Unvented roof deck question
Good afternoon,
About 5 years ago, I had spray foam insulation installed under my roof deck while my roof was being redone. I have a small cape cod in Lansing, Michigan built in the 1940s. My intention was that closed cell foam would be installed from the peak to the eave under the roof deck. Unfortunately, that didn’t happen.
Over the past couple years, I have noticed condensation and humidity issues in the upstairs bedroom in the summertime. The humidity level varies between 55-70% relative humidity and it stays about 70 degrees (with a small window AC unit). In the winter, there are no humidity issues. I do not have any ductwork upstairs.
After tearing down part of the ceiling (old condensed paper ceiling tiles), I noticed that the foam insulation above the ceiling is open cell and wasn’t installed under the roof deck. Instead, it is over top of the ceiling, leaving a small insulated triangle at the peak of the roof. Ultimately, this appears to have been an issue where the installers didn’t have full access to this area, so they sprayed what they could from on top of the roof deck. The open cell foam is not wet though. There is also old mineral wool batts directly above the ceiling tiles–the foam is above those.
Now, because of the humidity and condensation, I am planning to tear out the ceiling and have closed cell foam installed under the roof deck as I originally planned. I was hoping you guys might be able to give me some advice:
-The estimator gave me a quote for 5″ of closed cell in order to meet the minimum amount for them to take the job (originally, he suggested 3″, but that was well below the minimum). This equates to approximately R-32.5. The Michigan Energy Code states R-38 for the roof. So, I would probably need 6″ to achieve that. However, the estimator stated he’s never seen someone do 6″ of closed cell and he’s been doing it for a long time. Do you think 5″ is enough?
-After the foam is in place, I am going to drywall the ceiling. Should I put register vents in the ceiling to allow the air above the drywall to be conditioned with the rest of the space?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
55-70% humidity can be a simple case of sometimes inadequate dehumidification by the AC (a normal issue). You can fix with a dehumidifier.
Make sure you understand exactly why there is condensation before you spend money on a "fix".
I assumed part of the humidity was natural, but that it is being exacerbated by the way the open cell was installed above the ceiling. I agree with you that I could have inadequate de-humidification. The room is about 325 sqft and the AC unit I have in there is designed for 350sqft I believe.
The condensation is occurring in spots where foam is missing or where the warm air above the ceiling is meeting the cool air in the bedroom. If I put a dehumidifier up there, it would pull the moisture out, but I'm not sure how that informs what choice I should make on a "fix".
Wouldn't closed cell help with the humidity in this situation?
If I'm understanding your setup correctly (mini attic with oc SPF on the floor of it), the only thing you are missing is a set of gable vents.
No need to rip everything out and re-insulate unless you have your HVAC up inside this mini attic.
No, that's not quite right. Here's a crude diagram to describe it better.
What you have should work. My guess is there is not enough roof ventilation.
If you have a weather station with a wireless outdoor sensor that measures RH, put one up into the attic area there. Check the dew point (you can calculate it from RH and temp) in the morning after sun has hit the roof a bit. If the dew point is higher than outdoor dew point, you need some more ventilation.
> condensation...stays about 70 degrees
Sounds like you have an air infiltration problem (and some over-cooling). Better air sealing is the best way to address this (pressure control is a less desirable alternative if that doesn't completely fix it).
> Wouldn't closed cell help with the humidity in this situation?
Probably not. There is nothing wrong with open cell on the floor of a vented attic.
M. Walker,
I agree that this is not an insulation problem. I disagree with Akos -- increased attic ventilation won't help.
Jon R. is correct that better air sealing should help. It's also possible that your air conditioner isn't doing a good job of dehumidification.
I guess the weather there will can hit 70F dewpoint on hot muggy days, can't see this condensing enough water unless it is consistently above that. We get similar weather in Toronto, never seen condensation on leaky ceilings unless there was an AC duct above it.
Another source of high dew points is accumulated water being evaporated by solar heating and then driven inwards. GBA sees occasional reports of this (condensation when the outdoor dew point/indoor temp doesn't justify it).
Yup, which is why I suggested the OP to check the dew point in the attic after the sun has heated it a bit before doing anything.
I'd extend the measurements to the entire roof line (ie, also the knee wall/storage area that has open cell foam against roof sheathing).
非常感谢你们所有人你的回复。我真的一个ppreciate it. I am not in the building or construction industry, but I learn so much from reading this board.
I think I understand what everyone is saying--correct me if I have it wrong. The insulation I have is not necessarily the problem. It is adequately blocking the radiant heat from above it into the conditioned space. If I am having a problem with condensation, there are two possible sources: (1) the hot air above the ceiling insulation cooling down at night and dew forming, or (2) hot air from above the ceiling leaking/infiltrating the conditioned space and condensation occurring at that intersection. The open gable vents at the ends would allow whatever the humidity is naturally to come in.
I do not think this is an issue with dew. There is no discernible liquid water on the insulation or anywhere else in this area and the insulation is not wet. There is also no liquid water in the storage areas.
The remaining issue, air leakage, would likely occur at the edges of the insulation or spots where there is no or less insulation (which is what is happening). This issue could be resolved by removing the ceiling tiles and mineral wool batts, adding foam where necessary, and ensuring that the hot air above the ceiling can't infiltrate the conditioned space. Lastly, I may need to add a dehumidifier in the room to help get rid of the excess moisture.
如果我仍然想把我挖出来,做closed cell foam under the roof deck after this discussion, do you think there would be significant problems with that or areas to watch out for? Would 5" be enough? Is any air passthrough to above the ceiling necessary in that setup?
Thank you all again.
I think that is a good summary. Issue 1 is not actual dew forming but higher moisture content in the air which will eventually move through the open cell and cause condensation.
Pulling off the problematic area, removing the open cell foam and re-sealing with closed cell would fix your issue whether the issue is 1 or 2.
If you want to tear and re do with closed cell, 5" is more than enough. Keep in mind that you will also have to spray the gable ends as well as close the gable vents. Usually you can have a gap between the ceiling and cc SPF, not sure if that attic area is too much though.
Hello again--
Over the weekend, I tore out the old ceiling and insulation to prepare for closed-cell foam. I found mold in the areas of concern (above the open cell that I tore out). I thought you all might like to see, so here's a couple pictures.
This might be a good read, sounds like the same problem you had. Replacing with the open cell with closed cell should fix it.
https://contractors.efficiencyvermont.com/Media/Default/bbd/2018/docs/presentations/efficiency-vermont-bbd-why-are-attics-insulated-with-open-cell-spray-foam-so-damp.pdf