Airtight drywall: Why seal double top plates?
airfix
| Posted in General Questions on
I’m trying to understand the airtight drywall approach and why it is important to seal across double top plates. If the sheetrock itself is air tight. If the corners and joints of the sheet rock are taped and mudded to make them air tight. If the electrical boxes are all sealed to the sheetrock and wire penetrations to the box are sealed. If the sheetrock is sealed to the subfloor. Why is it important to seal the gap between the double studded top plate? The gap in the top plate is behind the air barrier?
Here is the advice I see about air tight dry wall, I’m just trying to understand the logic.
Cheers
Steve
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Replies
Hi Steve,
Airtight drywall is creating an interior air barrier, which is part of the air sealing that a high-performance home needs. If you are installing air-permeable insulation in the wall cavities, those cavities should also be air tight. That includes sealing seams and penetrations at the bottom and top plates. And that work needs to be done before the drywall is installed.
Here's another article on the topic that may be helpful: https://www.greenbuildingadvisor.com/article/how-to-hang-airtight-drywall
And here are some videos on the topic: https://www.greenbuildingadvisor.com/green-basics/video-how-to-hang-airtight-drywall-1-of-3
I've not had a chance to look at the videos yet but I reviewed the article. If the drywall is sealed like I stated in my first post, then the cavity sealing you mention must be for sealing to prevent air ingress from the exterior because air ingress from the interior is controlled by the drywall?
I am planning to tape and seal my sheathing. Actually on my drawing I specified the sheathing as my primary air barrier and my drywall as my secondary air barrier. So it seems to me if I tape all the sheathing joints, tape the sheathing to the roof deck, caulk the sheathing to the sill plate, caulk the sill plate to the foundation caulk the windows and wall penetrations and use ccSPF on the underside of my roof deck than I have my primary air barrier fairly robust.
Using the drywall sealed per my first post and my sheathing sealed as above, in theory no air can get into the cavities, so again why is it important to seal the double top plates?
I'm just trying to understand the logic, it seems the cavities that you say need sealing are between two air control layers and so using more caulk in the cavity joints wont help, or am I mistaken?
Steve
It sounds like you're thinking about the exterior wall to top plate connection, which there is truth to what you're saying. But then introduce interior walls which are not air sealed. Also consider the intersection of interior walls with exterior walls. When you add in that complexity, even just the way top plates intersect, it's time to get the caulk gun out. It will cost an extra 10 bucks and 20 minutes time.
I think It's a redundancy thing.
I taped my exterior sheathing, and my drywall is mudded as well, but I also caulked my framing for fun because I'm on a messy job and it was low cost redundancy.
I'm not at the level where I get to play with blower door tests on multiple projects, so I can't tell you exactly what works and what doesn't. But I can tell you that contractors, subcontractors, and the foot solders doing the work are not usually great. No one on site actually understands the purpose of air sealing. We don't get to take the workers to building science seminars nor would they want to go. So when the job boss turns his head or isn't there that day to babysit, it's probably not going to get sealed well.
Jamie B,
That's exactly where I am now. I'm in the middle of framing and my workers goal is to frame the house on time. Air sealing is not their top priority even if it is mine.
Caulking all the joints is not so easy though because of our ceiling height much is out of reach for me so I'll have to convince my contactor its a worthy goal.
Btw I did my own back yard caulk and tape test that I'll post when I can find a spare minute.
Steve
This is pretty typical for any kind of specialized construction work. I consult on critical facility systems (telecom sites, hospitals, stuff that absolutely has to work all the time). One of the big problems is that many contractors don’t understand the complexities of systems, especially when they don’t work with the particular system very often. I always say it’s similar to building an oil refinery: sure, it’s mostly pipe work, but it’s way more complex than anything a regular plumber ever sees.
If you’re specifying design features that your contractors aren’t familiar with, you need to pay extra attention to having very good details and callouts in your plans, be extra diligent with supervision, and be careful writing your contracts as insurance against any problems that come up. Using specialized contractors that ARE familiar with the specifics of less common systems may cost more, but it’s about the best you can do if you’re trying to get things done properly.
Bill