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Building Science

Three Reasons to Remove Attic Floor Insulation in a Sealed Attic

Attics insulated with spray foam have different characteristics from vented attics

If you put spray foam insulation under the roof deck, should you remove the cellulose or other existing insulation from the attic floor?
Image Credit: Energy Vanguard

I get asked a lot of questions about spray foam. Do I need an ignition barrier? Should I use open-cell or closed-cell spray foam? Will open-cell spray foam really rot my roof?

But the question I get more than any other on this topic is about whether or not the insulation on the attic floor should be removed when insulating the roof deck in an existing home. As you can tell from the title of this article, my answer is to remove it. Here are my three reasons, in increasing order of importance.

1. To prevent moisture problems

Think about temperature. Think about dew point. If you leave the insulation in the attic floor after insulating the roofline, the attic will be cooler in winter than if you remove the insulation.

But a common reason to put spray foam on the roofline is to avoid having to air-seal the attic floor. Thus, the air in the attic is connected with the air in the house. That means it’s more humid than outdoor air and more humid than vented attic air. Cold air is dry air, you know.

But now the attic isn’t vented to outdoors. The attic is much warmer than outdoors in winter but significantly cooler than the living space if you leave the old insulation in the attic floor. That makes the surface of the spray foam cooler, possibly even below the dew point.

Removing the attic floor insulation will solve this problem. It is not, however, the first solution. As Dr. Joe Lstiburek says, we shouldn’t be calling these things sealed attics or unvented attics. We need to think of them as conditioned attics. Once you deal with the air up there, this problem goes away, with or without the insulation in the attic floor.

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4 Comments

  1. User avater
    Dana Dorsett | | #1

    For the attic to be a wintertime dew point problem...
    ... it has to be really thin insulation at the roof deck, or really fat insulation at the attic floor. In US climate zone 6 as long as 50% or more of the total-R is at the roof deck, the average temp in the attic will be above the dew point of the conditioned air, and little to no moisture accumulation occurs. By climate zone zone the minimum fraction of the total that needs to be at the roof deck is roughly:

    Zone 7 - 60%

    Zone 6 - 50%

    Zone 5 - 40%

    Zone 4A & B -30%

    Zone 4C - 20%

    Zones 1-3 - 10%

    So R20 on the attic floor and R20 at the roof deck isn't likely to become a moisture problem for any location south of zone 7, but putting R20 at the roof deck with R38 at the attic floor (roof-R = 34% of the total) could spell winter moisture trouble in zones 5 and higher, but it can be monitored. In many cases the thermal bypasses from the fully conditioned space undercut the effective R of the fluff on the attic floor, resulting in higher attic temps than the simple-math model might indicate.

    If installing the IRC 2015 code minimum R at the roof deck, it's highly unlikely that any pre-existing attic floor insulation would push the attic temps into the moisture accumulation zone.

    If the floor insulation is full of bat crap or reeks possum pee, sure get rid of it. But if it's reasonably clean there's no reason not to keep it. Although the insulation on the floor DOES provide a thermal benefit. But code compliance purposes adding the floor-R to the roof deck-R is disallowed- you can only count the roof deck R when there is a thermal bypass as large as an attic.

    Pulling back the floor insulation at the edges to get the better air seal with the roof foam is legit, but it can be pushed back once the foam is in place.

  2. Kohta Ueno | | #2

    Actual Problem Case (Cold Climate)
    An actual unvented/conditioned attic problem case I have seen was in Zone 6 (Maine), with ~2" closed cell spray foam at the roofline, and ~R-30+ left in the attic floor. This was a "what were they thinking?!" kind of case--I can't really explain the logic of the installer. As you would expect, the dewpoint in the attic was similar to interior conditions, and in winter, the surface of the underside of the ccSPF (and framing) were cold enough to cause condensation and mold growth. The moisture problem was solved by cutting vents back in at ridge and eaves. Unfortunately, it did not do anything to solve the air leakage problems at the attic floor.

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

    Response to Kohta Ueno
    Kohta,
    Thanks very much for the comments and photos. This case shows that (a) methods that seem obviously risky to those of us who think about dew points don't necessarily seem obviously risky to many builders and insulation contractors, and (b) scenarios that seem unlikely are discovered all the time in the field.

  4. D Dorsett | | #4

    And yet...
    ...that ~R12" foam + ~R30 fluff would probably have worked just fine from the warm edge of zone 4 on south.

    In Zone 6 removing the R30 and leaving only R12 at the roof deck would have been pretty energy piggy, nowhere near code-min!

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