A friend owns a cottage and would like to insulate the roof. The problem is the limited amount of space underneath the ceiling, and the view of the exposed rafters he’d like to preserve. That’s what got us thinking about insulating from the outside, on top of the existing roof sheathing and underneath some new shingles. Is this possible? What’s the most economical procedure?
Yes, this certainly is possible. In fact, it’s an excellent way to get the job done. It preserves the present ceiling appearance, and delivers great thermal performance, without the need to fumble with a vapour barrier. There are a couple of ways you can do it.
The first is the method employed by timberframe builders, using a manufactured insulation product generically called stress skin panels. Plug this phrase into any search engine and you’ll find suppliers. Stress skin panels are factory-made insulating sandwiches of plywood or waferboard bonded to a 4, 6 or 8-inch thick layer of rigid foam. Order the panels to fit your roof, then hoist them up top and fasten them to rafters with long, 1/4-inch diameter bolts.
Another approach is to simulate the stress skin panel with site-assembled sheet goods and rigid foam. Layer at least 4 inches of extruded polystyrene foam on to the roof deck, then top it off with a layer of 1/2-inch plywood. The biggest challenge with either of these approaches is finishing the thickened eaves area. But that’s nothing compared to the hassle of covering an interior ceiling with insulation.