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Community and Q&A

Air Sealing Attic in Older Home

DerekG | Posted in Energy Efficiency and Durability on


I have a new-to-me home built in 1900 located in southeast Michigan. As you’d expect for a house of that time, it’s balloon framed with plaster and lathe walls and ceilings. In the mid-90s, the house underwent some significant renovations and as a part of that, the plaster was covered over with drywall. The attic is vented with gable vents and has mushroom cap vents as well.

I’m now working in the attic to address some ice dam issues that came up over the winter (<1 inch of insulation in some spots) and have started to learn about air sealing. 

I’ve sealed up the ductwork in the attic, and am moving on to the electrical boxes, and bath fans, and can lights. I’ve also installed insulation baffles in each rafter bay (I have no soffit vents at this time, but I am planning on adding them this summer). The gable walls are blocked at the top with wood (Unknown if they are blocked at the basement). I am now getting ready to seal the top plates where they exist. Before I do, I have a few questions.

1. Should I be sealing the top plates considering I have drywall over plaster/lathe? I have heard that several quarts of water can diffuse through drywall and wonder if sealing the top plate would lead to excess moisture building up in the cavity. There are no bulk-water issues that I know of, and there is insulation in the exterior walls, as revealed by thermal imaging (unknown what kind).

2. I am concerned about using expanding foam to seal electrical boxes and wire penetrations in the ceiling. Would fire-rated caulk be an acceptable alternative?

3. I have a few interior walls that are balloon framed as well, with their stud bays open from the attic. My plan for this was to fit 1.5in of XPS foam, secured with adhesive and then seal around the rigid foam with expanding foam. Is this a sound approach?

4. I am planning on building a box to cover my bathroom exhaust fans out of rigid foam. Is that still best practice or is another material recommended?

Thank you all for your time, and the countless responses you’ve posted on here. I’ve learned a lot from GBA since discovering it a month ago.

All the best,

GBA Prime

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  1. Jon Harrod | | #1

    Hi Derek,

    It sounds like you are going about this in a thorough and thoughtful way. In response to your questions:

    1. Air sealing top plates will not lead to issues of moisture accumulation in the wall cavities under normal conditions. In a typical older house, a lot more moisture is transported by air leakage than by vapor diffusion. Sealing top plates is generally recommended to prevent attic moisture issues. That being said, if there is a wet basement or dirt-floor crawlspace that is adding a large moisture load to the house, be sure to address that.

    2. High-temperate caulk is an appropriate material for sealing electrical boxes. Some state programs require this instead of foam for this application.

    3. Yes, this is a widely used approach. I would recommend polyiso rather than XPS because is it compressible enough that you can cut it a bit large and get a good friction fit. XPS will snap if you put too much force on it. 1.5" is the best thickness, thick enough to have some strength but thin enough to cut accurately with a pocket knife.

    4. This is a great approach. Again, I like polyiso because it takes tape better than XPS does. You might want to pre-build the box using some expanding foam at the corners to give it a little extra strength. Since you will be insulating over it, scrap plywood or OSB are also options.

    1. DerekG | | #2


      Thanks for your reply! I wound up using the 1.5" XPS since it is what I had on hand, and went with the high-temperature caulk. I appreciate you taking the time to look over my question and reply back.

      Thank you again,

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