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sweating beams

gerretw's picture

sweating beams (post #185991)

gerretw on Thu, 03/11/2010 - 22:06
in
I did a job a while ago - it has a cathedral ceiling with a 6 x 12 beam at the top, and 2 x 10 roof rafters hung from the sides of the beam. There is R-30 batts between the rafters. The bottom of the joists are covered with 5/8" drywall. The bottom of the beam is exposed. 4/12 pitch.

So, drops have been forming on the bottom of the beam which drip off to the floor. The same situation occurs in another room which has a 4 x 12 beam. My theory is that the beam gets cold at night - it has been in the high 40's / low 50's at night here in LA - and the beam holds the coldness and water condences from the warm air in the morning onto the beam.

The drips do not occur during rain, so that is eliminated as a posibility. They also do not form in the summer - it is a winter occurance.

Am I correct in my assumption, or is there another possibility I am not thinking of?  If you look at the attached photo, you can see the water glistening where the drywall meets the beam.

Looking forward to your comments.
G

-A +A

ha ha (a subject box in the reply)............. (post #185991, reply #1 of 6)

by calvin in reply to gerretw [original] on Fri, 03/12/2010 - 03:18

I tried to look closely and the picture is real good, thank you.  I see the droplets but I also see slight openings between the drywall and the beam.  Is it possible this moisture is dripping down from / through those openings.  If that is the case I would think this is  formed by the warm moist air escaping through the crack, condensing  and then dripping back through.

Is there visqueen above the drywall?

Is the roof vented?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 

Certainly it's condensation.  (post #185991, reply #2 of 6)

by DanH in reply to gerretw [original] on Fri, 03/12/2010 - 05:59

Certainly it's condensation.  Calviin's correct to wonder whether it's occurring on the exposed beams or instead up in the ceiling -- I'd concur with his suspicion.  A 12-inch beam shouldn't be cold enough on the bottom to cause condensation, even in Minnesota, much less LA, so likely hte moisture is dripping down from above.

Your solutions are to either lower the humidity in the structure or somehow keep humid air away from cold surfaces.  The most obvious possible fix is to caulk the joint between drywall and beam, but do that only if you're confident that there are no other air leaks into the ceiling, as otherwise you'll only trap the moisture up there.


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

I didn't see you mention a (post #185991, reply #3 of 6)

by Scott in reply to gerretw [original] on Fri, 03/12/2010 - 15:05

I didn't see you mention a vapor barrier. Is there one?

 

We've got catheral ceilings throughout our upper storey, and I can tell you that the vapor barrier work was no fun to do, but it had to be done and done well. Otherwise I knew I'd be facing stains in the drywall and moldy lumber.

sweating beams (post #185991, reply #4 of 6)

by gerretw in reply to Scott [original] on Sat, 03/13/2010 - 08:20

No barrier other than the roofer's paper.  I had never heard of putting a moisture barrier on the roof - I only used Tyvek on the exterior walls.  We are going to caulk it and see.  I expect there is a gap in the insulation at the ridge and when the warm air rises into that gap and hits the cool air then the water condences on the beam.  It seems closing up the gap between the drywall and beam will stop the warm air from migrating upwards. 

Thanks - G

Is that the only migration source? (post #185991, reply #5 of 6)

by calvin in reply to gerretw [original] on Sat, 03/13/2010 - 10:23

It seems closing up the gap between the drywall and beam will stop the warm air from migrating upwards. 

Are there any other penetrations in the lid?  Can Lites or something like that?

Any soffits that are below that might have cans?

Chimney chase around a fireplace insert adjacent to the vaulted ceiling?

Basement or crawlspace below-with wiring, duct, etc penetrations into any perimeter walls?

Many places for warm moist air to migrate-finally collecting at the top, with basement or crawl providing much source for potential water vapor.

If you seal that location up at the beam, you may still have the problem-only this time it's been damned up and forgotten-until something more drastic happens.

Best of luck.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 

This may just be a localized (post #185991, reply #6 of 6)

by davidmeiland in reply to calvin [original] on Sat, 03/13/2010 - 11:22

This may just be a localized issue. The warmest air in the room is migrating up to the beam, some of it is moving thru the gap, and the resulting condensation is dripping back thru. The cold lines along beams like this are easily visible with an IR camera. As you say there might be other paths for interior air to get into the ceiling and up to the beam, and these should be sealed if possible, although some will be difficult or impossible to address completely.

If he seals the gap, and there is another source of warm wet air, a bigger problem might develop and this should be monitored after the fact. Probably easiest to use a pinless moisture meter that can take reading thru materials like drywall. I have been using a Protimeter Surveymaster and it's a great tool to have. I know at least one guy who embeds USB thermo/hygro sensors in wall and ceiling cavities and retrieves them later to download the data. It just depends on how bad you wanna know!