I am having a difficult time finding information on exterior cladding options for SIPS construction.  What is acceptable and what to stay away from.  Can anyone point me in the right direction?

Posted By okeydokie on 12 Oct 2010 10:20 AM
I am having a difficult time finding information on exterior cladding options for SIPS construction.  What is acceptable and what to stay away from.  Can anyone point me in the right direction?

Its all good, you name it we've done it!............ Brick, stone, vinyl, stucco, hardie, synthetic tile blocks

so screwing a heavy substrate like cement board into the OSB facing then coating with stucco is not too heavy of a cladding system? i was concerned that weight of the cement board/stucco combination might not pair well with OSB

Posted By okeydokie on 12 Oct 2010 04:24 PM
so screwing a heavy substrate like cement board into the OSB facing then coating with stucco is not too heavy of a cladding system? i was concerned that weight of the cement board/stucco combination might not pair well with OSB

I would not use cement board, I would use wire lath and 7/8" cement stucco. there is no concern with weight which is all in shear and can be applied to OSB  or metal skins

you wouldnt use cement board for the reason i stated? too heavy, not enough attachement support with the OSB?

The mechanical support is one issue, but protecting the OSB from moisture deterioration is another. OSB is more sensitive to moisture than plywood, and it's worth taking measured to ensure it dries quickly should it ever get wet, and limit the wetting potential.

The most reliable approach to keeping it dry under any clading is to design in a "rainscreen" , which means the cladding gets back-ventilated with an air gap between the cladding & OSB, with both bottom & top vent openings to maximize the rate of drying from any rain/dew/other moisture that gets in there. With stucco & cement board, or any masonry facing this can be critical to longevity, since these types of siding can harbor a LOT of water (they're sometime referred to as "reservoir cladding") and exhibit very high vapor drives toward the OSB when the sun heats it up after a dew or rain-wetting event. With both top & bottom venting the natural convection drives that are generated by sun-warmed siding are enough to keep up, purging the moisture to the outdoor air rather than into the OSB.

A minimum back-ventilation gap of ~3/8" (10mm) would be required BY CODE in Canada for any type of siding, but even an inch wouldn't be overkill with stucco or brick. By contrast vinyl siding has very low moisture holding capacity, and is usually inherently back-ventilated, but giving it 1/4" of standoff from the OSB wouldn't hurt.

The other approach to dealing with vapor-drives from reservoir cladding is to put a modestly vapor-retardent layer such as 1/4"-1" of XPS insulation between the OSB and siding. With 1/4" fan-fold XPS you minimize the total wall thickness, and have a ~2-3perm vapor retarder that lets moisture out, but limits the rate at which it can move in or out, so the peak drives from sun warmed wet cladding are largely "unseen" by the OSB. The success of this approach may be climate-specific, whereas rainscreens work pretty much everywhere. (Just be sure to limit critter access with screening at the vents to keep it from becoming a nesting spot for bees/ants/lizards/whatever.)

Wall cladding options for SIPs are basically no different that stick built structures where the structural sheathing is OSB.  In Galveston County Texas more than 90 of new residential construction uses OSB for structural sheathing.  One of the main advantages to using structural sheathing, I prefer C-D X plywood personally, it that in increases the design load of the cladding  over attachment to framing studs without structural sheathing.  In other words the cladding IS the structural sheathing with these types of designs.  It is legal, but I would not do it on my house and would not recommend for yours.  

Products such as Hardie Plank are very popular on the Gulf Coast and most of the time is attached over OSB structural sheathing.  It if works so well on stick built, I would suspect it would work just as well on SIPs.

Best of Luck,

Matt Phelps