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Attic floor with Rockwool need vapor barrier if air sealing?

user-6959320 | Posted in General Questions on

Hi Everyone,

   I’m a little confused between advice I’ve been getting locally vs reading up about need for vapor barrier or not.  I’m hoping one of the experts here can take a moment of their time to school me on current theory and practices.

House is ~35 y.o. Zone 6 w/3 attic partitions.
Just replaced the roof, and the (3) static vents with all new ridge venting this year.
The 2 unconditioned attics had a fair bit of mold, which I’ve remediated. Not sure if its due to the very low intake/exhaust sqf of previous venting, or the fact that the 50 3″ intakes were mostly covered by insulation….

Sorry, getting back on topic-

Locally I’ve been told I should have a vapor barrier.

Online, I’ve read that air sealing is much more important, and if done basically obviates the need for a vapor barrier.

Since I’m pulling all the old fiberglass, and will be sealing light cans and penetrations, can I skip the vb, or do I need one, or should I install Membrain?

Next year I plan on removing popcorn ceilings, and will be painting with a vapor barrier/retarder paint anyways, and installing continuous soffit venting.

I really appreciate you taking the time to help me out.


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  1. Expert Member
    Peter Engle | | #1

    As you've read, air sealing the attic floor is the most important step. Far more moisture is transported via airflow than by diffusion through building elements. It should go without saying, but also make sure all bathroom vents, appliances and such are vented directly outside, and not near any soffit vents.

    Your clogged intake vents and few roof vents certainly contributed to your issues. Full ridege and soffit venting will overcome all but the worst moisture problems, and all of them are caused by air leakage.

    If you will be applying vapor retarder paint to the interior of the drywall soon, you don't need another vapor retarder in the attic. For this winter, keep the indoor humidity on the low end of the comfort range, around 30%. Don't use a central humidifier - room humidifiers as necessary. Go up and look at the sheathing on the coldest mornings before the roof warms up. If the sheathing is visibly wet or has frost on it, you've got more work to do.

    1. Jon_R | | #2

      Consider installing humidity meters as a more reliable way to detect excessive attic humidity.

  2. user-6959320 | | #3

    Gentlemen, thanks very much for your time and expertise.

    I did check last winter before the roof was replaced and never saw condensation on the rafters or plywood. Existing ventilation was (2) 1 sqf(?) static exhausts in this attic and ~30-40 3" metal screen intakes. Since most of the intakes seemed to have batts partially blocking them, and they're the screen types which have very little open space to begin with, it would seem to me to be ventilation wasn't good.

    Still pulling fiberglass so not sure if there are issues with wall plates. I'm following this quick video on top wall plates and exterior-

    I'm planning on using the baffles from Lowes I'm thinking just staple and then seal to prevent wind washing?

    I've got baseboard heating, and no humid/dehumidifiers in use. Not sure if I need to worry about that, unlike furnace heat I've had in the past, the baseboard doesn't seem to affect humidity much, that I've noticed.

    Again, your help is greatly appreciated!

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