing sealed by your insulators before the drywallers arrive.
I wish I'd thought of that
I wish I'd had thought about the 2x4 pieces between the studs at the bottom plate before I did my ADA. The problem is that once the floor sheathing is in it covers up 50 to 75 percent of the bottom plate exposure and there just isn't enough exposure left to reliably seal to the drywall. It potentially ruined an otherwise good sealing job of mine. Now I'll probably have to seal the baseboards to the wall to seal it. I think probably the easiest way of getting around the whole problem there is just to double up the bottom plates. Much, much easier than cutting individual pieces between studs.
Edit: I guess I should add that this may not be a universal problem. I put floor sheathing over the original diagonal planks. The original bottom plates rested on the original diagonal planks. So my (and other peoples') situation may be different from the norm where bottom plates rest on the top layer of floor sheathing.
NICE DRYWALL JOB, POOR INSULATION JOB JUDGING FROM PHOTOS #9 AND 10, OUT OF 11 TOTAL. I AGREE WITH MALCOLM - DRYWALL HANGERS WANT TO HANG IT AND HANG IT QUICKLY. MOST CANNOT BE RELIED UPON TO DO THIS KIND OF CAULKING. THE CONTRACTOR WOULD HAVE TO WORK DIRECTLY WITH THEM TO ENSURE A GOOD JOB.
Not the Future of Air Sealing
There are just too many penetrations in the drywall.
Air sealing the outer surface of the sheathing is more cost effective. The is also less chance of future homeowners violating the air barrier.
There is also no point in air sealing the drywall if the air barrier is the outer surface of the sheathing.
Once you have a good air barrier in place, it's hard to improve a the ACH50 measurement with another good air barrier.
Kevin, I agree with you for
Kevin, I agree with you for new construction. It generally seems to be more effective, simpler, cheaper and less labor. But sometimes when you're doing a remodel, moving walls, gut jobs etc, it's much easier to integrate the air sealing with new drywall. In the particular case where there is no exterior work needed to be done on a preexisting home then air sealing the exterior for a remodel would be a major headache.
I'm building a new home myself and will be employing the belt and suspenders approach by having multiple air barriers, one of which being airtight drywall.
Is it worth drywalling all the exterior walls and ceiling before interior framing? This would eliminate all interruptions at partition walls and theoretically reduce the chance of leakage.
This will most likely speed up hanging drywall, but could cause inconveniences for wiring. Seeing as how I'll be doing the work myself I can ensure quality either way and I really don't mind if it ends up taking me more time. I just want to be sure I'm really not screwing something up for the benefit.
Doing all your own work is the only circumstance I think it might make sense to drywall the shell before interior partitions. It's not only wiring that becomes more complicated:
- The interior partitions are usually connected to the trusses above with clips to deal with uplift, and the fasteners for the drywall held back from the walls 12" to 16". Being continuous, your ceiling drywall can't flex and move with the walls. You need to c0me up with another strategy to stop the walls lifting or the drywall cracking at the intersection.
- Roughing in plumbing vents and any other services - ducts, etc. - that penetrate the drywall is now a lot less straightforward. Especially as you can't see the blocking or trusses above to plan routes.
- You are going to end up with a lot more butt joints to mud on the drywall without interior partitions to break up the spans.
None of these are insurmountable, just worth thinking about in advance. if you did this regularly you would work out the kinks, but I bet the first house would be a real headache.
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