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Attic air sealing: exterior top plates are inaccessible

tylertervooren | Posted in General Questions on

I am diligently spray foaming every top plate, chase, gap, dropped soffit, penetration, etc. throughout the attic of our 1960s ranch before adding add’l insulation.

One problem I’ve run into is the exterior top plates. The low slope of the roof makes them all but inaccessible. Even with a long tube on the end of my spray foam gun, I don’t think I can adequately seal them.

To make matters worse, most of the area of the plates are covered by blocking for the rafters (100% hip roof). Even if I could get at the inside joint, there’s no way I could get at the outside joint between the plate and the sheathing. Fuggetaboutit!

Any pro tips to get at this joint? I have almost 230′ of exterior wall.

What about changing strategy and working to seal all wall penetrations (caulking holes inside electrical boxes and around box perimeters, pulling base trim and sealing between the wall and the floor, etc.)?

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  1. Expert Member
    BILL WICHERS | | #1

    Do you have a frieze board you can remove on the outside without too much trouble that would gain you access to the outer top plates?

    Sealing all the wall penetrations and the bottom of the drywall will help too if that’s all you can reach. You’ll want some non-intumescent silicone fire caulk to seal the electrical boxes from the inside.


    1. tylertervooren | | #2

      Thanks, Bill. That's a good idea. I could remove the soffit panels and probably get at them from the outside. I would wait until I'm ready to totally replace it all because it's very thin and the fascia board it interfaces with the fascia, so I think I would destroy it taking it down.

      Maybe this will be an "air sealing phase 1" sort of job.

      1. charlie_sullivan | | #3

        No harm in doing what you can with a long-nozzle spray foam gun for now, and then coming back and doing it better from the outside. And doing to outlets, etc. on the inside as well is also a good idea. I'm planning to replace the outlets in my 1970 house as they are rather worn out, and plan to air seal as I work through them. If yours haven't been replaced already that might make sense for you as well.

        1. Expert Member
          BILL WICHERS | | #5

          Charlie, when you replace all your receptacles, get spec grade (commerical) devices which are a step up from the cheapies. If you get the kind with a clamp on the back that is driven by the screw, you have the best out there, IMHO.

          Spec grade receptacles are far better quality and hold tighter, and stay tight longer. The screw-driven clamp for backwiring makes connections easy. I’ve replaced every receptacle in my house with these.


  2. AndyKosick | | #4

    There is a long barrel angled gun that may work for you, I have one and it's done things that nothing else would. That said, it's a $200 item. (I do this for a living) There's a couple places online that have them but you have to know to look.

    The other option is to get drywall knife (6 or 8 inch) on a long handle, there are also some scrapers that screw on an broom handle. Use it to smear fiber-reinforced duct mastic all over the inner plate to drywall connect. It's a real pain but it works. If there's blocking, I'd just plan on getting the outside connection from the outside someday when you're there anyway.

    1. tylertervooren | | #7

      Thanks, Andy. Will investigate both of these options.

  3. user-723121 | | #6

    Tyler; If you have soffits that would be the best access for sealing over top plates. You will also want to get air chutes in place from this vantage point. I have used rigid foam tightly fit over the top plate between the ceiling joists and extending in a bit over the drywall. With air chutes in each rafter space you can fill the void between the rigid foam and air chute for an airtight, high R assembly. Wall sheathing can extend up between the rafters for wind wash protection.

    1. tylertervooren | | #8

      Thanks. I think this is the route I'll end up needing to take. Will probably skip it for this project and just move "replace soffit boards" up on the priority list so I can get to it sooner.

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