View Full Version : What would cause a wet spot on ceiling (no rain)?

04-17-2002, 07:08 PM
For the past 2 years, I have a wet spot (condensation?) that develops on hot/sunny days and somewhat dries during the evening. It doesn't do this in the winter and rain hasn't nothing to do with it. Oh, I live in the south. Anyway, this occurs on mainly 2 of the 8 triangular drywall panels that form this 12-13' cone shaped ceiling (follows the roof line). Could it be that insulation baffles weren't used? What if air leaks from inside the house into the space behind the drywall (the peak of the ceiling where those 8 triangles meet isn't sealed well at all)? Could there be something damp (insulation) behind the drywall from an old roof leak or something like that? Would any of this cause the condensation? I have a picture on the link. I'm obviously not a professional and I appreciate any help. I called an insulation contractor and drywall contractor locally and neither sounded very confident of the source of the problem. Thanks.

Ceiling Pic (http://purvis.engulfed.net/Ceiling.jpg)

04-17-2002, 07:30 PM
Good picture. Have you inspected the roof at the peak? Are there any water pipes or air conditioning ducts running in the area of the spot? Moisture from air leaks would show up in the winter, not summer. Solar-driven moisture from damp insulation would be a possibility, but eventually the moisture would be driven out. That ceiling and fixture look old enough to have been built before the days of insulation ('70s), by the way.

Bob H
04-17-2002, 07:54 PM
Perhaps a starting point might be consulting with a "building professional" or an engineer.

Glenn A. Davis PE
04-17-2002, 09:46 PM
Bob H

Dang Bob. There you go again. What are you getting at?


Mike OHandley
04-18-2002, 12:05 AM
Hi Glenn,

I guess we should feel priveleged. Out of 14 forums on this site, we're only the 2nd that has ever been graced with Bob's razor-sharp wit and obvious command of the written word.

We are indeed beneficiaries.


Mike O'Handley

04-18-2002, 06:57 AM
I've inspected the roof at the peak and it looks OK. Oddly, I have another ceiling like this in part of the bedroom and it has never had a problem (the roof is more shaded there though). There aren't any pipes, etc that run through those cavities. There is insulation there as I can see it through the soffit vents. I don't see any insulation baffles though (would it matter)? I'm thinking of putting a vapor barrier paint over the area and sealing up the drywall at the peak/center. Do you think any of this would help? Thanks.

Mike OHandley
04-18-2002, 07:41 AM
Hi Chris,

What kind of upper ventilation have you got there? Looks to be like warm air is moving up into that roof plane and then condensing behind the drywall. Since you say it happens only in the summer and you're in the south, shall I infer that you run your A/C all say long in summer?

Two ways this could be happening:

1. Warm outside air is moving into the soffit vents, rises toward the peak and then can't get out because there is no upper vent or because the framing members come together and essentially form a box to trap it. The warm air condenses on the backside of the cold drywall (A/C on)and soaks through.

2. Insulation has been stuffed into those pointed rafter bays, there is no ventilation at the top and air leakage from the inside moves through the unsealed ceiling plane during the day when it's hot and is trapped there. At night, once the temperature drops to dew point, the air trapped above the ceiling condenses in the insulation and drips down onto the ceiling and soaks through.

Since you say this only occurs in summer, my uneducated guess would be the first scenario. The same thing often occurs along the wall/ceiling intersections when there are soffit vents but no baffles and the insulation is in contact with the underside of the roof. Cold night air cools the roof and then causes condensation to occur wherever the insulation is touching the roof skin. It drips down onto the ceilings and staining occurs.

My opinion. Worth price charged.


Mike O'Handley

04-18-2002, 09:05 AM
I appreciate your offerings. I don't think warm air can enter the soffit vents and rise to the top because it appears (looking through the vents), that insulation is stuffed into those cavities. It seems they would obstruct airflow and I don't see any baffles. Is that not correct? Is the solution pulling down the drywall and installing baffles? That would still leave air trapped in the top where the rafters meet -- right?. Is there anyway to vent those triangular cavities? Is sealing the peak where the drywall comes together worth trying? This was obviously a poor design but it seems to be fine on the section like this in our bedroom. Anyway, what should I try? I would like to know and understand the solution before I call anyone to fix it. Thanks once again.

04-18-2002, 05:29 PM
Chris, where in the south do you live? Have you noticed any correlation between the outdoor relative humidity and the water spot? What is the solar orientation of the water spot?

Are you sure rain has nothing to do with it? A roof leak at the peak receives much less water than a leak lower down. One scenario that occurs to me is a roof leak which is just bad enough to wet roof sheathing and maybe framing but not drywall; then the sun comes out, heats up the sheathing and drives the accumlated water to the drywall, where it condenses and runs down.

And you didn't answer Mike's question: What kind of upper ventilation have you got there? What is the peak covered with? You can vent the peak with a cupola if necessary.

How about letting a water hose trickle continuously at the peak on a cool day to be sure the roof is OK.

04-18-2002, 06:59 PM
I live in GA. It has been mid 80s all week. Every day for the last week and a half, I've seen the spot. I took the pic in the original post yesterday. It hasn't rained in days. I also don't see the spot in the winter when it's cool. The spot faces East.
Anyway, I don't have any upper vent. Is that what I need to do? Thanks.

04-18-2002, 08:54 PM
I wonder if you are missing insulation in this area and the cool air(air conditioning) is condensing moisture on the hot area at the top of the roof. Is the spot actually wet or is this a discolaration?

If your soffits are filled with insulation, you are not getting air flow up to the peak.

I think the conditions that Mike O'H described are the probable causes.

04-21-2002, 08:29 PM
I have to agree with SDA with these additional comments:

if the insulation contractor happened to be using fg 16" o.c. I bet if you measured where the "V" shape of the moisture stain meets the outside edge of the triangular shaped wall panel it will measure 16" across. My hunch is that above this "line" there is either no insulation or the fg contractor stuffed it so tight that it effectively isn't there and here is how to confirm this without tearing the place apart (you might have to do this anyway).

I've posted a link to an organization that has trained professional infrared thermographers (Bob H - these guys might be considered a subset of professional engineers) and they should be able to recommend one who is certified in your area.

Crank the a/c up (temp down) as high as it will go on a hot day and have him use the infrared camera to document where you have missing insulation. Retrofit accordingly.

Suggestion: in the future anyone going to build such a complicated framing structure that is nearly impossible to insulate and ventilate effectively with traditional methods might want to look into using a qualified spray foam insulator...

Good luck - hope this helps!

John Snell Infrared INstitute (http://www.snellinfrared.com/)

05-03-2002, 09:12 PM
I've seen this happen on my mother-in-laws vaulted ceiling with a light hanging off a chain as well. Apparently the warm moist air in the attic was condensing on the cold metal from the lights canopy at the ceiling level. She keeps the ac cranking all summer. The ceiling box let in a lot of air around it. After caulking the gaps around the ceiling box and stuffing a little insulation where the wires came into the light box the problem went away. She also only had the problem in the summer. I got up there to check it one day and the top portion of the chain was a little rusty and damp. That's when I figured out the problem. This might be what's happening to you.

Kenny Mac
05-07-2002, 03:50 PM
Duck Leakage!Duck Leakage!Duck Leakage!Duck Leakage!Duck Leakage!

As SDA explained or maybe the airhandler is exposed to the extreme temps and moisture brought in from the carness.....
RE: "Joes Top Ten Stupid Things to do in the South" AT "Building Science.Com"

Or maybe a leak or your primary condensate line might be clogged up.

And crazy as it seems the refrigerant lines insulation maybe be missing.

With all due respect, on your next home "DO NOT LET THE HVAC GUYS PUT THE HVAC OUTSIDE OF THE CONDITION SPACE!!!!!!!!