It can be difficult to get these installations on residences with no decking beneath the roofing to work properly. However, you're on the right track. Adding a vapor barrier, if it can be done, would be helpful. Insulation also helpful. Increasing ventilation helpful. You have the two gable vents. No other vents? Any chance of eliminating them and adding soffit vents as intake and ridge vent as exhaust? If not, you may consider a power exhaust fan in one of the gable vents. The moisture leaks into the attic must be diverted, as you know. Adding ceiling fans in the house, or keeping the fan on your furnace running continuous could actually help by keeping some moisture inside the living space rather than having it migrate to the attic. Finally, an option would be to spray closed cell foam insulation on the back of the roofing.
Todd Miller
2016-01-08 05:07:03.000000
Hello,
I have been reading over lots of condensation problems with more questions than answers.
Here is my situation.
A new white metal roof installed in October 2015. Roof is attached to purlins. Also 1/2" white foam underlayment between rafters. And that is it
for the roof side.
Ceiling has just batt insulation.
Both gable ends have venting.
A couple of things I noticed was that the two bathroom vents vent to attic, which needs to get fixed to blow outside and some of the foam underlayment is falling down in some spots. Need to get fixed.
Since it is just getting below freezing here in TN the past couple of days, there is condensation forming. I know alot is due to the vents and underlayment falling down. If I fix these two things, will it help it out enough?
I would be willing to take out the batt insulation to put down some vapor barrier. I have 4mil plastic.
My roofer says I would need another 6 inches of blow insulation over the ceiling floor to help stop it?
I cant rip off the roof, but what is the best cost effective method to help stop or slow this down?
Thank you for any assistance.
Jeff Miller
2016-01-07 21:57:39.000000
Sounds like you have a mixture of a vented roof design with an insulated roof application. Not quite ideal but often done with metal roofs.
Todd is spot on. Space needs more venting to drive convection and remove the moisture.
If you are going to do anything to the attic floor, you need to air seal and insulate. Air sealing will keep a majority of the moisture in the space that is lost via air loss and the additional insulation will help with efficiency. Do not, I repeat, do not...insulate without the air sealing.
If you insulate (without air sealing) the attic will get much colder and the dew point will move accordingly. With the roof surface being closer to the ambient temperature, you will get more condensation vs. less. You have to air seal first, then insulate and in conjunction with ventilation modifications is ideal.
https://www.energycodes.gov/sites/default/files/documents/BECP_Buidling%20Energy%20Code%20Resource%20Guide%20Air%20Leakage%20Guide_Sept2011_v00_lores.pdf
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