GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Bathroom fan after attic was sealed

Jpearl1 | Posted in Green Building Techniques on

We have open cell spray foam r44 on the roof deck in NY. Near NYC
i realized one of the bathrooms doesn’t have a fan and after showers the humidity can get trapped in the attic. Having an electrician put a 110 CFM fan in ducted outside. He wants to duct into a soffit and drill a hole. How do I have this done without breaking the “seal”?
Eaves are covered with foam. Should have I have him foam the edges of the duct end when he’s done?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Patrick_OSullivan | | #1

    > It is sept 2023. In late may 2023 we had open cell spray foam installed through an energy rebate. We had it installed on roof deck/rafters.

    > I live in NY 2300 sf house.

    Is the roof vented? No one should be installing open cell foam below a roof in NY if it's not vented.

    > Ever since then we had higher humidity and stale air. I couldn’t breathe in the house.

    Somewhat surprising because the time period you've described is a period of high outside humidity in NY and any air leakage stopped by the addition of foam would have been high humidity air ingress.

    However, if the foam lowered your cooling load significantly such that your air conditioner is running much less, it's possible that could lead to less dehumidification.

    Do you have any quantitative data about these changes? Specifically, humidity, CO2, etc.

    1. Jpearl1 | | #5

      I had a small humidity monitor right by the hatch. It was approx 70% RH. This was originally a vented attic. Gables, soffits and two box vents up top. I’m not sure about a ridge vent. The humidity in AC season was always in the mid 50s in the house. Now it’s mid to low 60s in the house. It’s a 4 ton trane 13 seer that was fine before. Per my sensi it would run let’s say 300 hrs a month now it’s half of that which is great for bills but it’s not doing the job. The spray foam company left the box vents open after install and only put small fiberglass batts over the gables. I started to feel the house being stuffy and was getting dizzy. I went up to remove the fiberglass off the gables to get some fresh air going until we can mitigate. Prior to this we just had some old batts on the floor between the rafters. I have saved on energy bills but the house isn’t providing same comfort and am seeing issues with molding separating and some brown stain age around wall ends as well as mildew smells. Not sure what to do. Was advised by a company who did a blower test to install and erv. Said the house is really tight and he would never have done this. He said he would have blown in the floor. I feel like I was taken advantage of here and cannot mitigate. It doesn’t seem like it can be removed and worried we condemned this house.

  2. Deleted | | #2


  3. Expert Member
    BILL WICHERS | | #3

    It's much safer to use close cell spray foam under the roof sheathing, since open cell has been known to have issues with moisture retention.

    It sounds to me like they insulated the roofline of a vented attic here, left the attic floor alone, and closed off the gable vents. My guess here is that there were probably some soffit vents left open, or the attic was otherwise connected to outside air. Opening the gable vents helped get the moist, stale air out. If there weren't any mechanicals in the attic, I see no reason to have insulated the roofline, I would have air sealed the attic floor instead, then added additional loose fill insulation to the attic floor, most likely blown cellulose. It's very important to air seal the attic floor when doing a project like this if your goal is to maximize energy efficiency.


    1. Jpearl1 | | #7

      Yes the handler and ducts are up there. The attic was vented before. Changed the whole dynamics.

    2. Jpearl1 | | #8

      We are keeping the foam because we don’t have any choice. Right now all vents are sealed back up. We are going to install and ERV or aprillaire 8145. Do you think this will help with the circulation and moisture buildup?

      1. Expert Member
        Michael Maines | | #10

        It would be wise to install a variable permeance membrane under your rafters to limit how much moisture can diffuse through the foam into your roof sheathing.

  4. pnw_guy | | #4

    Before you rip the spray foam out, have you tried putting a dehumidifier up there? And/or a small intake+exhaust fan to circulate a little bit of fresh air through there?

    As others have said, it's supposed to be completely sealed, but if you never circulated much fresh air through there after the installation was complete, doing that might be a good idea even now. And a dehumidifier would help, too.

    1. Jpearl1 | | #6

      No. Thinking to do an ERV. Right now nothing going on up there. Spray foam company basically saying we are nuts. I think it was bad install.

    2. Jpearl1 | | #9

      We are thinking to have ERV or the aprilaire 8145 fresh air ventilator put in with a new 3 ton Bosch AC. The air in the house is stale. What fan are you referring too?

      1. user-5946022 | | #11

        Your original post does not seem to match the replies, so I'll presume you changed the original post.
        First, the concern with open cell under roof sheathing (the plywood) in your climate is that warm air rises as does moisture, so warm and/or moist air from the house rises, the moisture permeates through the open cell spray foam and condenses on the very very cold underside of the roof sheathing and eventually rots the sheathing.

        While it is good to try to remove unnecessary moisture source by installing an exhaust vent in a bathroom with none, this will not address the underlying issue.

        It is unclear from your posts what problem you are attempting to address with an ERV. You state the air in the house is "stale." That is certainly possible in a tight house. How tight is your house? This is typically measured by a blower door test. In my are the results of a blower door test are pasted to the inside side of the electrical box cover. Post back the results of your blower door, in the format of ACH x/50pas, where x is the result.

        You should also measure your air. A device such as an Airthings view placed in a sleeping area can tell you the CO2 and how much it fluctuates between day and night, and also provide temperature and humidity readings. Based on those results you would decide what the next step is.

        Replacing your AC only makes sense if you need to and won't help in the winter. What type of heat do you have? The way to figure out if a 3 ton is correct is to do a Manual J load calc on the house. If your house is very tight, it is doubtful you need a 3 ton system for 2400 sf.

        Finally, connecting the ERV to the AC is never the right answer because you don't use AC in the winter. Connecting the ERV to the HVAC is often also not the right answer. A separate system is typically best. Figure out your air quality to determine if you need an ERV, and based on that figure out a standalone ERV.

        Concurrently, strongly consider the advice to place a dehumidifier in the attic.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |