Condensation on Vaulted Ceiling - Pole Barn Style Framing 1

[RFreund]

I might be posting this in the wrong spot so feel free to redirect me.

I looked at a townhome/condo building where a resident was complaining of water dripping from their roof beams, ceiling and top of walls. The framing is similar to a pole barn where there are large roof beams with 2x4 or 2x6 purlins spanning over the roof beams. There is sheathing over the purlins and shingles on top of the sheathing. Gypsum board (aka gypboard, drywall) is fastened to the bottom side of the purlins. There is evidence of water dripping from the beams and water staining on the wall. They notice this happening in the spring and during the summer during hot days. I believe they have also had issues during the winter, but not as bad.

Here are my thoughts:
Warm moist air is condensing on the cool surfaces - ceiling and the beams. During the summer the outside air is humid and the inside air is cool and should be drier than the outside air. So this means that either:
[ol 1]
[li]There is warm moist air inside the house that is rising to the ceiling and condensing on the beams/ceiling or[/li]
[li]Outside air is getting inside through the roof[/li]
[/ol]

#1 Seems plausible because the return air is about 6' lower than the ceiling.
#2 Seems odd to me because I would think that since warm air rises, the air would be leaving the home. Not entering it. Also if this is the case I would think the air would be condensing on the exterior side of the gypboard (which it may be doing).

As for the solution - I'm not sure there is an 'easy' solution. There is really no way to vent the space between the drywall and sheathing due to the nature of the framing (2x4 @ 24" oc perpendicular to the beams). Therefore, I'm thinking removing the drywall and then using closed cell spray foam insulation. Then replace the drywall.

Any thoughts?
Thanks in advance!


EIT

http://www.HowToEngineer.com

 

[Teguci]

Where is the insulation and the vapor barrier? I would think that the air inside the house is cooler and dryer than outside and the condensation happens when hot moist outside air is trapped (mostly near the eaves) under the roof and then condenses at night. Attic space with proper ventilation allows this hot moist air to equalize with the outside and not condense on the underside of the roof. Attics without ventilation get plywood/underlayment that is soaking wet.

Could try cutting in some exhaust vents near the peak of the roof.

 

[XR250]

Somehow the roof framing is getting to the dew point temperature of the inside air.
I can't tell from your post what seasons this happens and what geographical area you are located.
If the unit has a crawlspace, check to see if it has a vapor barrier.
What is the relative humidity inside the house when this happens?

 

[Archie264]

Joseph Lstiburek is the guru on this topic. His website and youtube are replete with his discussions of the topic. This lecture is eyeopening:

https://www.youtube.com/watch?v=rkfAcWpOYAA

The whole thing's worth watching but you can fast forward to about the 17 minute mark for your situation.

In short he makes a compelling case that moisture moves from hot to cold and the location of the vapor barrier -- or materials that function as vapor barriers -- needs to be carefully thought out.

 

[BridgeSmith]

Sounds like you are likely on the right track with moisture condensing between the sheathing and the drywall.

I'm not at all familiar with residential detailing or construction, so this may be a lousy idea, or there may be lots of better ways to accomplish the same thing, but I'll just throw this out there:

One possible solution is to drop the drywall and add furring strip across under the purlins to provide a path for air movement between the purlins and new drywall up to the ridge, and then vent it at the ridge.

 

[RFreund]

Thanks for the replies!

A couple follow-up responses and a sketch of the roof framing.

@Teguci - I'm not sure if there is a vapor barrier. Attached is a sketch that shows my understanding of the insulation installation.

@XR - This is happening when it is hot outside spring/summer (no water when raining). We would probably need to do further testing to determine the actual RH when this happens, but it was around 45% and outdoor humity was around 65% on that day. This occurs in southern WI (zone 6 I beleive).

@Archie - I'll look into it.

@HotRod - That was also a thought I had.

I'm just not sure if there is any other solution than to make that an "insulated roof panel". But maybe it's not worth it.

Section:

EIT

http://www.HowToEngineer.com

 

[azcats]

Are there several units in the condo complex with the same problem? I'm wondering if the homeowner has habits that lead to excessive indoor humidity like drying clothes indoors or use of humidifiers. Or perhaps an undiagnosed plumbing leak? Or other mechanical problem that leads to AC not pulling out enough moisture or bringing in too much outside air?

There was once an office that we occupied in which the AC unit pulled in too much MUA. When it would get humid outside the unit couldn't remove all the moisture and it would get really humid in the office. Then the water started to condensate inside the ductwork and saturated the fiberglass ductwork and started leaking/dripping into the office space.

 

[dhengr]

RFreund:
I would really read, study, listen to what Joe Lstiburek has to say on this subject. As Archie264 suggests, he has really made a career out of these issues, and knows what he’s talking about. The comment that… “framing is similar to a pole barn,” and assuming that some of the finish detailing is too, to fit and match, doesn’t exactly connote air-tight, moisture vapor resistant construction. I would open the ceiling and walls up in a few places where there are problems to really get a handle on the actual construction and detailing. Right now, that ceiling is not vented to relieve any moisture vapor build-up and the dew point is happening someplace up around the underside of the sheathing. The roofing and sheathing heat drastically during the day, and cool almost as drastically (quickly) at night, while the insul. and everything below changes much more slowly, and this causes the condensation. Where any vapor barrier should be, and where any building sealing/caulking, etc. should happen depends on where the moisture vapor is coming from. It looks like they got by ‘on-the-cheap’ with the bldg. design and process and now they get to address the errors of their ways, which may not be so cheap. If the problem is only happening in a few locations, I would study roof orientation, and look for poor construction and finishing details which are allowing moisture vapor movement. This can be happening from the living space into the ceiling construction, or from outside into the ceiling cavity.