Ceiling air sealing
B_Bush
| Posted in Green Building Techniques on
I’m researching have a home built and looking for ways to increase air tightness. I’m planning a vented attic an possibly some vaulted. While looking at ways to air seal the ceiling I thought about the use of sheet plastic stapled to the sealing and extended a foot or so down the walls. I’m on the border of 2a/3a. I was hoping to hear some opinions on the idea.
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Replies
Brad,
If you are still in the planning stage, I suggest searching GBA using the phrase "pretty good house." That search will turn up lots of useful information. (But you definitely don't want to use poly as an interior air barrier.)
You also should read this article on insulating cathedral ceilings (https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling) and this article on installing airtight drywall (https://www.greenbuildingadvisor.com/article/how-to-hang-airtight-drywall).
As you move forward with your project, consider hiring a builder who is enthusiastic about building energy efficient and comfortable homes. Finding this type of person will help you avoid stress and budget overruns.
Hi Brad.
Using plastic sheeting for an air barrier means that you also have a class I vapor retarder. So before you go that route, it is important to make sure that having this makes sense in your assembly (probably not in your warm climate). Some builders use variable-perm vapor retarders as ceiling air barrier for a more forgiving vapor control layer. This project shows an example of that approach: A Pretty Good House in the Sierra Nevada
With just a little bit of extra detailing with sealants, a drywall ceiling can work really well as an air barrier. The fewer penetrations you make through the ceiling, the better. So, for instance, many builders who are using the drywall as an air barrier on the ceiling avoid recessed lights. In addition to the articles Steve provided, you may find this helpful for a look at all that needs ceiling on an attic floor: How to Insulate an Attic Floor.
Some builders sheath the ceiling with OSB and tape the seams before installing drywall. The idea here is that the OSB is a more durable air barrier for the long haul. Here's an example of a project that took this approach: Lessons From a Practical Passive House.
Still with OSB, the minimal penetration rule applies. In the Pretty Good House story linked above, you'll see one more option, to frame a chase below the ceiling air barrier where you can run mechanicals, install lights, etc.
I looked into the Intello Plus in the Pretty Good house article. I've read the article before and didn't remember that part. I searched that product and the price is way too high. The plastic is a vapour barrier but is that an issue? In my climate vapour drive is always outside to inside and the differential is very high. The plastic would block the large amount of vapour being pushed through the drywall. I assume the drywall would also stay at a much lower indoor moisture level.
Hi Brad.
I don't know enough about your build/assemblies to say anything with much confidence, but poly sheeting is a particularly bad idea in cooling climates where A/C is running often. The poly is cooled by the A/C and becomes a condensing surface. In other words, the inward vapor drive that you describe, and humid air will condense against the poly. Because the drying path is inward, the condensation doesn't dry. The assembly stays wet. Again, unless you have an assembly that I don't understand, I don't recommend interior poly in your climate.
I'm curious- why the reticence about plastic on a ceiling? The ceiling always has more insulation than walls, the surface is guaranteed not to be a condensing surface (isn't it?), the vented attic means that the assembly can readily dry upwards, so where's the concern? Is it trapped water if the roof leaks? But even smart barriers won't help there?
I understand the problems that can arise with plastic on walls, with trapped moisture between inner and outer surfaces, but I'm not understanding the danger with this 1-surface ceiling assembly.
I didn't consider vapor trapped under the plastic against the drywall. It will happen if the conditions are right. I assume the plastic temperature would be in the 80 degree range assuming 72 degree interior temps and its covered with insulation. I also assume 50% interior humidity levels which again I assume the ceiling drywall would be. I am starting to think the idea is skirting to close to disaster to be a viable solution.