In search of an airtight ceiling, I have heard talk of sheetrocking the ceiling & dropping in a ‘second’ ceiling
Svig
| Posted in Green Building Techniques on
In search of an airtight ceiling, I have heard talk of sheetrocking the ceiling and then dropping a ‘second’ ceiling where lighting and maybe HRV ductwork etc. could be installed. I am wondering how this was accomplished.
For my home I plan to build, a nine foot ceiling in a one story home with trusses, plastic barrier and then sheetrock attached to the trusses. Are 2 x 4’s hung down about six inches with some kind of joist hangers, and another layer of sheetrock installed with lighting and HRS ductwork in between?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Steve,
First of all, the use of an interior polyethylene vapor barrier is not recommended in most U.S. climates, especially if the house will be air conditioned during the summer. If you live in northern Canada or Alaska, the polyethylene may make sense. For warmer regions, however, it's best to skip the poly.
Q. "Are 2 x 4's hung down about six inches with some kind of joist hangers, and another layer of sheetrock installed with lighting and HRV ductwork in between?"
A. Yes, that's one way to do it. It's also possible to install a conventional suspended ceiling (metal rails with drop in panels).
For more information on this issue, see Service Cavities for Wiring and Plumbing.
Steve, Another variant is to frame up a 2"x4" ceiling (or floor depending how you look at it) bearing on the interior and exterior walls and sheath the top with OSB or plywood. This gives you a continuous air barrier and a platform on which to easily assemble the truss package.
Martin, Extreme Northern Minnesota, and I am planning to just use the plastic on the ceiling with a vented attic, which I thought was normal. Batts between the trusses and blown cellulose on top although I am not totally sold on that. 2 x 6 batt insulated walls, osb sheathing, with 3" exterior foam. 30 x 50 south facing home on a slab on bedrock. I am not really interested in a suspended ceiling, more looking for how others went about suspending 2x 4's to attach another layer of sheetrock. Trusses would be clear span 30', sheetrock then a center wall the entire 50' length.
Malcom, that sounds very interesting. As I stated above the home is 30 x 50 with a close to center wall running the 50 foot length so that will give me 16' span in the front and a 14' span in the back, which might be too far for sheetrock to hang on? But I will give that some thought, the house is in the design phase right now.
Steve,
If you are in extreme northern Minnesota, the polyethylene is OK. Go forward without worries.
Steve, Once the trusses are in place most builders secure the 2x4" ceiling joists up into the trusses to reduce the spans.
If you do as Malcolm suggests in 2), it makes the REMOTE method look attractive. You've got a flat, temporary roof, just airseal the whole thing, then put the trusses down on top. I have to believe that the framers would like working on a flat surface. And while REMOTE can work in many places, it was developed in cold climates. (As far as I remember, REMOTE (Alaskan) came out of PERSIST (Canadian) approach.)
This method ticks a lot of boxes for me, including very robust airsealing and build-in service cavities.
Aside: I used to work in International Falls, MN. While fun, I don't miss it this month, and I'd definitely call it extreme.
Steve,
Another slight variant to what Malcom described is to use ceiling joists and rafters rather than trusses. Put up the ceiling joists, sheath and airseal the seams and corners (at the joist/wall connection), then put up the rafters. As Malcom mentioned - it's "pretty sweet" to be able to install rafters while standing on a solid deck (so I hear). Insulation can either go all above the ceiling joists, or a mix of above and below. If insulation is all above, then you obviously have to insulate the "band" area of the joists. Ceiling joists can be dimensional lumber, I-joists or open web truss. Open web truss is particularly useful if you want to run a significant amount of ductwork without having to drop soffits. See http://www.prometheanhomes.com/portfolio.html for some photos of A+ work using these techniques.
Steve, I missed the bit about perhaps wanting to run HVAC in the space between the ceilings. If you need a lot more space, maybe even enough for equipment as well as ducts, you can use scissor trusses and air seal the underside, then frame a flat ceiling underneath.
I think there was an article here on GBA about trusses designed so that the bottom had a chase built in that functioned in much the same way. I've never seen them though.
John, That attic you linked to is so nice you'd end up wanting to finish it ;)
Thanks for the excellent comments, everyone.
Andrew, I need to investigate REMOTE and PERSIST, I have seen them mentioned from time to time. And I.F. is the exact extreme area I am speaking of. I am in Alaska now, heading south to extreme MN this summer.
John, I can only hope to have some photos that look as nice as those in your link!
Malcom, I am planning for mini-splits with electric baseboard/something back up for deep winter, so I think all I need room for is HRV ductwork, which appears to be about 2" ID, maybe 2 1/2' OD? That and lighting, maybe water lines and wiring. So I don't think I will need all that much room, 6" should suffice. I will have a utility room for the HRV to live in and need to get the intake/exhaust around to their spots. I just really like the idea of nothing but insulation above the upper ceiling line, maybe a plumbing vent the only roof penetration.
Malcolm suggested scissor trusts with flat ceiling and using the space between for ducts, etc. I'm doing something like that in my house. The bathrooms, kitchen and mechanical rooms are all on the same side of the house. We have flat ceilings above those areas, with conditioned space above for HRV ducts, plumbing stacks, minisplit lines, etc. That utility space has enough height for someone to crawl around in, should we ever need to. We'll put a few lights in as well. The rest of the house has cathederal ceilings.
The roof insulation is all inside the raised heel scissor trusses. We put an air barrier membrane attached to the underside of the trusses, strapped it and hung drywall, all before installing any interior partitions. The cellulose was blown in through an access port in one of the gable ends. The only roof penetration is a single plumbing stack. We'll terminate and cap the radon pipe inside the utility space. Once the house is done,we'll test for radon and only if necessary will we install a fan and run the radon pipe outside.
Stephen, your layout sounds similar to mine. Verrrry interesting!
Steve-here's a photo showing the utility chase above the kitchen area.