Q: I was reading one of your postings recently that had to do with roof venting. I have a question that you may, or may not, be able to help me with. We have a home in Raleigh, N.C., built in the ‘30s. It has a beautiful slate roof. It underwent major renovation and additions about eight years ago, all of which also had matching slate applied. Well, on these hot days, the upstairs in the original part of the house has trouble cooling. I noticed there are no powered through-the-roof fans as we had in our previous home. It gets mighty hot in the attic as there does not appear to be any ventilation up there. Do you recommend these powered roof fans to attempt cooling the attic? The heavy-duty slate roof potentially complicates any type of installation I would think.
A: Attic ventilation is important for most roof attic areas, but slate roofing is different. Slate is a natural stone material that is quarried and then split by hand. A quality slate shingle should last for more than 100 years with proper maintenance of the fasteners and flashings.
Because slate is a natural stone, the heat and humidity normally associated with an attic will not harm the slate. Your problem appears to be one of cooling an upper floor area or finished attic space. Cold air is denser than warm air, and it is a challenge for the furnace blower to push the dense air any higher than one story. If your furnace fan is located in a basement or cellar of any two-story home, it will be difficult to cool the upper floors with just the one unit.
Solutions range from adding another unit for the second floor, to rearranging the ductwork to include a zoning system, to adding additional insulation to the attic area. The high cost of adding a zoning system often includes replacing the ductwork and the furnace and fan system because the fan will be required to work harder and longer. Adding a spray foam insulation to the underside of the roof slats (slate shingles often use slats in place of decking) will reduce the heat gain of the attic space. Because slate shingles are held in place with metal fasteners, some experienced roofers hesitate to recommend foam insulation. They are concerned that any possible moisture accumulation could cause the metal fasteners to prematurely fail. A sure solution would be to install a second cooling unit for the second floor or finished attic space. A second unit could be a window air conditioner or a split system unit, which uses a ground-based air conditioner and a window or wall-based fan system. No matter what you choose, make sure the finished area is well insulated to meet local standards and that the conditioned area is properly air sealed.
C. Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Reach him at d.Barnett@insightbb.com.