Replies

1. 
   Jon_R | Aug 23, 2021 09:47am | #1

   > How is this spot ~3° cooler
   Evaporation causes cooling.

   I suspect infiltration from above. Infiltration will cause condensation as warm/moist attic air is cooled to your chilly 71°.

   Some things to try:

   Air seal that area better
   Run the ceiling fan blowing upward
   Don't run exhaust fans (eg, kitchen or bathroom)
   Increase interior temperature
   Check that there are no roof leaks
   Air seal any ducts in the attic
   Create slightly positive building pressure or slightly negative attic pressure
   Somehow reduce vapor diffusion from the attic into the ceiling

   https://www.buildingscience.com/file/5864

1. 
   Jake_Rice | Aug 23, 2021 11:04pm | #3

   Thanks for the commentary Jon
   I have ordered a few tanks of closed cell foam to isolate the attic from the cathedral ceiling.

   I have ridge vents on my roof but very few soffit vents. I was going to add a few more soffit vents but that would hinder me correct? How does one go about increasing the building pressure?

   Thanks.

1. 
   charlie_sullivan | Aug 24, 2021 09:38am | #7

   The A/C system could be rebalanced to ensure a slight positive pressure. Or just meticulously air seal any supply ducts in unconditioned spaces, while being a little less meticulous about return ducts.

   Or you could install a very small supply-only ventilation system.

   Or you could install an ERV ventilation system, balanced to provide a little positive pressure.

2. 
   Jon_R | Aug 24, 2021 10:47am | #8

   The first step in changing interior/attic pressure would be to measure it. The device I have is no longer available, so maybe someone has a recommendation for one.

   Few soffit vents should help reduce attic pressure.

2. Expert Member
   BILL WICHERS | Aug 23, 2021 11:12am | #2

   Do you have an air conditioning duct in the general area of that cold spot? Air likes to follow along smooth surfaces (a sort of laminar flow), so a nearby vent might be cooling that spot on the ceiling more than you'd expect. A simple baffle or other diverter would fix that, you just need to stir the air (turbulent flow) so that it gets diffused enough that it doesn't excessively cool any one spot.

   I agree with Jon that air sealing is another big suspect here. You could potentially have a leaky duct that is pressurizing the cool part of that wall, for example.

   Bill

1. 
   Jake_Rice | Aug 23, 2021 11:10pm | #4

   Bill
   I have ordered a few tanks of closed cell foam insulation to isolate the attic from the affected area.

   I’m about to reroof my home and part of the ceiling doesn’t have a ridge vent but does have soffit vents, should i add a ridge vent?

   Thanks for the feedback.

1. 
   charlie_sullivan | Aug 24, 2021 09:34am | #6

   Spray foam might not be the best solution. I don't understand the layout your attic and ceiling and where you are planning to use the foam but my concerns include:

   * Spray foam need to be applied properly to cure; if it doesn't you can get persistent odors. A DIY kit can be hit or miss. Problems are rare, but really bad when they occur.

   * The blowing agents in the DIY kits are extremely potent greenhouse gases; more than 1000X worse than CO2.

3. GBA Editor
   Martin Holladay | Aug 24, 2021 05:30am | #5

   Jake,
   You might want to read this article: "Summertime Condensation Near the Peak of a Cathedral Ceiling."