Jeff writes from Breckenridge: "I live in a mountainous (elev 9,700') region in Colorado. Cold winters, low humidity. I have a detached 2-car garage. I had a contractor add R-30 with 4mil poly on the ceiling, and R-15 with 4mil poly on exterior walls. I have been heating the garage to about 40 degrees with an electric fan heater. I have started to see some condensation behind the polyethylene vapor barrier. It appears that the condensation is starting on the roof joist/framing near the intersection of the ceiling and wall, and dripping down between the poly vapor barrier and the insulation (unfaced batts). I cut open the vapor barrier on the upper wall near the ceiling, and this has helped dry out the area somewhat. But my vapor barrier is now no longer sealed. It is confusing, b/c I thought water vapor would travel from warmer to cooler places. With that said, I'd think the condensation would be found in between the insulation and the outer wall, not on the inner (warmer) side of the insulation. My wall construction is 4mil poly vapor barrier- R-15 unfaced batt- OSB- Tyvek housewrap- cementeceous siding."
Answer: Are you sure you have adequate ventilation in the garage attic? Check to make sure your attic ventilation is not blocked and that there is an equal amount in the soffit as there is in the roof. Consider using RAFT-R-MATE® attic rafter vents at the eaves where the insulation meets the roof deck. You may also have excess warm air infiltrating into the attic from the garage via cracks or gaps around electrical and/or plumbing fixtures. For example, any a pipe, electrical wire or junction box that penetrates the ceiling should be sealed with spray foam or caulk.
You will need to do some math to calculate your ventilation requirement. The minimum ratio is 1/300. That means you need one square foot of ventilation for every 300 square feet of attic space. We recommend a ratio of 1/150.
Is the ceiling properly insulated? If the moisture is forming in one area, there may be a problem in that area alone.
