Condensation Under Vapor Barrier

Topic: 
Insulation and Energy
Question: 

I have condensation (water drops) on underside of plastic vapor barrier in attic on top of 2x6 attic floor beams. When I put new installation in attic floor, I apparently mistakenly rolled out a continuous plastic sheet and shoved down in cavities between beams putting installation bats on top of plastic. I stapled plastic on top and sides of 2x6s. How do I get out of this problem? I also started to put plywood down on top of beams for a flooring. Every one is telling me I should have put bats down first and plastic second. I don't get it. I have forced hot air heat with a built in humidifier - won't the bats get wet if I reversed? Can I slit to plastic only the tops of the beams to get the moisture out. lay plastic on top.. or just put the flooring down pressing down on tops of the beams? I don't want to redo job if I can help it.

Answer: 

From your address and the time of the year of your questions (winter), I am assuming that the condensation is occurring during cold periods. This suggests that the heated air on the inside of your home has a moisture content and dew point that is higher than the temperature of the plastic vapor barrier in the attic. To solve this problem I would suggest the following steps:

Inspect the house for sources of elevated moisture: wet crawl spaces and basements, faulty furnaces that vent moisture and carbon monoxide into the house, un vented bathrooms, high humidifier settings... Make sure that the vapor barrier is on the warm (inside) surface of the insulation. Eliminate air spaces between the vapor barrier and the wall surfaces and the insulation. Avoid double vapor barriers - is the plywood acting as a second vapor barrier? Are portions of the vapor barrier exposed to cold air? Is the insulation on top of the vapor barrier uneven?

As for your desire to solve this problem without re-working the attic, let me stress the importance of eliminating the condensation problem. Such ongoing condensation conditions are common factor in major repair projects. Such condensation often results in framing damage.?

Based upon your description, my best guess is that warm air travels up the sides of the 2x6 ceiling joists, and that the temperature of the vapor barrier at that point is bellow the dew point.

Thank you for using the Express Service and for supporting The Sound Home Resource Center.

George