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TOPIC: Condensation Issues

Frost under sheathing, damn!

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Hi everyone, This summer I had a new standing seam roof installed. The home has cathedral ceilings with 2x10 rafters. The makeup of the roof goes like this; drywall, plastic, 6" foam, 1" air gap, 3/4 plywood, 15lb felt, standing seam roof. Now this simple gable roof has only a 4/12 pitch, and ventilation is achieved through soffit vents at bottom, with a continuous ridge vent at top. I noticed yesterday that icicles where forming behind the fascia board, and after further investigation found that condensation is occurring directly on the underside of the plywood. The temp throughout the house is 68-70, humidity is 30. outside the temp is 29 and humidity is 70. in our small attic space the temp is 39 and humidity 28. I am at a loss, my wife is even more upset, as we spent a lot of our money hoping to finally do away with our shingle roof and its ice damming issues. Any help would be appreciated. The underside of the plywood is covered with frost and mold. Thanks in advance, Mike
Guest User

12/6/2010
Does the gable roof have the same foam insulation or is it a more traditional attic space?
Eric Novotny
An informed customer is the Best Customer!

12/6/2010
One side of the roof is insulated with foam as I described, this side is 25ft X 45ft, all cathedral ceiling. The other part is 25x15, attic crawl space (3ft) insulated with 24" of batts over the ceiling. All the plywood visible from the attic is damp/frosty/mold covered. I just went outside and removed the soffit while waiting for my contractor to call me back and there is ice formed behind the fascia board and looks to be going under the metal roof. ?????????? -Mike
Guest User

12/6/2010
The roof with the attic is the section that is frosting, correct? The 24" batts are doing nothing to control the migration of warm moist air into that attic and are creating the moisture/condensation issue you are having. Adding ventilation will certainly alleviate the symptoms of the problem but not the issue at hand. That attic space needs to be air sealed and all the envelope bypasses need to be corrected. Keeping the warm conditioned air from making into the attic will eliminate the the moisture source and issues. You may need to treat the plywood with a mildewicide to kill the mold but it should not spread any further once you control the source. Contacting an energy auditor or a specialized insulation contractor should solve your problems.
Eric Novotny
An informed customer is the Best Customer!

12/6/2010
I spent some time crawling around in the attic this afternoon and discovered that the insulation under the ridge vent was soaked. I'm thinking that the ridge vent may be the culprit. The mold is concentrated from the top down as well as the frost. my guess is that the sheathing has been getting wet since summer. I have calls into the contractor and will be getting some more eyes on the situaltion. Thanks.
Guest User

12/6/2010
That does not mean the leak is from the ridge vent. It is very possible that the warm humid air is migrating up and coming in contact with bulk cold air at the ridge vent and condensing there. Get an auditor out there to take a look at the space and do some blower door testing on the home. That will tell you how much connection there is with the rest of the home and what likelihood of the building to attic connection being the culprit. If you just have your roofer look at it, you could be trying to fix something that is not the source of the issue.
Eric Novotny
An informed customer is the Best Customer!

12/6/2010
Hello, I'm on the other coast, about an hour west of Seattle, but am experiencing similar issues with a very similar cathedral roofing system. The house is a new construction monitor barn style. Two stories in the center, two side extensions that join the main house half way up the second story. All roofs are cathedral. Metal snap-loc style roofing, felt paper, OSB sheathing, 1" air space, 6" of ridgid foam, forced tight and well sealed, all above the finished ceilings. Some ceilings (kitchen and bath) are drywall, the rest are 6" wide T & G. This is the second year the house has been heated during the winter, but the first with folks living in it, cooking, bathing, creating moisiture, etc. Temps were below freezing for the past few weeks. Heavy morning frost, humidity above 70 percent outside, not much rain, etc, but as soon as the first warm day came we got water dripping randomly out of the T & G pine boards. Mostly on the side facing east. Could be the west side never warmed up enough, since it stays colder and is more protected by trees, and the temps didn't stay warm for long before dropping again.... Wasn't a lot of water, but enough to concern me, since I think it must be interior warm air freezing under the sheathing and then dripping back on the ceiling once warmed. Every rafter bay has full soffit vents at the bottom and and exit venting at the top (ridge and side wall both) according to the roof manufactures instructions. Basically a 1" cut in the OSB sheathing to let the air out and under the flashing. One thing to note is that when the building inspector was told I'd be using a full 6" of ridgid foam, he did not want a plastic vapor barrier between the pine and foam. He was worried it would act as two vapor barriers, trapping air between them. We did tuck tape the seams. Also, this foam was wedged in pretty tight to the rafter bays and beveled to the roof pitch to really fill those rafter bays, leaving just the 1" void for Venting (6" of foam into a 2x8 rafter). Need some advice, short of tearing out the pine ceiling to ad a plastic vapor barrier or sheetrocking over the T & G to act as a better barrier.... Would raising the temp in the house, when the outside temp drops below freezing (usually only a few weeks each winter here), hurt or help the situation? Thanks Brian
Brian Henry

1/5/2011
Your building inspector was incorrect in this application. The thing to do would have been to apply a full, unbroken vapor retarder and not tape the seam in the foam so that any moisture that did escape past the vapor barrier would have an opportunity to dry to outside. You are in pretty much the same predicament as the above poster. As much as you think the foam was applied tight in the rafter bays, it is not air tight by any stretch. The soffit ventilation is not enough in this case and is unfortunately right in between the sweet spot where condensation is forming. To the extent that it would foster convection air movement, more heat loss would drive more make-up air to enter from the eaves. You are right in the middle area where condensation is forming. You can try to lower the humidity and see what that does. Are there any recessed light fixtures or penetrations in the T&G where the condensation is showing? I would also like to see what the connectivity between the two other elevation to the center section is like before making a final scope recommendation. Removing the T&G is a real PITA as well as getting the vapor retarded applied properly and sealed. Attach or email me some pictures and I will make more specific recommendations. [email protected]
Eric Novotny
An informed customer is the Best Customer!

1/5/2011