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Community and Q&A

Insulating Attic Floor vs. Ceiling

rhl_ | Posted in General Questions on

Pick an arbitrary old house, is there a consensus that floor vs attic is better.

assume no mechanicals in the space, also assume the space itself is not useable as extra space.

Old house means the roof deck is not vented.

— some color, I think floor is better because heating and cooling extra space is wasteful, and therefore gives you lots of extra space for adding extra insulation.

what are the downsides?

One thought: if the attic temperature is hotter than the outside more often in the summer, it means you have a higher cooling load?

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  1. SierraWayfarer | | #1

    Climate zone 4a? The attic is uninsulated now?

  2. Expert Member
    NICK KEENAN | | #2

    Conditioning space is not what consumes energy, it's the surface area of that space that loses energy. All other things equal, the floor of the attic is going to have less area than the roof and sidewalls.

    Floors are also the easiest thing to insulate, you can just pour in loose insulation and spread it.

    1. SierraWayfarer | | #3

      DCC has that right.

      Are there ridge and or soffit vents? Will the existing ceiling support insulation? How tall are the eaves, how much insulation can be packed under them and still allow a ventilation channel to the ridge?

      1. rhl_ | | #5

        No vents. No eaves.

  3. Expert Member
    BILL WICHERS | | #4

    With no mechanicals in the attic, it's going to be cheaper/easier to insulate the floor, and as DC mentioned, less surface area = less area for energy transfer. The attic floor is easy to insulate with loose fill (blown) insulation, which tends to also be the cheapest insulation.

    I would do thorough air sealing job on the attic floor, then install at least code minimum levels of blown cellulose insulation. I like to install some of the cheap box store yard sticks around the attic prior to blowing insulation as reference sticks, and wrap some colored tape around the level you want for your insulation. This makes it easy to track progress during the job, and easy to see when you've reached your target depth too -- even from a distance.

    If the attic is hotter in the summer, it WILL increase your cooling load somewhat because it will make for a larger thermal differential across the insulated area. Adding ventilation to the attic will help with this.


    1. rhl_ | | #6

      This is the plan -- sorry this question was posed slightly more abstractly than my specific plan.

      My plan is (and we are mid execution), is to
      1. remove all the existing old insulation
      2. then air seal the floor area using blower door guided air sealing, cover the lights, etc.
      3. dense pack the floor area against intello,
      4. dense pack gable walls also with intello, overlapping the intello
      5. install loose fill insulation above to achieve > R-49, My spec was for R-100, (I know this is largely un-necessary, but I figure it cant hurt, and the labor/material cost for cellulose is quite minimal in the grand scheme of things).
      6. Small section on the middle of one part of the ceiling will still be walkable so we will use polyiso boards to get us up to R-49. (need about 4" of polyiso). Also figure the moisture risk is quite minimal with this since its not overall the full area of the ceiling.
      7. I have a few other ceiling sections in a flat roof, that connect to these roofs, that will get closed cell SPF, (and yes it all passes the pencil test).

      1. Expert Member
        BILL WICHERS | | #7

        If you can vent the attic, that might be easier/cheaper than the intello. With a vented attic, it's not really important to have a vapor barrier on the floor, you just need a good air barrier.

        R100 actually CAN potentially hurt you, just not the way you might think. All that extra insulation has WEIGHT, and can be an issue for the attic floor, or the ceiling underneath. Drywall is especially at risk here, since it has limited structural strength to support the weight of insulation.

        You might want to consider elevating your attic walkway so that you can skip the polyiso and just use blown insulation exclusively. That's probably what I'd do myself.


  4. rhl_ | | #8

    Ceiling below is metal lathe and plaster. I dont think this will be a big issue. 1/2" Drywall can hold 40lbs/sqft (im sure we are higher for metal lathe/plaster).

    at 3.5lbs/cuft (which is not close to right for loose fill) , we would have a little over 14lbs of weight per square foot of insulation. Far below anything close to a limit..

    Does my math sound wrong?

    1. Expert Member
      MALCOLM TAYLOR | | #9


      A 1/2" drywall ceiling can hold about 1.5 lbs/sqft. 40 lbs is the loading for a residential floor.

      1. rhl_ | | #10

        Ok, fair enough. That was a quick google search. How much can a 3/4” plaster and metal lathe hold? According to this

        It’s 15psf?

        But people do dense pack attic floors... at 3.5lbs/ft^3 your saying the drywall couldn’t hold up with 2x6s ?!

        Also if you dense pack 2x6s to ~R-30 that is 1.75psf to get to ~R-100 you would need 20” more of loose fill which at 2.0psf (conservative), is no more than 4psf, giving you at most 5.75psf?

        1. 1869farmhouse | | #11

          1.5 lbs per sq ft is BS. Yea that’s what “they” say, but I’ve blown r40-r60 cellulose and rockwool in probably 20 attics - most with 1/2” Sheetrock with zero issues.

          1. Expert Member
            MALCOLM TAYLOR | | #15


            Just like "they" say the loading for a residential floor is 40 lbs, but you may be able to load it to three times that without failure. If Ryan wants to go with what "sounds right" rather than the values the codes and industry have come up with he is of course welcome do so, but right now he is throwing around figures that are nowhere near correct.

        2. Expert Member
          MALCOLM TAYLOR | | #12

          "your saying the drywall couldn’t hold up with 2x6s ?!"

          I'm not saying anything at all about y0ur drywall, plaster and lathe, or insulation. All I pointed out is the 40 lbs per sq foot was a mistake.

          1. rhl_ | | #13

            It may be, but, 1.5psf doesn’t sound right either.

        3. Expert Member
          BILL WICHERS | | #14

          I went and grabbed my USG "Gypsum Construction Handbook", which is the bible of drywall, basically. On page 74, it lists allowable loads for unsupported insulation as follows:

          For 1/2" drywall and 24" support spacing, insulation should not exceed 1.3 pounds per square foot, or 2.2 pounds per square foot with 16" support spacing
          For 1/2" ultralight (sag resistant), or 5/8" drywall, insulation should not exceed 2.2 pounds per square foot.

          Those are the numbers from the book. I would not exceed those numbers.

          Regarding the "I've done ... and been OK", you may have been lucky. I've had customers say things like that in the past too. As builders, engineers, architects, we have to build things that we can be confident will be safe under ALL conditions. That is what the tables in the books are for -- to help us to achieve that goal of building safe structures. Yes, there is usually some extra margin in those numbers FOR SAFETY, and if you push it, you might be OK, but you might also have built yourself a time bomb that will fail at some point in the future. DO NOT COMPROMISE STRUCTURAL SAFETY. Follow the tables. Keep things safe.

          BTW, those tables are only valid when everything is properly installed. A classic way to weaken drywall is to overdrive the screws and break the paper on the surface. Be careful about that -- the paper is important to maintain strength of the panel, screws that break the paper are far more likely to fail with the drywall popping off the screw.


          1. rhl_ | | #16

            Ok I brought up drywall erroneosly as I have metal lathe and plaster at 3/4” thickness, I figure it’s loading is higher than drywall.

            The random google result I referenced says 15psf. Is that likely wrong ? If so what’s the better reference?

          2. Expert Member
            BILL WICHERS | | #17

            Wet plaster thickness is not nearly as well controlled as a factory made product like drywall, so I'd be very conservative with any load ratings there. I agree plaster and lathe is likely stronger than drywall, but exactly how much will depend on the thickness, attachment to the structure, etc. Somewhere in the 5-1o pounds/square foot is probably safe, but I'd be conservative and stay towards the low end.

            Remember that neither drywall or wet plaster are intended to be structural materials -- they're just finished surfaces. Any ability those materials have to hold up other stuff is essentially a free bonus and should be treated as such -- you should avoid relying on these materials for support as much as possible.


          3. 1869farmhouse | | #18

            Interestingly ran across this article from our resident expert Martin today - which far more succinctly seems to echo my sentiments. I’m not saying to overload the structure, but the rock won’t sag. Run a bead of construction adhesive if you’re really worried.


          4. Expert Member
            BILL WICHERS | | #19

            Be careful with the construction adhesive trick. My house was built that way, and the adhesive has seperated nearly everywhere, causing noticeable sagging. I've been working to correct this, but it's especially difficult on the upper floor ceiling since replacing the drywall requires removing the old insulation (blown cellulose) and replacing it. It's a very messy project. I've had the same issue on the walls.

            My recommendation would be to stick with screws, but install more than usual. If you want to use adhesive too, go for it, but consider it a backup to the screws.

            I also strongly recommend using 5/8" drywall on at least the upper floor ceiling (I use it everywhere) for the extra strength for insuation support. The extra cost to go to 5/8" is usually less than a buck a sheet.


          5. 1869farmhouse | | #20

            I’ve never seen adhesive used without screws, only in tandem with them. When I said “run a bead” I was more talking about an additional bead of adhesive on either side of the joists where they meet the drywall. I did that once for a customer that wanted r-60.

            Unless the drywall gets wet, I just don’t see the rock itself being the failure point - it has to be the attachment points. And 5/8 might be only moderately cheaper... but I sure wouldn’t hang it. And almost any crew I’ve ever worked with say the same or charge an enormous premium for labor. That stuff is heavy and the crew is normally on stilts in new builds.

          6. Expert Member
            BILL WICHERS | | #21

            5/8" drywall is the commerical standard, where it's used is required. For 4x8 sheets, 1/2" is about 44 pounds, 5/8" type X is about 70 pounds. The lightweight 5/8" stuff is about 64 pounds. That puts 5/8" drywall at about 40-50% more weight. If you're using a lift, it's not a huge difference.

            I've never seen a crew charge any different for labor for 5/" drywall, they all say it's about the same work to hang, and no difference at all to finish. It surprises me you've had crews that consider 5/8" drywall a big deal.

            1/2" ultralight has the same load rating as 5/8" for supporting insulation above a ceiling. The "failure" is usually not so much as a structural failure as it is aesthetic -- sagging. Glue, or any number of screws, can't help with that. A double layer of drywall would be better, or some slats above the drywall to help share the load of a heavy layer of insulation.


          7. 1869farmhouse | | #22

            I of course only know what I’ve worked with - which is residential. If any of those guys pulled out a sheet rock lift, they’d be thrown off the site haha. It’s a speed game.

            The little bit I have been around commercial - again at least around here, is union guys. So in that world it makes sense that it wouldn’t matter so much, nobody is actually lifting it over their heads and they are using lifts. Speedy guys on stilts vs slow deliberate work with machinery, I could see that. But one more time, I only know my little corner of the world.

          8. Expert Member
            BILL WICHERS | | #23

            5/8" is a code requirement in commercial work. Commerical work is way more concerned with fire risk than residential. Basically commercial guys have no choice but to use 5/8" drywall nearly everywhere, so maybe they're just more used to it?

            It's all about speed commercially too. I like 5/8" in residential builds since it makes more solid walls, and less sound transmission. If you want to beef up the ceiling for insulation support without dealing with 5/8" drywall, you could do a double layer of 1/2". It would take more time, but you'd be working with a material your crews are more familiar with.

            My personal opinion would be to tell the residential drywall guys to go to the gym and get ready for 5/8", but that's just me ;-)


          9. Expert Member
            NICK KEENAN | | #24

            I've never seen a drywall guy who looked like he needed any gym time.

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