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Blown-in cellulose in a finished garage ceiling (conditioned space above)

user-1107451 | Posted in Energy Efficiency and Durability on

Hi all,
Been putting this off til warmer weather and nows the time I push the button or not
Ok I have a 4 yr old 2 story colonial in NY (20 miles N of NYC) so the usual questions we see on the forum, why is the room above the garage so cold.
So Ive cut some holes and found the builder had put F/G Kraft paper batts in R30
What I have found is they werent attached to the floor above so now when I look in the hole I can see all the way down the floor joists (wooden I beam type), the batts are sitting on the ceiling of the garage so even with them being R30 there is a 5 ” gap between the batt and celing below it
So reading the forums here and elsewhere, I dont want to pull the ceiling (if I can help it) and the cost of foam is putting me off doing it that way, now my idea is this (flame away people, be gentle with me Im a newbie).

The idea I had was to use a 4″ hole bit and drill every 4 ft (length of each batt) reach in push the batts hard up against the floor (dont care about compressing it) as I would then reseal each hole and leave the end open and pump blown in cellulose to fill the void so I have 12″ of insulation and a V/B (Kraft on the batts hard up against the floor.

Now I have air sealed around the edges of the roof but the air gap in the floor is whats killing me.

Now I’m also going to do the garage walls with blown in as well , builder missed that, not sure if was cutting corners or just not a Green Building Advisor reader 😉
I’m trying to make the house as tight as I can without having destroy the house.

I’ve already sealed the top plate and cut and fitted foam insulation in the sill plate in the basement so the garage is the next thing.
So people let me know if my idea is valid or Ive missed something I welcome ideas

Any advise would be appreciated as I look at my damn utility bill ever month and shake my head and swear at the thieves

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  1. jklingel | | #1

    Can you access from above via 3" holes and blow in cellulose? I would think you could push the blower hose in several feet on either side of the holes, so not that many holes would be needed per bay. If you've air sealed well now, I don't see the space above the fg batts as being horrible, as it should be rather dead air. If the batts are poorly fit, then cellulose will sure help. Have you seen improvement since you air sealed? Just thoughts from a diy guy.

  2. GBA Editor
    Martin Holladay | | #2

    In a situation like this, it's very important to air-seal the rim joist areas, and that can be hard to do unless you cut holes in the ceiling at the perimeter of the garage for access.

    Once you're sure that the perimeter of the floor assembly is air-sealed, I agree with John that it will probably be easier to blow insulation on top of the batts, not under them. Don't worry about the kraft paper -- it doesn't matter where it ends up. Your OSB or plywood subfloor above is a perfectly adequate vapor retarder.

    Once you have done all of the above, you could install a continuous layer of rigid foam on your garage ceiling, followed by new drywall, if you really want to do it right.

    Here's a Fine Homebuilding article with more information: How to Insulate a Cold Floor.

    You might also find this article helpful: How to Install Cellulose Insulation.

  3. user-1107451 | | #3

    Yes Ive used a C.Saw set for 5/8and cut a perimeter large enough to fit my head in and put rigid foam and spray foam the edges to air seal it, but the floor is still cold after I have taped and mudded the perimeter back up, so thats why I wanted to see if the cellulose would work .

    The most important thing you have said is dont worry about the Kraft paper, so that brings me to a question that could save me a whole heap of time doing the blow in.
    If I dont need to worry about the Kraft paper not being against the sub floor, I can just shove the blown in cellulose on the top of the batts and dense pack it that way (rather than my idea of drilling holes and pressing the batts against the floor.
    Now my question, Ive read in so many places the Kraft paper has to be either up or down but not in the middle as it will lead to moisture issues.
    I assumed this would be an issues in walls or roofs but from what you are saying because its not in a place that will be an issue from warm air/moisture Im safe.
    If that is true I can just pump on top of the batt and be done with it.

    I was also looking at the foam and dry wall if the cellulose doesnt work but was trying to avoid having to get drywall up 12 ft in the air and only myself to do it, Im waiting for my brother to come over from Australia on holiday to help me with the cellulose.

    I appreciate your replys and the article link, Id prefer to research and do it right the first time (even if I dont have the budget to spray foam as If I did Id get the ceiling dropped CSF put in and re drywalled but dont have the $$ for that unfortately)



  4. GBA Editor
    Martin Holladay | | #4

    1. The OSB or plywood subfloor is your vapor retarder.

    2. The kraft paper isn't a vapor barrier, only a vapor retarder, and its presence in the middle of the floor assembly does no harm.

    3. If you do your job well, there won't be much air or moisture moving through the floor assembly, and I can't imagine any problems that would arise from vapor diffusion through your floor assembly.

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