Use 7/16 OSB as air barrier in HVAC channel
idahobuild
| Posted in General Questions on
Putting AC duct in conditioned space using modified plenum trusses. For the lining, I am thinking of using 7/16″ OSB as the lining and sealing the joints with Prosoco FastFlash Joint and Seam Sealer. Is OSB an adequate air control?
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Replies
OSB makes a decent air barrier, but why not just use conventional ductwork formed from sheet steel? The smooth inner surface of steel ductwork is a big plus for air ducts for various reasons (less resistance to airflow, less accumulation of dirt over time, etc.). If you want to use a rigid material, I'd try something with a smooth surface like masonite, then put the smooth surface towards the interior. Note that any wood product will be much more prone to air sealing issues than steel ductwork is.
BTW, you may also have issues with fire code here. Air handling spaces often require the use of nonflammable materials.
Bill
Sorry. I should have been more clear on this. The truss chase will simply hold the ducting, not serve as ducting.
Ah, then I see no issue using the OSB. Why bother air sealing it though? Use mastic on the ducts, that will work better. You're much better off adapting your design so that the ducts are INSIDE the conditioned space, so I'd try to wrap your insulation and air barrier around the ductwork so that the ducts are effectively contained within the building envelope.
Bill
It will be part of the ceiling air barrier.
Bill,
It sounds like the OP is lining the truss plenum as a part of his overall air barrier for the house. When you build with truss plenums and are using the ceilings as air barrier, the air barrier generally goes across the ceiling, then up into and across the plenum, back down and across the rest of the ceiling. Effectively, the ducts in the plenum ARE in the conditioned space. You can use OSB for the lining, but some commodity-quality OSB has been shown to leak a fair amount of air in blower door testing. The facing for ZIP sheathing is far more airtight for a small upcharge. Ducts should still be airtight as always.
I may not be visualizing the OP's situation exactly. What you are describing with the air barrier wrapping around the ducts so that the ducts remain inside the conditioned space is what I was getting at as a desireable goal. I don't like ducts running outside the building envelope, and try to avoid that situation as much as possible.
Bill
Bill, he's talking about a system like this one: https://www.greenbuildingadvisor.com/article/hvac-for-a-slab-on-grade-house.
I'm one of apparently few people who have seen commodity OSB not perform as an air barrier, so I would use Zip sheathing, which is a reliable air barrier. Or use taped drywall, as in the article above.
Thanks for posting that. That makes more sense.
I'd probably use plywood or drywall for that. OSB should work too, but, as you said, it's not as reliable of an air barrier in a more critical location.
Bill
Yep, Micheal and Peter have the proper design in mind. For some reason, I couldn't remeber the word 'plennum' in OP.
Now, it sounds like It'd be best if I coated the already-purchased OSB. I have some Prosoco MVP and I could easily run a roller over the OSB before running it up the ladder. Then use some Prosoco Air Dam or Joint and Seam Filler to get in the corners later....but before hanging the sealed ducting.
Any thoughts on this...?
Any sealer would probably work. Any leaks in OSB are likely to be pretty small, I'd be surprised if a regular primer/sealer wouldn't seal things up enough to be OK, but anything like the MVP you have that is intended for the purpose would be sure to work.
Bill