Innovative Insulated Panels
(Page 4 of 6)
October/November 2005
By Steve Maxwell
When conventional roofs are finished with drywall on the underside of the rafters, ventilation channels must be built into the structure to deal with internal moisture. Although it’s possible to combine conventional insulation with eaves-to-peak ventilation passageways, it’s not easy or efficient to build. If you simply stuff the rafter cavities full of fiberglass and cover everything with polyethylene, then you’re creating a potential moisture trap that will promote mold growth and seasonal condensation.
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SIP roof sections, on the other hand, become impervious to moisture when the joints are sealed during construction. They don’t need ventilation passages, and they’re strong enough to function without rafters. SIP roof sections are hoisted into place with a boom truck and set down on a bead of polyurethane foam at the bottom, top and panel edges. Washer-head screws secure the roof panels.
The main drawback of a SIP roof is the high temperatures that occur from sunshine hitting the roof surface. Because the roof is not ventilated, daytime temperatures can soar high enough to destroy conventional asphalt shingles in just five or six years. Fiberglass shingles, on the other hand, are much more heat tolerant than organic versions, and some brands have a 50-year warranty, even on a SIP roof.
MAKING ROOM FOR WIRING
Wiring is one of the main concerns people have with SIPs, but it really isn't a big problem. Check with your SIP manufacturer to find out the recommended way to install wiring; some can install pre-cut chases (a groove in a wall made to provide wire space) in the panel. Other manufacturers, such as the one I used, Thermapan, recommend simply cutting vertical slots routed through the interior surface of the OSB to cut-out pockets that accept outlet boxes and switches.
If your SIP manufacturer can include chases, make sure you specify the locations of your wiring. But if you choose to prepare the panels for wires yourself, start by cutting a plywood template to guide the travel of the router, speeding the operation. Vertical cuts in the OSB are completely safe and won't compromise the wall's load-bearing strength. Just be sure to avoid horizontal cuts because they greatly reduce a panel's strength. Extend slots to 2 inches deep, using a hot knife to remove foam, then install wiring and electrical boxes. Apply foam to the wire slots after inspection, then install interior wall surfaces.
BUILDING NEW TRADITIONS
Despite the technical advantages of SIPs, some people still feel hesitant about using them. This fact certainly made me stop and think before I chose SIPs for my own project. But if tradition alone were the way toward a better world, then all we'd need is more of what we do already.
To make the world truly sustainable, however, we need to change the way we build in order to conserve as many resources and as much energy as possible. To that end, resource- and energy-efficient building materials such as SIPs play a small, but important, part of what real-world sustainability looks like.
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