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I purchased an 55 year old two story home. Upstairs there is a knee wall attic space. There is air coming into the joist area and out of the pot lights downstairs. I can feel the cold breeze with my hand and i can see the pot lights when i look between the joists in the knee wall attic area.

What is is the best way to block the airflow?

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I think the proper solution to this is to use an air-tight recessed can and trim; this would however require replacing the fixture. The complexity of this depends on how accessible it is from above (I don't know of any air-tight rework fixtures). You cannot safely add insulation around the light due to the heat build up.

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Building Science has a Attic Air Sealing Guide that covers sealing recessed lighting as well as sealing knee-walls.

If your can lights are in the conditioned space (i.e., the floor/ceiling between your first and second floor rather than in the attic itself) then sealing the joist cavities in the attic space should be sufficient. No need to do anything with the cans.

The relevant information starts on page 32 of the PDF. The two options given are to fit blocks of rigid foam or solid wood into the joist cavities (from the attic side) and seal the edges with caulk or other sealant, or to stuff rolls of fiberglass into the cavities as backing for a layer of spray foam insulation. In both cases you follow this with insulating sheathing to cover the attic side of the kneewall, and additional insulation between and on top of the joists in the attic.

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