What is the most current “best practice” solution to the type of underlayment between SIP and the selected siding.
The standard 7/16″ OSB sheathing on the SIP panel is bound to be moisture sensitive. What is the most effective underlayment or barrier to ensure adequate air flow between the selected siding and the SIP unit? Also: Which siding options (vinyl, cement board, etc.)can be fastened effectively to the OSB, given the absence of studs in the structure, which, if they existed would provide addequate support for applying almost any type of siding.
Some options include, Tyvec, 15lb or 30lb felt, “Home Slicker” plastic mesh, Cedar Breeder, or some combination???. Matching the long term “health” of SIP panels (primarily the OSB), to Siding manufacturer’s requirements appears to be a real challenge.
Any guidance is appreciated.
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Replies
Norman,
If you want "adequate airflow" between your siding and SIP, use vertical furring strips, Home Slicker, or Cedar Breather to create a rainscreen.
Either asphalt felt or plastic housewrap will work fine as a WRB. Your installation details (lap details, flashing, etc.) are more important that which type of WRB you choose.
Your concerns about OSB's ability to hold siding fasteners is probably misplaced. If you have further questions on that issue, you should direct your questions to your SIP manufacturer.
We do engineering and the extra construction detailing required for SIP and other alternate building systems. So we have tried to keep up with the technology improvements as they come to market.
The good thing about SIPs is that if properly installed with attention to flashing and roofing details, moisture can only happen on the outside of the wall panel and hopefully only on the outside of the house.
Today's APA rerated structural OSB has water proof glue and is a different product than what was sold in the past. That same OSB type is used by most SIP OEMs. But you should check with the SIP OEM.
With the right OSB/SIP wall behind it, use a basic type membrane 'home-wrap' or alternate 'drain-wrap' products from companies like DuPont.
Either of these will work with cement lap siding and cerment vertical paneling. Each have an air gap behind them. The lap siding gap runs horizonal with each piece. But be aware of limitations of top nailed lap siding in wind storm zones.
The vertical paneling is molded with a small offset provide a small air gap. You can also install verical paneling with a bigger vertical 1/4" or 1/2" vertical gap with a vertical 2" galvanized galvalum colored flashing behind each joint. Can take additional nails for higher wind loads and looks contemporary.
In the event that moisture condenses in the air gap behind the siding, it should last only as long as the consitions allow it to remain as liquid water, then vaporizarion/dry out or breathing will occur as soon as the temperature rises or the outside RH drops.
This should happen with either sidings of the above over a one of these breathable membranes on a SIP wall.
Gary: Thanks for sharing your research. Very helpful. You do raise the "second" issue concerning the holding power of 7/16" OSB in "high wind zones". I have seen only one company, Certainteed, warranty its cement board product ("Weatherboard, applied with screws, not nails") for 140 mph winds without insisting on a 3/4" penetration into the basic SIP framework. Is your caution concerning top board nailing (as regards wind resistance) based on anecdotal evidence or on your gut feeling?
Most manufacturers that I've encountered will not honor their vinyl or cement board lap siding unless it is nailed to the studs (which, of course are non-existant in the SIP's) or furring strips (not a practicle solution). Incidentally, is the home wrap you refer to a Tyvek type product?