Air Sealing Home with No Sheathing


Old 01-19-17, 11:34 PM
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Air Sealing Home with No Sheathing

Hi, I mentioned at week or so ago I was insulating my rim joists and discovered there are no rim joists, just the clapboards nailed directly on studs. Well, going into the attic it became clear to me that this obviously continues all the way up the wall, so no rim joist in the basement and no sheathing on the wall.

I mentioned that when doing the "rim joists" I was not going to seal at least the horizontal portions of the cavity before installing the rigid foam board. I'm keeping the rigid foam about half an inch from the clapboards to allow some drying airspace. I'm assuming most agreed at least not sealing the clapboards to the bottom plate of the wall and sill was a good idea. I'm also working in the attic to air seal.

Aside from the normal things to seal in the attic, I was looking at the exterior top plates and wondering whether or not I should be sealing the outside side of the exterior top plate (i.e. between the exterior top plate and the clapboards) or just the inside side. I was going to either spray those seams with foam or fill in the gaps with caulk. Concern is that I'm cutting off some mechanism by which those wall beneath are drying.

The walls very likely have no insulation and certainly no air or vapor barriers. The house was built in '33. When the siding is eventually replaced in the next decade, the plan is to do a thorough rehab of the walls with exterior foam, possibly sheathing them, etc., so I prefer to not mess with the walls until that time since, IMO, what I do now may either do more harm than good or may, at minimum, limit my options in the future. Therefore, at this time, I'm just sticking with air sealing and trying to do no harm. So, if the walls have no sheathing and no insulation, would your suggestion be to seal the exterior top plate on both sides, one side, or not at all?

Thanks for your suggestions.
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Old 01-20-17, 04:14 AM
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Attic space is supposed to replicate the exterior atmosphere as much as possible. Insulating the ceiling joists below is the best option, and sealing around light fixture holes, etc. helps keep the migration of the house heat down. The attic should not be sealed from the outside air. That air should flow freely from the soffits to the ridge vent (optimal) So I am not sure what you are sealing in the attic. Just don't stop air flow.

Anything you apply to the exterior of the walls will cause problems. Firstly the insulative value is minimal. Secondly you will change the profile of the house and your siding will protrude past all your window and door trim. Insulating the wall cavity is the best option. Either blown in insulation or removing the interior walls and installing a good insulation like Roxul or others will help better.
Old 01-20-17, 11:33 AM
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I wasn't going to seal the sill vents or anything to outside. I was just talking about sealing the top plates of the exterior walls that you can see in the attic and whether that's advisable if the exterior walls are not insulated and not in any way sheathed. The exterior walls are just clapboards nailed directly to the studs, then covered with a few layers of cedar shakes, then covered with vinyl siding. Concern is that by closing off all upward airflow in those exterior walls into the attic I'm somehow harming the interior of those exterior walls. Is this something I should worry about or just go ahead and do it?

I attached a picture to show you what I'm talking about. My house is of course finished, so assume that there's plaster in place, that the exterior clapboards extend above the top plate to the roof (at least on gable ends) and that you're looking at it from the attic. I plan to caulk or otherwise seal everything with black arrows from the attic, but I'm not sure about whether to seal the red arrow side of the top plate to the clapboards on the exterior walls. Name:  exterior-interior_wall_connection_w_single_top_plate.jpg
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Last edited by Towsonite; 01-20-17 at 01:10 PM.
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