Innovative Insulated Panels
(Page 3 of 6)
October/November 2005
By Steve Maxwell
The top of erected SIP walls are crowned with a layer of 2-by-4 or 2-by-6 lumber, just like any stud-frame wall; the difference is that this wood is installed into a recess in the panel edges. You can specify that these be prepared for you ahead of time at the SIPs factory, or you can cut them on-site with a hot knife (see “Tools of the Trade,” below). The double top-plate strategy is used on both regular walls and the angled top of gable-end walls. Apply foam before each layer of wood is installed to ensure complete insulation and an airtight seal, fastening the lumber to the panels with nails driven through the OSB and into the sides of the wood.
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MOISTURE ISSUES
The incredible strength of SIPs is entirely dependent on the bond between the OSB and foam core. If the bond breaks down, then you’re in big trouble. In a stud-frame wall, water leakage around doors and windows will lead to rotten sheathing, but there’s still a wooden frame to provide structural support. Not so with SIPs; that’s why it’s important to make sure rain never seeps into the panels. A double layer of exterior weatherproofing is an idea worth considering.
Instead of relying on the questionable value of your building wrap as secondary water protection, install a waterproof membrane, such as Grace Ice & Water Shield, next to and below the corners, doors and windows. This peel-and-stick flexible membrane usually is used on roofs because it seals around nails, and the same holds true on SIPs walls, too. If the caulking, flashing or weather sealant ever fails, your all-important OSB remains dry.
Damage from exterior water isn’t the only worry. There’s also the issue of internal condensation occurring along panel edges during very cold weather. If warm, moist indoor air is allowed to enter these cracks, it can condense into water, which can cause rot problems.
“There is no question that moisture migration is a concern,” says Shawn Kelly of Thermapan, “but this also holds true for all types of construction. Most SIP manufacturers recommend that all interior seams be caulked with a flexible sealant and/or vapor-resistant tape after assembly. Also, all tight houses need the assistance of an air-exchange system to control humidity levels within the envelope.”
FINISHED ATTICS MADE EASY
Even SIP suppliers will tell you there’s no significant technical advantage to using panels on a conventional roof. If you’re happy with an empty attic space with fiberglass insulation between the ceiling joists, then trusses are the fastest, easiest and least expensive roof option. But if you question this standard design approach in favor of a livable, finished attic, then a SIP roof makes a great deal of sense. They eliminate the need for the usual sort of attic ventilation; offer exceptional energy efficiency; and eliminate the need for trusses and rafters. Also, the panels are so strong they’re self-supporting over most typical distances.
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