GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Is insulating an attic floor worth it?

mfleck | Posted in General Questions on

Hi all,

I’m working on re-insulating my attic ceiling (cathedral-ish) and making it a finished/conditioned space. Currently it’s just used for storage. 

My question: is it worth insulating the attic floor with dense-pack cellulose if I’m also insulating the attic ceiling?

I’m guessing that most people’s initial answer is going to be a solid no, but hear me out:

Our home has steam heat, and there are no radiators in the attic. We spend almost all of our time on the ground and 2nd floor and will probably only spend a little time in the attic once it’s finished as a guest room. We plan to install a heat pump for cooling and most of our heating in the attic when it’s in use.  For these reasons, it makes sense to me to insulate the attic floor to help keep the heat that we’re generating in the most frequently lived in spaces. We’d open the attic door and turn on the heat pump when using the attic space.  Lastly, I recognize that this would be an extra expense, but I’m willing to make the extra investment if it will improve the home’s energy efficiency. 

As you can tell, I’m leaning towards doing it, but thought I should consult with the experts first. 

Thanks, everyone. 

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Jon_R | | #1

    You are considering making the attic semi-conditioned. Note that IRC 806.5.1 requires " The unvented attic space is completely within the building thermal envelope." This is violated with a sealed, semi-conditioned attic below an unvented roof (if that is what you have). My guess as to the reasoning: the potential for moisture problems with anything that makes the attic cooler (Winter) or doesn't have interior airflow to dehumidify (Summer).

    Edit: see

  2. Expert Member
    BILL WICHERS | | #2

    Code generally sets minimum allowable standards. In this case, I’d interpret that to disallow semi-conditioned attics UNLESS either one of the two insulated surfaces (roof or floor) met the ENTIRE required R value for insulation in that part of the structure. Basically you can’t split R38 into R23 in the roof and another R15 in the floor to make up the R38 total.

    I think if you put R38 in the roof, or R38 in the floor, any insulation you put in the “other” location would just be extra over the code-minimum required amount and you’d be OK. Note that I’m just using “R38” here, but you’d need to use whatever the minimum is for your climate zone. Any extra insulation will slow down heat flow, so you’ll gain a little bit of efficiency. Whether or not that “little bit” is enough to be worthwhile is up to you.


  3. mfleck | | #3

    Thanks, Bill

    I believe R49 is code for me here at the northern edge of NYC. I don’t think I’m going to be able to get to that R value, but it’s going to be a huge improvement over what was there when I bought the home (a thin, 3-inch batt of porous, dirty rock wool).

    I recognize that I can’t get to code by splitting—honestly that’s not my intent, just trying to do the best I can with the inches of insulating space I have.

    I guess my question is more: can I do any damage in insulating the attic floor—are there any unexpected risks that I should be aware of?

    Thanks again for the advice here.


  4. BrianPontolilo | | #4

    Hi Matt.

    Maybe I'll learn something today, but I don't think there are any risk in insulating the floor other than spending money on an improvement that doesn't actually help with efficiency.

    I assume that you have a plan for insulating the roofline, but you may consider reading this article, because it is a tricky project: How to Build an Insulated Cathedral Ceiling

    And keep in mind that air sealing is as important as insulating, particularly at that attic ceiling.

  5. Trevor_Lambert | | #5

    I think you'll find that a small amount of insulation in the attic floor will make a difference, given the relatively low Delta T. I'm presuming you'll want the attic to be something reasonable, like above 40F.

  6. mfleck | | #6

    Thanks for responses.

    Brian, thanks for the advice. I’ve definitely read the cathedral ceiling article a few times. Very helpful. My plan is 1 inch air gap under sheathing created by cobbling in 1-inch foam with spacer. Then 3.5” of rock wool unfaced, followed by 2 inches of polyiso under the rafters and drywall. I think that’s the best I can do.

    Sounds like there are mixed opinions on insulating the attic floor as well. In my mind it seems like it would help keep the heat we generate in the space we live in most, but I guess in reality that may not play out.

    Any advice on air sealing attic? Is there a good article on this?

    1. BrianPontolilo | | #7

      Hi Matthew,

      The good news is that your drywall ceiling makes a great air barrier, with some caveats, like, don't poke holes through it for recessed can lights, air seal electrical boxes in the walls, and seal the perimeter with caulk or foam. Here's another article for you about airtight drywall: Airtight Drywall

  7. Expert Member
    Peter Engle | | #8

    You can also tape the seams of the polyiso insulation. Foil tapes stick like crazy and make it relatively easy to air-seal the underside of the roof.

    There is some risk in insulating the attic floor and maintaining the attic as a semi-conditioned space. Many people find that attics run a bit more humid than the rest of the house once the roof is insulated. if you insulate the floor, the attic space will run both cooler and more humid and in winter, that can be a recipe for mold growth.

    Use some easy numbers and take a look: Your roof stackup is about R-25. If you fill the floor cavities with cellulose, that might also get you close to R-25. So potentially, the attic will run halfway between inside and outside temperatures. When it is 10F outside and 70F inside, the attic will drop to near 40 degrees. Not in danger of freezing, so good right?

    However, if your interior air is 70F and 35% RH (not unusual conditions), the dewpoint is 41 degrees. Uh-oh. You are right at the cusp of getting condensation on the inside of your vapor barrier. Just a little bit more interior moisture, and you might be in trouble.

    So, keep you indoor air a little bit dryer, your attic a little bit warmer, and/or your floor insulation a little bit thinner.

    Of course, unless you've got pretty good separation between the attic and the rest of the house, convection will keep the attic warm enough by itself. You only really get into trouble when the attic is truly closed off and unconditioned.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |