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Better to compress attic blow-in fiberglass, or remove it?

cconti | Posted in General Questions on

I’m going to be laying a subfloor in my attic, where fiberglass has previously been blown in between the joists. 

 The blow-in insulation is currently thicker (deeper) than the joists, and rises several inches above the top surface of the joists. To lay the subfloor, I’m going to need to either compress the fiberglass between the joists, or remove the amount that extends above the top of the joist. 

 I know compressing fiberglass decreases its effective R value, so I’m wondering whether it’s better to compress what’s there, or remove it and have less fiberglass, that is not compressed! 

 Any advice would be appreciated!

(Edit: I know that either of these options, compressing the insulation or removing it, will decrease the R value and are therefore bad from a thermal performance perspective. I plan to add mineral wool batts between the rafter and gable end stud cavities to remedy this.)

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  1. GBA Editor
    Martin Holladay | | #1

    Compressing fiberglass insulation decreases the R-value of the insulation layer, but increases the R-value per inch. So clearly, of the options you list, compressing the insulation is the best approach.

    That said, here is an even better suggestion: Add 2x4 or 2x6 framing, perpendicular to the existing joists, to deepen the framing cavity, before you install the subfloor.

    One final point: Don't make plans to use the attic for storage unless you first assess the structural capacity of the joists.

  2. Expert Member
    Dana Dorsett | | #2

    Adding batts to the gable ends of a vented attic that has insulation at the floor does almost nothing for the thermal performance of the building.

    Most blown fiberglass isn't very air retardent and loses performance to convection during the heating season. Install a perpendicular set of framing lumber as Martin described, choosing the size so that the total depth would meet or exceed the IRC prescriptive R-value for your location. Blow cellulose up to the top of the new framing, which will restore the full R-value of the fiberglass due to the better air retardency of the cellulose.

    For R38 (climate zones 2 & 3) that would be about 10.5" total depth, so if the joists are 2x8 you can get there with 2x4s perpendicular to the joists. For R49 (climate zones 4 & higher) it takes about 13.5", so it would need another set of 2x8s. If the joists are 2x10s or 2x12s it would take 2x6.

  3. Jon_R | | #3

    Even if you block attic vents, a semi-conditioned attic (some insulation on the floor, some above) is not a good idea.

    Be sure to air seal.

  4. cconti | | #4


    Many thanks for the replies.
    The idea was to keep open the possibility of fully finishing the attic at some point in the future… which would of course require insulating the rafter cavities (likely keeping the roof vented with rafter cavity vent inserts behind the batts?).

    Because of that, adding framing to the floor isn’t really an option, because of the 3-1/2” of ceiling height lost.
    (Also, the insulation in the rafter cavities seems like it would make the additional insulation in the floor redundant?)

    Many thanks for all the advice.

  5. Expert Member
    Dana Dorsett | | #5

    How deep are the rafters, and which climate zone?

  6. walta100 | | #6

    What is the purpose of the sub floor? If it is cold storage just compress the insulation, the difference will not be noticeable.

    If your new subfloor is part of a plan to make the attic into living space. Then there are lots of questions that need answers about permits, structure, air barrier, insulation and roof venting.

    Most of the time the existing ceiling joists will be too small to properly support the live load of the floor above.

    Without a window large enough to exit thru you will not be able to get a permit. Even if no permit is required it is a bad idea to build a death trap of a room with only one possible exit.

    Will the new be tall enough to meet the code requirements?

    How will you keep the warm air under the new floor from escaping into your vented attic?

    Do you have the room vent your roof and insulate properly?


    1. cconti | | #8

      “Even if no permit is required it is a bad idea to build a death trap of a room with only one possible exit.”

      I don’t understand this. Every bedroom I’ve ever slept in has only had one exit (and most, while they’ve had windows large enough to exit from, have been many feet above the outside grade (think second, third stories, etc.) such that without a ladder of some sort exiting out them would be just as deadly as a house fire.

      1. brendanalbano | | #9

        We are perhaps veering a little off topic here, but egress windows aren't just to allow occupants to escape (which as you've pointed out might not be viable without a ladder), they are also there to allow a firefighter to access the space to rescue people inside, and the firefighters have ladders.

        Back more directly to the question of what you should do about your insulation, I agree with Walt's advice to assess the viability of a future attic conversion to living space before proceeding. If you know you aren't going to convert the attic in the future, that leads you one direction (compressing the insulation is fine, maybe you don't care so much about head height and can add some more thickness to improve the insulation, etc), and if you know that an attic conversion is viable and that you are likely to want to pursue that in the future, you can plan accordingly.

  7. cconti | | #7

    We’re in Zone 5.
    And the existing joists are only 2x8.

    The existing blown-in fiberglass is *at most* an average of about 12-14”, perhaps 15” in certain spots where it is mounded. The house (built 2001) is definitely insufficiently insulated (that’s one of the reasons I’d like to air seal and insulate the attic space (probably having to keep the roof deck itself vented for moisture reasons, given that I don’t know what kind of membrane is stuck to the top surface of the roof deck?).

    (I don’t know the span of the 16”o.c. 2x8 joists, because there is an existing section of subfloor in the center of the attic, so I can’t tell if there is a beam there or not.)

  8. Expert Member
    Dana Dorsett | | #10

    If the joists are 2x8 it's unlikely that the rafters are any deeper than 2x8. Compressing the fiberglass to 7.25" will result in approximately R28-R30 on the floor.

    With only 7.25" of rafter depth you can install 2" of HFO blown closed cell foam (R14) and 5.25" of compressed R23 rock wool (R22 at 5.25") The total center-cavity R would be R36-ish, with 39% of the total R outside the first condensing surface. That isn't quite up to the IRC prescriptive ratio, but close, and good enough if you're not dead on the cold edge near zone 6. If you ARE on the cool edge painting the ceiling gypsum with vapor barrier latex would limit moisture accumulation in the batts.

    The floor insulation can simply be left in place if you're now heating the attic. When it's time to re-roof adding 2" of roofing polyiso or a 2.5" polyiso nailbase panel would be sufficient to beat code minimum performance on a U-factor basis, due to the R11+ thermal break over the rafters.

    It doesn't matter what membrane is above the roof deck- roof decks don't really dry toward the exterior anyway (especially when covered in snow or wetted with rain or dew, which is a large percentage of the time).

    If you go with soffit to ridge venting you'll only have 6.25" of space, and even 6" of HFO blown foam won't get you to code min on either an R-value or U-factor basis (but it WOULD empty your wallet.) At 2" the foam is still about a half-perm or so, a class-II vapor retarder with is a sufficient drying path for the deck, but slow enough to be protective.

    If you skip the venting and install 7" of HFO blown foam it's almost a true vapor barrier, but would make code on an R-value basis, but would underperform due to the R8-ish thermal bridging through the rafters.

  9. walta100 | | #11

    You need to consult an engineer!

    But a quick look at the tables tells us without a bearing wall in the center your 2x8 16” apart are limited to 11 foot 1 inch of span.

    It is a wild guess but my bet is the 2 rooms below are each 12 foot wide.

    Building a room with only 1 possible exit is a bad idea. If my choice is stay and burn to death or jump out of a second story window. I for one will be jumping, yes I will likely me injured but I will survive.

    You are not adding value to your home making changes that do not comply with the bare minimum
    requirements of the code.


  10. cconti | | #12

    Hi all,

    As is so common, what started out as a simple question appears to be ballooning into a much more complicated one.

    My wife’s and my new house (built, it seems, pretty cheaply in 2001) is 29 feet front to back. The second floor ceiling/attic floor framing is 16”oc 2x8 joists running *apparently unsupported* across that entire span (unless there is a hidden beam flush with joists, which I doubt given the relatively cheap construction of the rest of the house... which I’d have to pull up existing attic subfloor to find out).
    The attic rafters are also 2x8 16”oc.

    Given this 29’ span of 16”oc 2x8 joists, which is almost 3 times the allowable 11’1” span for living space Walt pointed out, it seems clear that serious additional framing would be necessary to turn that space into living space.

    How would that framing best be done? With a 29 foot unsupported span, it seems even huge, 16” or 18” manufactured I-joists sistered to the existing ones would be insufficient. If one was to install a beam, how would that be done?

    One way or the other, it doesn’t appear that converting that space to living space is going to happen anytime soon. So that leaves the space as cold storage. Given that framing though, is it even advisable to use that space for light cold storage? Is there a data table that exists somewhere detailing how much weight such a span could support with acceptable deflection?

    Many thanks for the advice, which has now strayed so far from the original question...

  11. Expert Member
    Peter Engle | | #13

    I suspect that there is some sort of intermediate support, if only because you can't buy 29' long 2x8 lumber. Even with a midspan support, the 2x8 rafters are barely sufficient to support the weight of the ceiling and insulation. They are not sufficient to support any additional weight including subfloor and even light storage.

  12. cconti | | #14

    Given the insufficient strength of the existing framing, how should I go about reframing? Sistering? Or a second layer of framing, perpendicular to the first, as Martin described at the top?

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