Sealing and insulating a dropped soffit; sagging drywall?
Hi,
I have 8′ ceilings, except for one bathroom, which has a 7′ ceiling. The ceiling is drywall, with an exhaust fan and a ceiling light in it. Whether coincidence or not, I get ice dams near that bathroom.
Should I air-seal the low 7′ ceiling itself, and then pile insulation directly on that drywall?
or
Should I create a layer of drywall, foamboard or plywood that sits 1′ above the 7′ ceiling, to create a new surface that is on the same level as the drywall for the other rooms, and then seal and insulate on that new surface? (leaving an empty, sort-of conditioned space between the existing drywall and this new layer) I found this idea on the PNNL (DOE) website, but I’m not sure I understand the benefits of this approach.
Separately – I know this has been asked before, but in case there has been more information since last time: My ceilings are 1/2″ drywall, 24″ on center. The drywall is about 45 years old. There is currently a small amount of sad fiberglass insulation on it. Should I have any hesitation to blow in R49 or R60 cellulose insulation – can the drywall take that load? I obviously don’t want drooping, sagging ceilings.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Sam. Air sealing is recommended. You should do as much as possible.See this article for more information: https://www.greenbuildingadvisor.com/blogs/dept/musings/air-sealing-attic
So you have a drop-ceiling in the bathroom to conceal the duct run from the fan to the outside? Can you see this area from the attic, or is it covered by a second layer of drywall? What kind of attic space do you have?
Your climate sounds cold. But where exactly do you live?
Not that cold. Zone 4, Virginia. R60 seems overkill to me, but maybe I'll go R49, so I want to make sure the ceiling can take it.
Yes, I definitely want to do air sealing. My confusion is whether I should perform air sealing on the 7' low ceiling, or build a second "ceiling" for that area at 8' and seal that, so the entire attic has a rigid surface at the same height. Maybe I'm not describing it well.
I actually have no idea why this bathroom has a lower ceiling. But yes, I can see this area from the attic. From the attic, you can tell this ceiling is a foot lower. just one layer of drywall right now, though part of this area also has a plywood catwalk. So there is about a foot of empty space between the drywall and the catwalk.
Not sure how to describe the attic space, otherwise. Uninhabited, and not used for storage (yet). Some fiberglass between the joists.
Thank you for the help.
Sam,
Either of the two approaches you mention can work, as long as the work is done conscientiously. In most cases, it's easier to create a new airtight ceiling at the 8 foot height (using drywall, plywood, or rigid foam) than it is to try to create an airtight ceiling at the 7 foot height.
The reason is simple: you may not have good access for sealing the 1-foot-high vertical "walls" of the dropped ceiling area. If these 1-foot-high vertical walls aren't properly air-sealed, all your work is for naught.
In any case, the steps are simple: Make sure that your air barrier is continuous rather than interrupted. Seal all cracks and leaks with caulk, canned spray foam, or solid materials like rigid foam. Then heap up at least R-49 of insulation (for example, cellulose insulation).
For more information, see Air Sealing an Attic.
Thanks, and you are right. That 1'-high vertical "wall" is simply missing right now. No drywall there. From the attic, I can reach my hand through that 1'-high opening and into the wall cavity, for the wall separating the bathroom from the adjacent room.
Do you have a bathroom exhaust and does its duct run through this space? I found that a tear in the plastic exhaust duct in our house was dumping warm, moist air into the space under the roof, causing ice dams and condensation under the roof. After replacing the exhaust with a through-the-wall type, I lined the ceiling with 2" xps, then drywalled. No more ice dams or water issues.