To SIP or not to SIP

What is the best way to create an insulated attic space in the cottage I plan to build next spring? I’ve been looking at using structural insulated panels (SIPs) for this job, but don’t know how or if these could be used to make a roof. - Andrea Tuck, Whitehorse

SIPs are typically made with two pieces of oriented strand board glued to each face of a piece of rigid foam. SIPs come 48" wide and in various lengths, from 8' to 24'. Although they’re most often used to build walls, SIPs also work outstandingly well for roofs over livable attic and loft spaces.
SIPs come in different thicknesses, the 81⁄4" size is used most often for insulated roofs. SIPs are strong enough to be self-supporting in many situations, so a group of 81⁄4"-thick SIPs can take the place of rafters and trusses. SIPs go up kind of like a house made of playing cards. All they require is a ridge beam to rest against along the peak while also sitting on top of exterior walls.

Except for the smallest structures, you’ll need to rent some heavy equipment to lift the roof panels and set them in place. Either an excavator boom truck or small crane are ideal. Bore 1"-diameter holes near the middle of each panel to accept lugs for hooking lifting chains onto. Before each panel is set in place, lay down a generous spread of expanding polyurethane foam on all joint surfaces.

SIP roofs are secured with long, specially made wood screws with large, flat heads. Drive these screws every 6" to 8" wherever the panel is resting on some part of the building framework. As an added precaution against air leaks along the eaves, cut angled strips of 2-by construction lumber to fit into the corners where the panels meet the tops of the walls. Then install these strips with more foam and screws. When you remove the lifting lugs, fill the 1" holes with more expanding foam, then trim the excess flush when dry.

SIPs can create open, energy-efficient loft spaces in ways that would be difficult or even impossible to complete if they were made with trusses. The only precaution you need to take is the kind of shingles applied to the roof. Since an SIP roof is completely unventilated, shingles get very hot during summer weather. Although ordinary asphalt shingles fail prematurely under these conditions, fibreglass shingles are heat-resistant enough to live out their full working life despite the heat.

Meet our expert
Steve Maxwell is a master woodworker