Airtight drywall in mechanical room?
New construction in zone 6b.
I’m planning to go with a belt and suspenders approach to my air barrier. My primary air barrier will be air tight sheathing. My secondary air barrier will be air tight drywall.
Now that I’ve seen the mechanical room and the equipment start to go in, it appears to me that it is not practical to drywall the mechanical room. I’d need holes in the mechanical room drywall for HVAC ducting, ERV ducting, make-up air, domestic plumbing, garden sprinklers, fire suppression sprinklers (required by code), electrical and who knows what else by the time I’m done.
How do people that do air tight drywall manage their mechanical room?
Steve
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>"How do people that do air tight drywall manage their mechanical room?"
People do it by putting the mechanical room fully inside of conditioned space, where only the exterior walls need to be made air tight, with no ducts and minimal plumbing (if any) penetrating that air tight exterior wall.
It sounds like you're putting the mechanical room in a garage or something, outside the pressure and thermal boundary of the house-proper?
Steve,
Probably a bit late for you, but I'd have been tempted to install plywood backing - but just where the wall was being penetrated by services. That way they could still accessible during installation, but also could be sealed before the drywall was put in.
Dana is correct, best to keep the mechanical fully inside the conditioned areas as much as possible. By keeping everything within the conditioned areas, there is no need to be concerned with leaks around all of the wall/ceiling penetrations.
If for some reason you do need to make a lot of penetrations between conditioned and unconditioned areas, you need to seal things "commercial style" with firestop materials. In this case, your concern is more with air sealing than keeping fire from spreading, but the same materials work for both. You can use caulk, but there are also fire stop "sheets" (reddish sheets of a stretchy, sticky, clay-like material) that can be molded around penetrations. You can also spray foam around the penetrations to get an air tight seal. There are a lot of ways to make good air seals, but it's best to just avoid the need if you can. This is similar to the "keep your mechanical out of the attic" issue, and for many of the same reasons.
Bill
My mechanical room is fully contained within the thermal envelope. There is one wall that is common to the outside. Most of that common wall is below grade and concrete. The concrete is wrapped in 1.5" of foil faced polyiso and framed inside that with 2x4's. There is about 12" that is framed on top of the concrete that my band joist sits on.
That wall I have no penetrations through at this point except my mains water. It would be fairly straightforward at this point to sheetrock that one wall. However that doesn't make sense if I don't sheetrock the ceiling and the other 3 walls because air will just migrate past the sheetrock.
I have penetrations through the other 3 walls and the ceiling though.
"By keeping everything within the conditioned areas, there is no need to be concerned with leaks around all of the wall/ceiling penetrations."
I don't know that I follow this point. Surely if air gets past the drywall at the penetrations you describe can't it work its way along floor joists to exterior walls and then up exterior walls and leak out the home? Granted I have my sheathing sealed but if my drywall was my only air barrier then the air has escaped my air control layer.
Why are all the electrical junction boxes to be sealed in walls if penetrations through the drywall don't need to be sealed? Those are all within the thermal envelope.
I'm confused.
Steve
If you do a good job of air sealing all of the exterior walls, and the attic floor, then air sealing of the interior walls doesn’t really matter. The reason is that even if those interior walls leak, there is still no way for the conditioned inside air to leak to the outdoors — it will be blocked by the air sealed exterior of the structure. This is the concept of the “thermal envelope”, at least as it applies to air sealing.
You just need to make sure you seal anything that leaves conditioned space and goes outdoors. If you have something that goes from one conditioned room to another through and interior wall, that penetration doesn’t need to be sealed. This is why you don’t need weather stripping on interior doors, for example.
Bill
>"By keeping everything within the conditioned areas, there is no need to be concerned with leaks around all of the wall/ceiling penetrations."
I don't know that I follow this point. Surely if air gets past the drywall at the penetrations you describe can't it work its way along floor joists to exterior walls and then up exterior walls and leak out the home?
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That would imply that the space on the other side of those partition walls aren't conditioned space, whereas you opened with "My mechanical room is fully contained within the thermal envelope." Both can't be true at the same time.
So which is it- the mechanical room is either inside the pressure and thermal boundary house or it isn't. Treating the partition wall as if it were the pressure boundary of the house and running ducts through it isn't going to change that. The exterior assemblies of the house are where the air sealing efforts are going to make the difference, not the partition walls.
My mechanical room is definitely 100% inside the thermal envelope. The pressure envelope I'm not sure about. Ignoring my primary air barrier (the exterior sheathing) and assuming my air barrier is 100% from the drywall then I think my mechanical room is outside my air barrier.
In order for it to be 100% inside my air barrier it needs to have 4 walls and ceiling covered with mudded and taped drywall, the drywall needs to be sealed to the floor and any penetrations need to be sealed to the drywall. That's the only way I see my mechanical room being inside the air barrier which is the drywall?
Said another way, looking at two rooms adjacent to each other 100% inside the thermal envelope. If the drywall is the air barrier in both rooms then any hole in either room's drywall on the adjoining wall is a hole in the air barrier (drywall). Then air can travel up the stud cavity, along the upper floor, floor joists out through the sheathing.
It was my understanding that the drywall needs to be air tight even on interior adjoining rooms. For instance you still have to seal light switch plates on a room that is in the middle of your house surrounded on all sides by other conditioned space?
Still confused.
Steve
Steve,
You should be able to draw the air-barrier on a section of y0ur house. If it is continuous there will be no secret paths for air to make it's way outside the conditioned area. If the drywall is the main air-barrier then there needs to be some care taken to seal the top plates of interior walls, but no reason to have those walls themselves air-sealed from adjacent rooms.
Malcolm,
When you said "If the drywall is the main air-barrier then there needs to be some care taken to seal the top plates of interior walls"
Did you mean sealing the top plate of external walls? Why would the top plate on internal walls matter if the sealing between internal rooms doesn't matter.
Steve
Top plates are the air pathway into the voids between joists. If the drywall is the primary air barrier and you’re not counting on the exterior sheathing to do the job, then you need to seal the top plates of interior walls to maintain your air barrier. Usually this means canned foam around wires and pipes that penetrate top plates, and acoustical sealant between the top plate and the drywall. If the top plates leak, air entering around electrical boxes and the like can get into the joist space and then leak out from there.
It’s best to seal the exterior sheathing since there are less potential leak spots there.
Bill
Sealing of the internal walls doesn't matter specifically because the top plates on the internal walls are sealed. If the top plates are not sealed, then airtight interior drywall matters. It is much easier to seal the top plates.
See the partition detail here: https://www.finehomebuilding.com/2014/03/05/a-practical-air-sealing-sequence
Steve,
You are correct that conditioned air can escape the conditional space and find its way into the attic through interior partition walls. That's why many builders who are using the ceiling of the house's top floor as the air barrier, install continuous drywall before framing the partition walls. Another approach is to air seal penetrations through partition wall top plates into the attic, and seal along the top plates themselves from the attic with caulk and/or canned spray foam before insulating the attic floor.