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Musings of an Energy Nerd

How to Insulate an Attic Floor

If you’re building a new home, remember to seal the air leaks before you insulate

You can't insulate your attic floor until the drywall crew has finished the ceilings. Before you insulate, it's essential to seal any ceiling air leaks.
Image Credit: Image #1: Buildipedia

Maybe you are building a new home with an unconditioned vented attic. The house is framed, sheathed, and roofed. The drywall contractors have finished their work, so now you’re ready to insulate the attic floor.

If you are an owner-builder, this may be the first time you’ve done this. So what do you need to know?

Air sealing comes first

Although it’s common to talk about installing insulation on “the attic floor,” most attics don’t really have a floor. They have floor joists (or the bottom chords of roof trusses) with drywall below. When you are in the attic, you’re usually stepping carefully on top of the floor joists, looking down at the back side of the drywall ceiling.

You’ll be installing insulation on top of this drywall ceiling. (In some high-performance homes, builders install OSB rather than drywall in this location, and then install a service chase below the OSB. If you’re insulating the attic, however, it doesn’t matter very much whether there is a drywall ceiling below or an OSB ceiling below.)

If you care about energy performance, now is probably a good time for a blower-door test. You certainly can’t insulate your attic until you’ve finished your air sealing work — once the insulation is installed, tracking down air leaks is a real pain.

In most cases, air-sealing workers enter the attic after the drywall is installed, and seal the obvious leaks. The blower-door test occurs after this work is done. On the day of the blower-door test, it’s a good idea to have a few people on site, equipped with caulk, tape, and canned spray foam, so that any unexpected leaks discovered during the…

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  1. User avater
    Paul Kuenn | | #1

    Gabled Luxury
    How we long for something other than 4:1 hip roofs. Certainly not any in NE WI where almost every house built in the 60s was a bird cut rafter-ed hip roof. I'm dreaming of someday seeing something like those photos you like to use for this Web Site. Need another Peshtigo fire to clear the ruins.

  2. Douglas Horgan | | #2

    When is R-38 the same as R-49?
    Answer: in Zones 4 through 8, if you have full-height insulation over 100% of the ceiling including the top plates (as shown in your excellent guide here), the IECC allows R-38 rather than the R-49 that is required if you can't achieve full depth insulation over 100% of the attic floor.
    It's an interesting exception...
    See section 402.2.1:
    This 2015 language is updated a bit from the 2012 version, but both are supposed to say the same thing: if you have full depth insulation everywhere, you don't need as much.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Douglas Horgan
    Of course, as I'm sure you know, R-38 is not the same as R-49.

    The intent of the code is to encourage builders to extend the insulation out so that it covers the top plates of the exterior walls -- an excellent goal. It's unfortunate, however, that the code writers decided to encourage what should be a standard practice by allowing builders to skimp on insulation depth.

    Here is the language of the code exception:

    "R402.2.1 Ceilings with attic spaces.
    Where Section R402.1.2 would require R-38 insulation in the ceiling, installing R-30 over 100 percent of the ceiling area requiring insulation shall be deemed to satisfy the requirement for R-38 wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves. Similarly, where Section R402.1.2 would require R-49 insulation in the ceiling, installing R-38 over 100 percent of the ceiling area requiring insulation shall be deemed to satisfy the requirement for R-49 insulation wherever the full height of uncompressed R-38 insulation extends over the wall top plate at the eaves. This reduction shall not apply to the U-factor alternative approach in Section R402.1.4 and the total UA alternative in Section R402.1.5."

  4. Jim Elliott | | #4

    Wind Washing Dams
    Greetings- Martin.
    I understand the reason for installing the wind washing dams to protect the insulation below the insulation baffles. The part I am missing is the requirement for these baffles to be caulked and airtight around the perimeter. If the junction of the wall OSB and the top plate is sealed like in the picture, then what value is the caulking around dams?

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Jim Elliott
    Of course, many builders don't bother to seal the perimeter of their wind-washing dams. However, if wind is allowed to enter through these unsealed cracks, the performance of your fluffy insulation will be degraded at this crucial location. (When cold winter wind blows through fluffy insulation, it reduces the insulation's performance, just as a cold wind blowing through a wool sweater will make you feel cold during the winter. If you put a windbreaker over the sweater, you'll feel much warmer.)

    If you live in a region with winter snowfall, the performance of the insulation at the perimeter of your attic is very important. Heat loss near the eaves leads to ice dams.

  6. Bruce Lepper | | #6

    Bad link
    For info, the link " Borrowing a Cellulose Blower From a Big Box Store" goes to the wrong subject.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Bruce Lepper
    Thanks very much for pointing out our error. I have fixed the broken link.

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