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Community and Q&A

Boxing in recessed lights

johndoe2015 | Posted in Energy Efficiency and Durability on

Hello. I have 8x airtite/IC rated recessed lights in my second floor. I have read some posts about boxing them in being an effective/safe way to airseal the leaky “airtite” cans. I would like to use 1″ polyiso foam and make a box 3″ away from the cans and spray foam/tape the joints. I would like to stuff rockwool inside of the box between the foam and cans and also outside/on top of the foam box. Is this plan ok? Thanks.

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  1. Expert Member
    BILL WICHERS | | #1

    I wouldn't put any rockwool inside the box. Even though IC cans are rated for that ("IC" stands for "insulation contact"), the rockwool won't really add much, and it will make the installation a lot more difficult. You can build the box with 1" polyiso, but don't use canned foam to glue it together, use something like Loctite's PL300 foamboard glue instead. Canned foam will try to push all the joints together as it expands, and will make a mess of your box.

    There is no problem putting rockwool -- or any other kind of insulation -- over the top of the box on the outside. I would seal the box to the ceiling though using the same glue you use to build the box. Remember to leave a notch somewhere for the electrical wires to get into the box, and put a squirt of canned foam in that notch as a final step to air seal the hole.


    1. johndoe2015 | | #2

      Thanks. I was not planning to spray foam the box together, I was going to use mastic aluminum tape but I will check out the PL300.

      Can the foam board be placed right next to the can's junction box if there is 3" of space between the foam board and can itself? Or do we have to have 3" gap between the foam board and the junction box/hangers?

    2. johndoe2015 | | #3

      Since you suggest not to stuff rockwool between the foam box and can, would it be fine to use 2" foam board for the box for added R-value?

    3. johndoe2015 | | #11

      I cut my foam boards and assembled one of the boxes. One side of the can only has 2" of clearance from the foam. Is that going to be a problem?

      1. Expert Member
        BILL WICHERS | | #12

        I don't see that 2" clearance being a problem, especially if you are using LED lights in the cans. It's even less of an issue if the "foam board" you're using is polyiso.


        1. Deleted | | #13


        2. Deleted | | #14


  2. Expert Member
    BILL WICHERS | | #4

    There is no problem going up to 2" foam board, but it sounded from your OP like you'll have insulation on the exterior side of the box too, so I'd just use that. As an example, a typical application would have a box over a recessed can for air sealing purposes in an attic floor, then loose fill blown in over the top of the box. The box doesn't need to provide much R value, since the loose fill insulation on top of the box is doing all the "work" of giving you the R value you want. You could put extra piles of insulation on top of the box, but in practice the relatively small area of the box means that the extra thermal losses from a slightly reduced amount of loose fill insulating material directly over the box won't have much effect on the overall thermal performance of the attic insulation. All that means is if you take R28 or worth or loose fill cellulose (which is around 8" or so), and put in about R6 for 1" polyiso, you are taking about R22 out of an R49 layer in the attic. Even if you do that over 1 square foot, the extra heat loss in that small area isn't going to amount to much when considered over the entire area of the attic floor.

    That's all a fancy way to say that going from 1" to 2" polyiso for the box doesn't hurt anything, but it doesn't make a huge difference in thermal performance either, if you will have some other insulating layer on top of the box anyway. In my earlier example, 2" polyiso would be R13, so you'd from from R22 to R15 removed from R49 worth of insulation, which is only a difference of a little less than 9%. If you then work this out relative to the rest of an attic floor, with 6 recessed lights of 1 square foot each, and a 500 square foot floor plate, that means you have less than 9% worse performance over only 1.2% of the floor area, which gives you an average of R48.82 over the entire floor, or about 0.37% difference in overall performance from a perfect R49 fill. While it's true that I just did a simple average here, and that doesn't give perfect results in these thermal loss situations, it will be very close in this case.


  3. andy_ | | #5

    The last time I did this I got so annoyed trying to get foam strips cut to make the box that I just pulled some scrap OSB and ripped it to width on the table saw and had a dozen boxes tacked together in a few minutes. caulked the joints and done. R value difference? Negligible. PITA factor? much reduced.
    Like Bill pointed out, the real insulation value is from on top of the boxes so don't sweat the inside too much. Besides, it's a lot easier to make or fix connections inside those boxes without a bunch of insulation in the way. Ever pull a can light from a finished ceiling with loose insulation above? It can get real messy.

  4. the74impala | | #6

    Or you could replace them with puck lights and make a much shorter box to go over them.

  5. EPMiller | | #7

    No matter what you do, and I second the OSB boxes, make certain you have enough slack Romex inside the box to work with when the cans have to be replaced.

    1. johndoe2015 | | #8


      They are new work boxes. Wouldn't I need to replace them from the attic? Also do cans go bad that often?

      1. EPMiller | | #9

        Cans may last a long time but think about the shift from cans to LED wafers, technology or style may bring something on that would require change. It appears that this is being done to make the cans actually airtight which infers the cable being sealed tight. Repairing cans is a pain in it's own right, (bad socket, a wire nut that wasn't installed quite right are two I can think of quickly) a too short wiring pigtail will bring curses down on you and it's so easy to do correctly the first time.

      2. Expert Member
        BILL WICHERS | | #10

        You can usually cut out a box from below if you're careful. When I have to do it, I try to lever/pry the old box out as much as I can, but sometimes I use a sawzall. Just be careful not to damage the wire, and remember that if you have to abandon the old box above the ceiling that's OK as long as you remove the wire from it and put the wire into a new box. You cannot leave wire terminating in a box that is inaccessible.


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