Drawbacks to air-sealing gable walls?
Am about to start air-sealing the attic in my 1950s brick home. There are two large gable walls, and in the attic, I can see down into the framing about two feet, until it reaches some blocking. There’s about an inch of open space between the temlok sheathing attached to the framing and the inner face of the brick exterior. I can feel quite a lot of air rushing up into the attic at every stud bay.
My neighbor, a retired mason, insists I should not block the movement of air into the attic; he says that’s what been keeping the brick exterior so dry, despite it being fully exposed to rain for the past 65 years.
The air-sealing article by Lstiburek though states pretty plainly that those spaces should be air-sealed.
My question: How do we know that blocking that air movement will not leave the bricks wetter and prone to freeze-thaw damage, as my neighbor thinks?
Thanks
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Replies
Dave,
If I understand you correctly, there are several layers to your wall assembly.
From the outside in:
Brick veneer
Sheathing (I don't know what "temlok sheathing" is -- did you mean to write "hemlock"?)
Studs
Empty stud bays (without any insulation)
Interior finish (probably either lath-and-plaster or drywall -- possibly both)
You definitely want to air seal the top of the stud cavities to prevent air movement through those stud cavities. Even better, you want to insulate those stud cavities as well as air seal the top of the stud cavities.
When it comes to the air gap between the brick veneer and the sheathing, however, your mason is right. That air gap should be left open at the top to encourage ventilation air flow.
Martin--thanks for your reply. I guess this is an advantage of doing things myself, because I'd think at least some insulation companies might have just stuffed batts in there and foamed them, or filled those areas with cellulose.
Temlock was a state-of-the-art sheathing used in the late 40's. I know what it is because it has "Temlock" stamped all over it.
Thanks again.
Dave,
Here in Vermont, we sometimes use hemlock sheathing... but never (to the best of my knowledge) Temlok sheathing.
Looks like the correct spelling is "Temlok." Apparently, it was a brand of fiberboard manufactured by Armstrong.
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Here's another photo.
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Ideally the brick veneer cavity would vent to the outdoors. As currently configured it only gets to the outdoors via the gable vents, after traversing the attic(?). If you block the gable vents, you impede that convective flow.
Cutting/drilling in vent holes in the mortar lines on any of the top three coursed of brick will allow air to convect from the weep holes from the bottom to the exterior of the house without needing to pass through the attic, at which point it doesn't much matter if the top of the cavity is sealed off or open.
Freeze/thaw spalling in places warmer than US climate zone 6 are fairly rare even with unvented brick veneer cavites, but the humidity levels in the wooden part of the wall assembly rises. Asphalted fiberboard like Temlock is fairly immune to moisture- you can even leave it out in the rain during construction. But it's very vapor permeable. In warm humid climates if you air-condition the house to a cool temperature you can end up with high enough humidity in the plaster or wallboard to cause rot & mold problems ESPECIALLY if you don't ventilate the cavity. The brick will hold on to quite a bit of moisture if you don't vent it, and when the sun hits it the moisture drives are pretty extreme. Even if the outdoor dew points are in the 50s F, the dew point of the air in the cavity can approach the temperature of the sun-baked brick, which is higher than the interior wall temperature.
So, even if the spalling risk is low, you still want to vent the brick veneer cavity, with weep holes near the bottom every 18-24", with corresponding vents at the top. That way when the brick warms to above the ambient air temperature convection will purge the super-saturated humid air out of the cavity as the brick sun-dries.
Asphalted fiberboard sheathing adds about R2 to the whole-wall R value, and was very popular during the 1950s, particularly under brick veneers due to it's moisture resilience. But when central air conditioning was retrofitted into those houses sometimes mold-bloom nightmares erupted inside the walls. By venting the brick both top & bottom you can avoid starring in that 1950s grade-B movie.
Where are you located?
Wow--that first pic of temlok is exactly what is on my house. I've had to make some holes in recent years through the walls, and parts of that logo were visible. Plus, there was a stable built here the same time as the house, and they used a slew of those temlok panels as a ceiling. It has held up fine after 65 years.
Dana--you are right that the interior of the bricks vents into the attic and out only through three large gable vents. The house has never had soffitt vents or a ridge vent. It doesn't seem to have been a problem. The roof is planked and covered with 60 pound felt, then clay tile. The roof is original and in great shape. In fact, I just had a roofer here who does mainly slate/tile, and he was amazed at how good the roof looked. I asked him whether to consider soffitts and a ridge, he sort of laughed and said "After 65 years with no problem, why?"
The only issue I see at present though is that there are no weep holes, at least not any above grade. But I can feel draft at the top of the walls in the attic, coming up inside the brick.
I am in Pittsburgh PA.
The house had AC added in the 1970s but when I bought the house it was essentially useless, so I removed it. The house was built with a house fan in the ceiling of the hallway, and that does a great job of cooling it off once the sun is down. I also added solar film to all the windows, which helped a lot too.
All advice appreciated.
Dave,
There are a few issues here. One issue is that it's impossible to change the details inside the ventilation gap between the brick veneer and the sheathing. Ideally, you would have an air barrier between the sheathing and the air space -- these days, that would be taped housewrap or taped rigid foam. Your wall doesn't have that. At best, it's got a layer of asphalt felt.
If air is rising in this ventilation gap, it's possible that the makeup air entering the bottom of the gap is indoor air leaking through the cracks between your sheathing boards. That's not ideal.
Many brick veneer jobs lack adequate air outlets at the top of the wall, and most of these installations still work OK. Now that you've provided more details about your house, I might be inclined to modify my advice.
What you end up doing depends in part on how much time and energy you have. It certainly wouldn't hurt to drill through the mortar at the bottom of the wall to create some weep holes that double as air inlets, and it certainly wouldn't hurt to drill similar holes near the top of the wall. If you are willing to do that, then you can seal the gap you see in your attic.
It's probable that your brick veneer will be fine if you seal the gap at the top, even if you aren't able to create the weep holes and outlets. (The fact that you can feel air rising from the top of the gap is a sign that you have a good gap -- one that isn't clogged by mortar droppings -- and that's a good thing, because such a wall is more robust than one with a gap clogged by mortar droppings.) One advantage of sealing the gap at the top is that you will probably be reducing the rate of air leakage through your insulated wall.
Martin--thanks again. The air gap beneath brick veneer is only visible from the attic along the gable walls. The non-gable walls, which have eaves, don't have any opening visible behind the brick veneer. I assume this is because the brick on the non-gable walls is protected by the eaves from rainwater, whereas the gable walls are fully exposed to rain.
Since there are no visible weep holes, I agree that the draft I feel might be comprised of a lot of interior air, though the draft feels too strong to be from just tiny cracks inside the living space. The area at the top of the bricks in the attic is full of cobwebs, and you can see them being moved pretty strongly by the draft coming up.
Either way, if the incoming air is coming somehow through the veneer from the outside, it is serving the purpose of helping keep the bricks dry, and so I shouldn't pursue it, correct? Whereas any air getting through the interior walls and through the sheathing is a problem. Also, all the air-sealing I'm about to do at the attic floor isn't going to help this particular problem. I didn't mention that the exterior walls had cellulose blown into them shortly before I bought the house. How good a job they did, I don't know. I do know it was professionally done, since I have a copy of the receipt. But it probably wasn't dense-pack, right? So likely isn't stopping any significant amount of air?
The only sources or air leakage into the exterior walls then would be outlets, switches, heating vents, windows, and doors. So I guess I'd need to air-seal these as much as possible, in addition to the attic? Even on the non-gable walls?
BTW--the house is a ranch but has three stories: a walkout basement/garage, the main floor, and the "attic," which is really an unfinished second floor; it has a full staircase and 7.5 feet of overhead clearance, so it's easier than average to work in.
Yes, air seal the hell out of it, and when you can't find any more obvious leakage points, chase the rest down with a blower door.
Very often there are major air leaks into brick veneer cavities along band joists between floors, and thus not something you can see and point a foam-can at, but those leaks can usually be dramatically reduced by drilling a 2.5" hole in the ceiling with a hole saw, inserting a rough mesh feed bag into the joist bay, and dense-packing the bag with blown cellulose. (It's usually done in the ceiling rather than the floor for ease of repair.) The leaks into the joist bays have a wide range, but flue & plumbing chases that bottom to top of the house are prime suspects for large leaks. Those too can be chased down and mitigated with blower doors, but it's also true that anywhere such paths exist in a three-story there is a lot to be gained from sealing them at both the top and the bottom, which are often more accessible than intermediate floors.
Even low density cellulose is VERY air retardent compared to R13 batts, even more air retardent than R15 batts. Framing detials matter, but if the stud bays stop at the tops & bottoms of the joists, the band joists are often left un-insulated (and thus more air leaky). Uninsulated band joists are pretty easy to spot using infra-red imaging cameras, with or without the house being pressurized/depressurized with a blower door. With a brick veneer house it would be more obvious from interior IR imaging than outdoors, since both the air flow in the cavity and the thermal conductivity of the brick will smear the temperature profile, making it less obvious. You can sleuth it out yourself with a $50 box store pistol grip IR camera by looking for big temperature differences at the ceiling near the wall and the mid-height at the wall. The thermal bridging of the top plate framing would induce some temperature difference right at the crown molding, but if the band joist is insulated the ceiling near the wall should be warmer.
Thanks again. It all makes sense, though now I'm not certain about the original issue, namely covering the air passages from behind the brick, up into the attic. Regardless of the air source being from the interior or the exterior, it's a lot of air movement constantly up and over the sheathing, and with only cellulose in the stud bays and no thermal break, that air movement has to be a huge loss of heat in winter. The gable walls are in fact colder than the non-gable walls most of the time, but whether it is due to this issue or something else, I'm not sure.
The thought is that if the airflow were stopped, the brick would probably be okay, but the framing might not be?