Last year an addition was added to my house. We wanted vaulted (cathedral) ceilings with exposed rafters. The builder put up normal rafters, decking and had blown in insulation with sticky stuff in it applied to underside of the roof decking. The sheetrock was then attached to the stringers which were even with the insulation. The end result was a non-ventilated (no air space) insulated roof. The addition is over a non-insulated ventilated crawl space.
The problem occurs every spring, early summer when it warms up. The moisture in the roof condesates and drips through the sheet rock. The first year the builder said it was due to the crawl space not being dry enough. We haven’t had any rain over 1″ since August of last year this year though.
What can be done to prevent the problem short of replacing the non-ventilated system with a ventilated one? Would gaps between the sheetrock and rafters exasperate the problem?
Replies
Dear Mike,
-last year an addition was added to my house.
-problem occurs every spring, early summer...
-first year the builder said...
I'm confused, but it sounds like a nasty flashing leak. You won't see roof condensation in warm weather ever unless there is a leaky a/c duct.
Close the foundation vents. You have the soil covered yes?
Yes, gaps in the drywall will help to let rain water drain out the roof, but that's all.
Regards, Fred
[email protected]
Thanks for the reply Fred; I'll try to answer your questions and clarify some things. First I don't think it is a roof leak since it doesn't happen when it is raining. Second the crawl space soil is not covered with plastic (I would presume this would need to be special type). Wet soil is not a problem in my area; (no lakes, streams, 150 - 200 feet to groundwater) but maybe some water could enter through holes in the foundation for the electric lines. They are sealed but maybe not tight enough.
I have one question though.
Supppose moisture were to enter the crawl space when it was cold. When it warms up would it dry out and exit with the air entering/exiting the crawl space or would it turn to vapor, go through the floor, through the cracks in the drywall next to the exposed beams, travel through the insulation and maybe condensate on the metal plates on the trusses. I can't quite get this scenerio to work since the condensation I'm familiar with occurs when it is cold.
Would the trapped air in some crevice or something up in the rafters be cold enough on a 85 degree to cause condenation? (Wild guess)
I guess I think something is happening with the crawl space/ non-ventilated roof but I don't know what. Thanks for your information and let me know if any of this makes since.
I don't know why contractors today still don't ventilate a roof. Your first and biggest problem is no ventilation in your roof. Your contractor was right about the crawl space being to wet, he should have addressed that when it was built with some 4 or 6 mil poly. The cause is relative humidity every thing in your house releases humidity you when you breath, washing dish's, laundry, showering and so on.
This is what is happening with your addition, all of the moisture in your house is going up to the roof ( the chimney effect ). At night when the temperature drops outside the temperature of shingles and plywood also drops and the relative humidity in the roof cavity gets to 100%. When that happens the moisture condenses on the under side of the plywood and in the insulation. Over a period of days there is so much moisture that the insulation will not hold it any more and that is why it is dripping through the drywall. In a roof that is ventilated the hot air with all the moisture rises out the ridge vent and pulls drier air in behind it from the soffit vents.
What you have to do is get rid of the moisture. Exhaust fan when showering, poly in the crawl space, run an air conditioner to take the moisture out of the air, anything that might help, and/or talk to your contractor and get him to fix the problem ( remove insulation and drywall replace with new and ventilate it properly
Once insulation gets wet it will not perform as well and can get wet enough to need to be removed and new installed, You might be at that point soon if you don't do something. One problem with the whole situation is the possibility of mold there are some mold's out there that can make you sick but those mold's are rare and I don't want to scare you. Two other reasons for venting a roof are; plywood even though it is extior glue it can't take that moisture over long periods of time and it will delaminate also with no ventilation the shingles will heat up faster longer and hotter and will result in premature failure of the shingles.
All roof's need to be vented with exception of one and that is when spray foam has been applied to the under side of the sheathing ( there is now where for the moisture to condense )
When I do a crawl space I put down some poly and a dust cap ( 2 inch's of concrete with a bull float finish )
Edited 4/4/2002 9:16:03 AM ET by RBROWN321