I have an attic in an older home (1910) which has no air/vapor barrier to seperate the living space from the attic. At this time the ceiling is sheetrock in a few rooms and lath and plaster in the rest. The attic insulation is 3 1/2″ of loose rock wool and 8″ of unfaced fiberglass. I want to seal the ceiling plane and add 10″ more fiber glass. If I clear out all the insulation and I cut 4 mil plastic sheeting and lay it between the joists and seal the plastic to the joists with one part foam or caulk, then reinstall all the insulation, would this be a good solution that would not have moisture entrapment issues?
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Jimmy,
Vapor diffusion through ceiling drywall or plaster rarely causes problems. Exfiltrating air, however, can lead to a host of problems. What you need is a ceiling air barrier, not a ceiling vapor barrier.
The easiest way to create a ceiling air barrier in an older home is to pull aside the existing insulation and fill cracks with spray foam. Get a two-component spray foam kit and go at it. Don't forget to address the cracks between partition top plates and the drywall on partition walls.
You will also need to install good weatherstripping on your attic access hatch.
Martin Holladay, senior editor
http://www.greenbuildingadvisor.com
Does the lath and plaster and sheetrock ceiling itself form an good air barrier? Is it only the edges and holes that need attention? I have read that both surfaces are fairly vapor permiable, does this mean that there is some air movement through them as well. I would imagine that any cracks in the lath and plaster would be necessary to treated so it might be best to cover as much of the attic side of lath and plaster so as to not miss anything. I have concidered spraying the whole attic with two part foam but I wonder if the foam can withstand the wood movement from a 120 degree summer attic temperature to the freezing damp cold of our winters?
Plaster and drywall, save at the edges and around light fixtures and the like, provide a perfectly adequate air barrier. Whether they provide an adequate vapor barrier depends on conditions (how cold does it get?) and what they're painted with. Several layers of old oil paint will create a pretty good vapor barrier, as will one layer of "vapor barrier paint".
Obviously, cracks may be a problem, but usually only in plaster, not drywall.
No hint as to where you live, but the insulation you're talking about suggests somewhere cold, so there could be some modest advantage to a vapor barrier below the ceiling insulation. However, much more important is to obsessively seal the ceiling against air leaks -- wire and pipe penetrations, the areas around ceiling light fixtures, flue chases, and odd architectural features such as dropped ceilings all provide paths for warm air to leak into the attic. In addition, especially on plaster/lath walls, the joint at the top of the wall between ceiling and wall header is often a major source of leaks.
After the new fiberglass is in (or even if you don't add it) an air barrier that ISN'T a vapor barrier (eg, housewrap or plywood, not plastic) over the fiberglass can improve its efficiency significantly. Or use cellulose, which doesn't need such a barrier.
The idea of housewrap is an interesting idea. Is this to prevent wind wash and is it nessary to cover the whole surface with overlaped and taped edges? If there is no vapor barrier under all the insulation, sealing the holes will be enough to prevent vapor build up because the houeswrap its self is so vapor permiable that it will dry out?
That's the basic idea.
That's the basic idea. Probably not necessary to tightly tape the seams, though it wouldn't hurt.
NO. I wouldn't. What you
NO. I wouldn't. What you suggest wyould be such a pain IMO!! I'd use vapor retarder paint on the ceilings (which is readily available). I'd seal gaps and holes going into the attic. I'd ensure the attic is properly ventilated. I'd probably leave the insulation in place and work my sealing around it.
similar but different vapor barrier/air barrier ques in attic
I've discovered an improperly insulated area in my attic above 2 bathrooms and against a family room (2 adjacent walls). There is 6 1/2" to 9 1/2" insulation against the roof, with no insulation and no vapor/air barrier on ceiling or behind heated walls. Therefore, this attic area is warm and cozy...hence, I'm paying to heat this wasted space (total is approx. 8' X 12'). I have access to all ceiling areas (2 X 6 construction) and walls (2 X 4's). Several questions emerge:
1. what do I do for air/vapor barrier protection? I can see all backsides of drywall in ceiling below and walls. Some walls were built with 2 X 4's on their sides, so I've already installed 1 1/2" polystyrene foam- nice and tight; the balance of walls are all standard 2 X 4. But what about a vapor barrier against the drywall? I won't be able to fit a barrier behind any of the wood framing. Is this an issue?
2. what should I do for insulation to bring it to code? I can install 6 1/2" fiberglass in ceiling. Then I was thinking of laying down 2" of polystyrene foam on top, possibly covered with OSB or similar for wear & tear & flammability. Same insulation with walls... 3 1/2" FG with foam over top.
I'm planning to remove the insulation facing the roof... after I insulate properly.
Gotta love the contractor that did this install in 1985.
What do you think?
Thanks.
Conceptually there's nothing wrong with insulating the roof instead of the ceiling. You don't lose heat in a volume (cubic feet) of space, you lose it in the area (square feet) of exposed wall or ceiling/roof. Depending on the pitch, the roof may have about 1.5 times the area of the ceiling below, but in the grand scheme of things that's not a big difference.
So the insulation you have is probably "adequate" (though you don't say where you live -- MN, WV, OH, NY?), and likely you'd be better off investing your time and money in obsessively sealing the "envelope" vs adding more insulation.
location
I live in upstate New York.
This portion of the attic adjoins the rest of the attic, which is well insulated (floor has old FG and new blown in cellulose done right) and well ventilated (eaves and ridge vents). The other day, outside temp was in 20's and 30's. The attic was 40 degrees and falling. Before I added blown in insulation, the attic was running in the 50's and 60's in the dead of winter. I was helping to heat NY.
So, to match the rest of the attic, it makes sense to insulate walls and ceiling and let the attic be cold in winter. The only other consideration is that I have a bath fan and (2) 4" merging flexible ducts that run through this section of attic that I'll have to insulate. I'm thinking of switching most of it out for rigid PVC pipe and then insulating it. The total run of the ductwork is close to 20 feet.
A house of that age probably has enough coats of ceiling paint to create a more than adequate vapor barrier.
As others have stated, air leakage is your real problem, and air movement carries vapor with it. Once you air-seal as has been suggested, vapor problems will disappear.