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Community and Q&A

Moisture Barrier Under Attic Rafters

PDS | Posted in Energy Efficiency and Durability on

We built our custom home over 18 years ago. We are in western Washington, Zone 4, where we typically see just a few weeks of freezing weather each winter. Most of the top floor is covered with 2×12 joists, insulated with R32 batting. Over that is a plywood subfloor, for walking and storing in the large attic. Over the attic is 2×12 rafters, for the 9/12 pitch roof.  Entry into the attic is through a folding ladder with built-in plywood that lifts against the framed box in the ceiling below.


When the Fire Inspector saw that we have a floor in our attic he said, “Oh, I see that since you have an attic floor, you intend to let people into your attic to store things, therefore you have to sprinkle the attic also, and since you cannot allow the sprinklers to freeze, you have to fully insulate the attic, and heat it as you do the rest of the house.” So we added another layer of R32 in the rafters, plus R17 in the gable ends. The attic also has two large 2-pane windows. But since it has never come close to freezing up there, we don’t actually heat the attic.


A couple years ago our 30-year comp roof began leaking and our roofer friend confirmed that the sheathing was sagging due to moisture. So they replaced many sheets of sheathing and put a new 40-year roof on the entire house.


He suggested that the source of the moisture was the ladder itself. It is over the master dressing room, which has our hand sinks. And this room is just one door away from our shower room (although both rooms do have vent fans).


So I have now weather-stripped the border of the plywood under the folding ladder, and applied rigid foam board as best I could between the plywood and the folding ladder. But I am not confident that this is completely sealing the moisture out of the attic. And I don’t want to replace the roof all over again in 16 years.


I have contemplated stapling a layer of polyethylene to the underside of the roof rafters and insulation, to keep any moisture from penetrating up to the roof sheathing. Do you think this is a good idea? Other thoughts?



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  1. Expert Member
    DCcontrarian | | #1

    "When the Fire Inspector saw that we have a floor in our attic he said, 'Oh, I see that since you have an attic floor, you intend to let people into your attic to store things, therefore you have to sprinkle the attic also, and since you cannot allow the sprinklers to freeze, you have to fully insulate the attic, and heat it as you do the rest of the house.'"

    I would have taken out the attic floor.

    1. handyhomehacker | | #3

      Geesh. I trust that the fire inspector advised that the floor boards could disappear, as an alternative.

  2. Expert Member
    DCcontrarian | | #2

    Almost certainly, what's happening is that warm, moist air from the interior of the house is seeping through the insulation and condensing on the underside of the roof sheathing.

    There are three basic ways of keeping this from happening:
    1. insulate the top side of the sheathing, between the sheathing and the roofing, to keep the underside of the sheathing warm enough that condensation doesn't happen. The time to do this is when you re-roof.

    2. On the underside of the roof use an impermeable, airtight insulation that is thick enough that the point that is in contact with air is warm enough that condensation doesn't form. The only insulation that is air-tight enough for this to work is spray foam, either a thin layer of closed-cell foam or a thick layer of open-cell foam.

    3. Ventilate between the insulation and the roofing so that any condensation that forms evaporates. Usually this is done by putting air channels between the underside of the sheathing and the insulation and vents at the eaves and ridge of the roof, although I'm reading more about people who do it by putting an over-roof between the sheathing and the shingles.

    There is high probability that #3 is how your house was originally meant to function -- with the insulation on the floor of the attic the entire attic was the vented space, with small openings at the eave and ridge.

    This is something we discuss a lot here. If you're looking for keywords to search on, what you've done is converted from a "vented attic" to an "unvented attic." The symptoms you've described are a classic sign of an improperly insulated unvented attic.

    Polyethylene as you've described isn't going to help, it's not going to keep the moisture out. It might move the rot to somewhere else but that's about it.

  3. Expert Member
    BILL WICHERS | | #4

    Since you said you "added another layer of R32" to the rafters, I'm assuming you insulated those with batts. Do you have vent channels above those batts? If you don't, that's probably the problem -- you don't want to build unvented vaulted ceiling assemblies (which is probably essentially what you have here) with batts. The usual way to do deal with this with a new project would be to use closed cell spray foam under the roof sheathing, which, being fully adhered and a vapor barrier in the required thicknesses, is the safest way to go in one step.

    If you want to keep insulating with batts, you really need vent channels above those batts and below the roof sheathing, and those vent channels need to run from soffit vents to a ridge vent. You can do this with site built vent baffles made from thin plywood (I like to use 1/4" waferboard for this because it's light and it's cheap), or 1/2" polyiso, or you can use factory made vent baffles. Any of those options works.

    If I were doing this myself, and I had enough room to furr out the rafters, I'd go with vent channels and batts since it's not that expensive, and it's easy to do as a DIY project. You can also put polyiso under the rafters (with batts between the rafters) to build up your total R value, which helps with thermal bridging, then tape the seams to help keep moisture from getting up into the assembly. The vent channels will deal with anything that sneaks up there to prevent your sheathing from rotting out. You need to either use polyiso rated to be left exposed (Dow Thermax, etc.), which is expensive, or use cheaper polyiso and drywall over that (an extra step, but likely cheaper). Either way works.

    Anything you do with the access stairs or other air sealing efforts is pretty much a band aid, and while it might help, it's not a reliable solution tp the problem you're seeing.


  4. PDS | | #5

    Thank you for your help. I tried to think of everything, but forgot to mention, that there are soffit vents and ridge vents. So there is generally a small air gap between the batting and the sheathing (which is mostly OSB, although the sections that were replaced are plywood). But there is nothing between the batting and the sheathing to prevent the batting from being pushed up against the sheathing. So I think that I need to pull down the batting and staple up something like Polyvent Baffles to the underside of the sheathing, then place the batting back up. Yes?
    But then, wouldn't a layer of polyethylene under the rafters/batting add that much more protection from moisture? Would Polyiso under the rafters/batting be worth the expense? (I'm not concerned about thermal bridging of the rafters, since this is already a 2nd layer of R32)

    1. Expert Member
      DCcontrarian | | #6

      A vent between the batting and sheathing is exactly what you want.

      Once vapor gets to the sheathing you want to let it escape, it will cause trouble if you trap it there. The place to control vapor is at the warm side of the insulation -- the ceiling below. The insulation there should be faced with the facing facing the warm side. If it's not a vapor barrier primer on the drywall may be called for, both drywall and latex paint are pretty vapor open. Also, the drywall itself should be as tight as possible to keep air from leaking into the attic. Any entrances into the attic should be sealed with weatherstripping.

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