Air Sealing Vented Attic in New Construction

[Morning Wood]

I've done some searching online and haven't found anything on methods for air sealing new construction. Does anyone have a tried and true method that seems to work well? I have recently tried two methods. One using luan plywood under ceiling joists and another using a membrain (certainteed) product. Both are taped and caulked. I haven't done blower door test yet on either, so I don't now how successful they will be. Anyone have another method out there? Or does anyone have areas that always seem to give them a load of trouble sealing? Thanks, Nick.

[CASHCOW]

Re: Air Sealing Vented Attic in New Construction

You'll find sea good air sealing guide here:

http://www.energycodes.gov/sites/def..._v00_lores.pdf

For which many of the challenges will depend on the quality of the design-build.

[Morning Wood]

Re: Air Sealing Vented Attic in New Construction

That link isn't really what I was looking for. I'm interested in discussing methods for air sealing. Not, what to air seal. That guide mentioned air tight drywall, which is nice, but not if you are looking to blower door test prior to insulation of the envelope

[Kevin_in_Denver]

Re: Air Sealing Vented Attic in New Construction

I've been pondering the same thing.

For a vented attic, I don't see any advantage to using Membrain. Clear polyethylene sheet would still make it possible to see insulation issues, and it's a lot cheaper. Maybe you didn't think it's as tough.
At the ceiling plane, you don't really need water vapor going in either direction, since both sides are ventilated well.

What's the advantage to using Luaun? Since it will later be covered with sheetrock, why not install the sheetrock just before the blower door test (and before insulation)? Any inspection issues are the same with plywood or sheetrock at that stage of construction, as I see it.

[worthy]

Re: Air Sealing Vented Attic in New Construction

Pretty much the same materials you'd use in a retrofit--mastics (for HVAC), small cans of polyurethane foam (eg., Great Stuff) and acoustic caulking and gasketing on attic hatches.

[ThingOfBeauty]

Re: Air Sealing Vented Attic in New Construction

I'm interested to hear how your lauan barrier works. Could be a good plan.

A somewhat similar approach has been used on some passivehouses, where you're shooting for 0.6 ACH50 or less.
A layer of sheet goods like OSB or plywood is applied on the inside of the structural members (truss roof e.g., or on the inside face of the structural wall framing) and sealed from the inside. In the examples I've seen, a set of joists are hung below this sealed plane, and wires and fixtures are all run inside the air barrier. Insulation can be installed on either or both sides of the air barrier but is usually on the outside.
So for a ceiling you might frame the walls higher than your desired ceiling by 4"+, then trusses, plywood the bottoms and air seal, then put 2x4s as ceiling joists. Or add room if you want recessed lights.

For a vented attic, I don't see any advantage to using Membrain. Clear polyethylene sheet would still make it possible to see insulation issues, and it's a lot cheaper...At the ceiling plane, you don't really need water vapor going in either direction, since both sides are ventilated well.

In our area (the somewhat humid east, with an air conditioning season), poly on ceiling = water on ceiling in summer.
Denver is probably a much different story, since it's so dry, but I would think 3/4 of the country you would not want poly anywhere in an assembly.

[NW Architect]

Re: Air Sealing Vented Attic in New Construction

I'd like to hear the luan experience, too. I've mused about putting in coil stock on the tops of interior partitions, to give a continuous, protruding edge to seal ceiling drywall to. But I wonder if electricians and plumbers will find it an obstacle to drilling, whether framers will find it terrible footing for wall walking, and whether it works.

Luan solves may of those questions, but I worry about a fractured edge and a fall from wall walking. And that takes me to a rip of plywood, and a bunch of expense.

I DID hear a solution at JLC NW last weekend. Cutting sill sealer into 1/3 width strips and stapling it to top and bottom wall plates before installing gyp, making a compression seal from drywall to framing. Anybody tried that?

[ThingOfBeauty]

Re: Air Sealing Vented Attic in New Construction

NW,
I like both of those for achieving a final air seal, but if you want to blower-door-test before hanging drywall, I think you'll need another plan.
Why not use silicone foam or EPDM gaskets to seal the drywall at the top plates, instead of making it up with sill seal?

Also, I have seen people enclose the tops of soffits, during framing, so ductwork can be run below the ceiling level. The house I saw they used Therm-o-cardboard, but I think I would just hang drywall if it were up to me. Even paper-free drywall would probably add up to the same (despite slightly higher material cost) because you can run it wild and join the drywall right to it, saving labor.

[NW Architect]

Re: Air Sealing Vented Attic in New Construction

ToB, just reporting on a thing I heard at seminar. I'm fond of foam weatherstripping tape with sticky on one side. I'd f'sure look at that cost vs cut up sill sealer if I were the builder.

Re your second paragraph, I do something similar in commercial work, for fire resistance. Wood framing for load bearing and fire walls. Those get wired and plumbed. Then in comes the sheetrock crew to rock all the wood and erect lightgage metal framing for all interior partitions. Those get wired/plumbed and drywall crew does a pass back to rock the metal framing and tape/mud/finish.

I still have no clue how to blower test a house without all its parts in place . . .

[KC_kid]

Re: Air Sealing Vented Attic in New Construction

During an energy audit of my home, the auditor suggested to create a good attic plane seal by removing all of the existing insulation, put down a 1-2" layer of spray foam insulation and go back with blow-in insulation to the preferred R-value.

Not sure if this applies to your specific blower door test before drywall requirement, just thought I'd share.

[BruceK]

Re: Air Sealing Vented Attic in New Construction

We do most or our air sealing for vented attic space from below. We normally put 1" rigid foam up before the drywall lid goes up. We fit the foam tight to all the exterior walls and partitions and then we caulk thoroughly around every plate line. We carefully seal all the mechanical penetrations in top plates. If necessary, we can add one inch material to the bottom plate to compensate for the thickness of the 1" ceiling foam. All joints in the foam are taped with high quality sealing tape. If we have to do anything up in the attic space, the work is minimal, like installing recessed can covers and foaming them in place. And we get the ductwork out of the attic with few exceptions. If you have to put ductwork in unconditioned space, seal it up to be airtight and cover it with plenty of insulation.

[Mark Parlee]

Re: Air Sealing Vented Attic in New Construction

Bruce
I would think it a lot easier task to spray a 2" layer of open cell after the lid goes in and less cost as well. Then you could take care of the rims at the same time.

[Kevin_in_Denver]

Air Sealing Vented Attic in New Construction

When there is a question about building science, I try to see what Joe L. says: http://www.greenbuildingadvisor.com/...-venting-roofs

Joe says "The single most important thing you have to remember about a vented attic is that the ceiling plane — the gypsum board layer, the drywall layer — needs to be airtight."

For some reason the OP doesn't want to do his air sealing work on the drywall, and is trying MemBrain or luaun. Why? Why not seal the drywall?

Your drywall screws will tend to poke holes in the airtight layer if you don't want the drywall to be the airtight layer.

So the cheapest answer to the original question is to use high quality tapes (SIGA) and inject foam where required. Use the drywall as the airtight layer. It can be done.

[Ted S.]

Re: Air Sealing Vented Attic in New Construction

For some reason the OP doesn't want to do his air sealing work on the drywall, and is trying MemBrain or luaun. Why? Why not seal the drywall?

Because he is trying to do a blower door test prior to drywall presumably so that he can fix what is necessary before the drywall. Once the drywall is up, it is late in the game to fix.

I commend him, he is in MA and we are headed for severe code changes in the near future. While I agree with you and Joe about an airtight lid, it's more complex then that.

Builders will need to develop means and methods to ensure compliance BEFORE waiting until the end of construction to test.

All the experts like Joe Lstiburk piss me off because they are too busy advertising how smart they are to be concerned about home builders trying to make a living and the practicality of their ideas.

Quite simply, airtight drywall only works in a voluntary capacity. In a production environment, which represents the vast majority of Americas housing stock, builders will either need to go to foam or develop means and methods such as the OP is pursuing.

[Dick Seibert]

Re: Air Sealing Vented Attic in New Construction

Quite simply, airtight drywall only works in a voluntary capacity. In a production environment, which represents the vast majority of Americas housing stock, builders will either need to go to foam or develop means and methods such as the OP is pursuing.

Better tell them to stay away from the foam, this in the paper the other day, now they're saying that foam causes childhood obesity, kind of like the anti-tobacco people went after tobacco with the Marlboro man smoking a limp cigarette, telling women they will get fat sitting on a foam couch or living in a foam filled house should do it. Environmentalists will stop at nothing to get their way, save the planet, and make money in the process.

Another option is to steer clear of polyurethane foam altogether and look for furniture made of wood, cotton, wool, polyester fiberfill and canvas, according to the Center for Environmental Health.¹

¹ http://www.sfgate.com/health/article...re-4994841.php