[Greenbuilding] Unvented cathedral ceiling condensation

[Michael O'Brien]

Hi, Leslie—

Just wanted to chime in to agree about the air leaks across the interior side of the cathedral ceiling. Not only recessed can fixtures, but often every wiring hole drilled through top plates, every vent stack and flue have not been sealed to block air leaks. The walls may be contributing, too, if there are penetrations in the top plates. 

The long-term fix is to take down the ceiling drywall so the leaks can be sealed and a proper vapor rertarder installed, but in the short term they could run a dehumidifier to reduce the water vapor in their occupied space. 

Sometimes builders will open up the blocks along the eaves and install some sort of vent at or near the ridge, but this may have the effect of sending cold air under the insulation and cooling down the interior surface of the ceiling, it takes some detailing to keep the vent air above the insulation.

Best wishes,

Mike O’Brien



On Dec 10, 2016, at 11:50 AM, Leslie Moyer <unschooler at lrec.org> wrote:

I have some nearby friends having a problem. I think I understand what the problem is, and even some possible ways to solve it, but I'm not certain I could give them advice that would fix their problem the best or cheapest way. I thought you guys could, though!  

They read this article & I think they will go ahead and pay to read the article referenced within it: 

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

We are considered a "hot humid" climate--NE Oklahoma 
…………………………….

We have a question regarding condensation problems in a cathedral ceiling.  We live in northeastern Oklahoma (zone 3) and just added a dinning room (cathedral ceiling) 15 x 19 addition.  The addition was just opened up to the main house earlier this week, and we got hit with (what are for us) very cold temperatures.  Thursday night had a low of 12 F.  By noon on Friday we noticed that water was dripping down the north side interior wall  (along the drywall).  The drip lines appeared to be spaced every 24 inches, or about where a roof rafter would be.

The ceiling/roof construction was constructed with 2 x 8 rafters and insulated with R30 Roxul (rock wool) insulation and is not vented.  The interior ceiling is wooden tongue and groove car siding.   The roof decking is LP TechShield Radiant Barrier (with the metal foil side facing the interior of the house, as described on the boards) with a metal roof (there is felt paper in between the decking and the metal roof on the North side, but on the South side we used double bubble).   We did not have any condensation issues on the south wall.

We have spoken with over half a dozen different experts, and we’re getting as many different suggested solutions.  We are desperate to fix this problem and would greatly appreciate any help!  Thank you!
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