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Community and Q&A

Blown-In Cellulose Over Non-Ducted Ceiling Cassette

PatriciaCranberry | Posted in GBA Pro Help on

My all borate cellulose is scheduled to be blown in this coming Friday, May 7th! Finally made it! “ALMOST” all air ceiling done along wall partitions, eaves, light boxes, etc. I’ve hesitated on the two SLZ ceiling cassettes and two bath fans (yes, ducted to outside).
1)Do I need to build a xps rigid box over these units and seal it to the attic floor?
2)Or can we just put non expanding sprayfoam at the ceiling/unit junction and let the cellulose do the rest?
3) I have some left over bats of mineral wool. Would I be wise to wrap around and on top of the the unit with some bats first, then let the cellulose fill the rest…then, if maintenance was needed, the cellulose could be moved aside/away and the bats removed? My first thought is it would be better to have the cellulose fill all the nooks and crannies around the devices rather than wrap bats around them.
4) Overall attic: I was planning on laying the left over bats of mineral wool (that I used for exterior/interior walls) on top of the blown cellulose in the attic. I should have returned it to the store for a credit but I was too tired before the drywallers came and closed up the ceiling so I just threw it up on the attic rafters…now it’s too big to keep packaged and remove thru the hatch door..  So I figure I will just unpackage and lay it over the cellulose (perpendicular to the ceiling joists). Other than weight, does this approach cause any concerns?
Would love some feedback/advice! Any tips for a first time green owner/builder approaching the finish line?

Thanks all!

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  1. PatriciaCranberry | | #1

    I read all the Related Questions--and this is what I gleaned: I should seal the unit at the attic floor and then use the roxul batts to insulate around and over the cassette...the batts would almost act like a dam to keep the cellulose from around the equipment (helpful for future access). Seems reasonable.

  2. PatriciaCranberry | | #2

    Am I talking to myself? Typical. As I read further I see Martin and Andy saying “ build an airtight box, seal all penetrations. Won’t my end dams between the joists and some Roxul batts around and over the equipment be okay ( as long as I air seal the rough opening to the attic floor)?

  3. Expert Member
    Akos | | #3

    The ceiling casette is something you might need to remove down the road to service, in which case you don't want a foot of cellulose to fall into the house.

    I would build a doghouse over it out of rigid foam, use canned foam to seal it against the ceiling, blow cellulose over the doghouse. Make sure to tape the seams of the doghouse, you want this to be as air tight as possible. Leave enough space around the unit that it can be removed if it ever needs to be serviced.

    Mineral wool by itself won't work as it is air and vapor permeable, you don't want attic air making it to the cold surface of the cassette in the summer time and condensing. Foil faced rigid foam is the best in this case.

  4. PatriciaCranberry | | #4

    Hey...thanks for reaching out...Tonight I measured it for a polyiso box with lid(nailed taped and foamed in place). Any reason I can’t fill the box with my left over batts? Would be easily removed if maintenance or service was needed.

  5. gozags | | #5

    Your insulation was blown in today? How did it go?

  6. PatriciaCranberry | | #6

    Sean: Way late seeing this....but it went fantastic! Coming up on my one year anniversary! All my prep work made the invoice much more agreeable...although I was physically exhausted! But I rest easy knowing I sealed it up!

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