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Community and Q&A

Attic Venting With Combination Spray Foam / Blown Insulation

AJMartin905 | Posted in General Questions on

We recently bought a house built circa 1900 in Northern Missouri.  The attic has zero ridge vents and zero soffit vents, just  one lonely little gable vent.  At some point, someone sprayed what appears to be rigid closed cell foam all over the top side of the second floor ceiling and rafters.  There’s several inches of either fiberglass or blown rockwool on top of that.

The roof is pretty new (standing seam metal) so I really don’t want to demo it.  It seems like a place for powered ventilation, but everything I’ve read says that’s a bad idea.  I’m not sure I could afford to encapsulate/condition it, but even if I could, wouldn’t the spray foam on the top of the ceiling be a problem?  In short, I’ve read a hundred articles and none apply to the situation I’m in – and I’m looking for help as it’s humid as heck up there.

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  1. user-5946022 | | #1

    To be clear on your situation:
    From inside conditioned space to attic on your second floor, you have:
    - Wallboard or plaster on the ceiling
    - Ceiling Joists (horizontal framing members that act as both a collar tie to the rafters and support for wallboard/plaster that is the interior conditioned ceiling finish material.
    - Spray foam which you believe is closed cell sprayed against the backside of the wallboard/plaster (and or lathe), and against the ceiling joists
    - Loose fill blown fiberglass or rockwool above the spray foam
    - Unconditioned & unvented attic
    Is the above correct?
    If so, I would presume the spray foam was intended to enhance the air barrier between the conditioned and unconditioned spaces.
    Does your attic have any HVAC equipment or ducts in it?
    Do you know the detail of your metal roof system? Ie is it, from attic side, wood decking (plywood or old boards), synthetic felt, metal roof panels, or is the detail more complex such that there is either insulation or ventilation between the decking and the metal roof panels?

    1. AJMartin905 | | #3

      Yes, your understanding is accurate. The attic space is unconditioned and basically unvented save a single, maybe 12” gable vent.

      The roof assembly is plywood decking with synthetic felt on top and then standing seam metal roofing. There is no additional insulation or ventilation as a part of the roof assembly or the house at all.

  2. walta100 | | #2

    Seems to me your current situation falls in the good to great category.

    The spray foam should give you a good air barrier if the foam is 2 inches thick that’s R14 + the fluffy insulation say 6 inches maybe R20 for a total of R34. I say that is very good It would be nice if it was R60 but the pay back from 34 to 60 is likely to be more years than you are likely to own the house.

    The only reason to add more vents would be if you have winter moisture problems in your attic.

    If your concern is the summer attic temps I say forget about it you have enough insulation that it is not a problem and the recommended amount of ventilation really is not going to change the attic temp. Attic vent are designed to remove moisture in the winter not cool the attic in the summer. Let’s say your attic gets very hot maybe it will be 130° and the AC has the house at 75° you have a 55° difference with R 34 in between. How cold can it get in the winter -20° and the furnace has the house at 70° you have a 90° difference with R 34 in between. Summer is not the problem winter is.

    If the problem is that the AC does not keep the upstairs cool. Is the ductwork in the attic? Was the duct work designed for cooling? Was a manual J done?


    1. AJMartin905 | | #4

      This is a comforting comment and I appreciate the input. There’s at least a foot of blown insulation over the spray foam, maybe more. And no, I don’t have any any HVAC or ductwork in the attic. I thought the problem would be in the summer. The humidity right now is definitely higher than inside the house, but it’s not crazy. 47% right now. So maybe there are no problems at all. I just read a lot about attic ventilation and everything I read indicated the attic should have ridge and soffit venting, of which I have neither.

      1. user-5946022 | | #5

        So before you do spend money on this attic, get yourself a few remote read humidity sensors, and stick them near the ridge of your attic. Monitor for a year.
        - A few because consumer grade humidity sensors are usually off, so a few will give you an average and deal with the situation where about one in 10 units is over 10% off.
        - Best to get a system that logs ; you won't be able to keep up with the daily swings.
        - You want to figure out the highest humidity you get in the attic over the course of a year.

        Then come back and report back to us.

        1. AJMartin905 | | #6

          I have a probe up there now and have been monitoring manually, but a logging feature would be nice. I just assumed no matter what, the attic needed more ventilation that the tiny gable opening, but maybe that’s not the case! It was 67% this AM with a temp of 34 F. Really the only penetration through the foam is the exhaust fan from bathroom. It runs outside, but looks like it could be sealed better.

          1. Expert Member
            Akos | | #7

            Gable vents only are pretty common around me (zone 5) even with 2.5 story houses that will have a lot more air leaks than your well sealed ceiling.

            As long as the attic dewpoint is about tracking outside dewpoint, you have enough venting. You have to convert the RH and temperature readings to dewpoint to check, RH by itself is not enough.

            My guess is that you are fine as is.

  3. seabornman | | #8

    I have owned 2 houses in zone 5 snow belt that had very little attic ventilation for different reasons. They both had ice dam and icicles issues. After intensive air sealing, both houses were totally cured of their problems. When I built my current house addition with a metal roof, I decided to ignore the experts, and only put one gable vent and soffit vents in. Again I went to great lengths to air seal. No problems at all. I believe attic ventilation is overrated.

    1. Expert Member
      DCcontrarian | | #9

      The purpose of attic venting is not to prevent ice damming, that's a side effect. The purpose of attic ventilation is to prevent moisture from accumulating in the roof sheathing, causing rot. That moisture comes from inside the house, so if your house was perfectly sealed you wouldn't have to worry about it.

      1. seabornman | | #10

        Yes, as I said no problems at all. That would include moisture.

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