QUESTION: Due to condensation in my attic in the wintertime, I had vents put in the soffits, an attic fan put in with a humidistat, and more insulation installed (R-30 on top of the R-10 or so that was already there). The roof is only 10 years old (prior owners had it done) and has a ridge vent. But I believe the area for air to escape should be two inches or so on each side of the ridge beam (top of the roof). Mine has only a half-inch or so.Do you have a suggestion as to what relative humidity the humidistat should be set? In the attic is a small dormer area of 10 or 12 square feet. In this area, I still see three or so roofing nails with a little moisture on them, despite all the improvements I mentioned. The moisture is so little that I thought I could ignore it. What do you think?ANSWER: For natural ventilation, the rule is one square foot of vent area for each 300 square feet of attic area if there is plastic vapor retardant in the ceiling, and one square foot of vent area for each 150 square feet of attic area if there is no retardant. Half the vent area should be near the roof peak and half at the eaves. Baffles should be used to ensure that eave vents are not blocked by insulation, and that wind does not enter the insulation, reducing the R-value.

Have questions for Alan J. Heavens? E-mail him at aheavens@phillynews.com or write him at The Inquirer, Box 8263, Philadelphia, PA 19101.