Air seal large gap around light fixture box
runner9
| Posted in Energy Efficiency and Durability on
I’ve made it to nearly my last electrical box in the attic that holds a light fixture. I’ve used fire caulk for all of them, but this last one has a big gap on one side, probably a half inch wide.
Caulk won’t work, it just falls in. I’m thinking either fiberglass mesh and duct mastic or UL 181 foil tape.
I have both of the above, I could buy Great Stuff Fireblock foam but since that still ignites at 240 degrees I doubt that’s the best idea.
Suggestions?
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Replies
Jeremy,
I'm not aware of requirements that an air sealing product around a ceiling electrical box needs to comply with fire-blocking standards. Now that I've written this, a few GBA readers (or you) will probably point out my ignorance, citing code references to support their position.
That said, most weatherization contractors or homeowners would seal this type of air leak with whatever product is handy: for example, canned spray foam or (better) a small piece of high-quality European air sealing tape.
Thanks Martin. I trust your judgement/guidance.
Last night while putting duct mastic on the bath fan vent duct seams I put a gob on the spot next to the light box.
From the current Consumer Safety Sheet for all Great Stuff products"
"6. Cured foam is combustible and will burn if exposed to temperatures above 240°F (116°C). Do not apply GREAT STUFF™ foam around heaters, high heat lamps or recessed lighting fixtures, radiators, furnaces or fireplaces where it could contact heat-conducting surfaces. Do not use GREAT STUFF™ foam inside electrical boxes or panels (applications around the boxes are permitted)."
Sounds like as long as none of the can-foam ends up inside the electrical box, Dow doesn't think it's a problem, unless it's not on a lighting fixture that can get hot.
Don't use housewrap tape or similar on a lighting fixture box without checking the service temp & ignition temp ranges of both the adhesive & backing. There are foil-only tapes with adhesives rated for use on flue pipes that are good to 600F which would be just fine in this application, eg:
http://solutions.3m.com/wps/portal/3M/en_US/NAHomeEnergy/Home/Products/~/3M-High-Temperature-Flue-Tape?N=7579606+3294348401+3294529207&rt=rud
(Some hardware stores carry it, but you can also buy it online.)