Post and Beam SIP House: Siding over existing vertical shiplap siding
Have to come up with a plan for 18 year old post and beam house with SIPs (urethane/OSB board interior/exterior) covered with Tyvek and vertical shiplap cedar siding. The wood windows have rotted sills, casing and frames due to the fact that they were never flashed and the Tyvek was installed after the windows. The OSB is slightly water damaged around some windows and at foundation in couple spots.
At foundation level there is issue with oversized foundation being flush with the shiplap siding allowing water to wick in.
Replacing all windows and doors now.
Plan is to remove complete window & exterior casing (5/4 x 4″ casing), repairing OSB board around window as needed blocking out void around windows, leaving cedar shiplap, securing with screws as needed into OSB. Applying new paper and flashings over shiplap siding and install new horizontal siding, corners and misc. trim rakes etc.
With urethne SIPs don’t believe there would be a condensation issue, and the cedar would be a better nail base than the 1/2″ OSB. Cost is an issue, using siding as a build out helps two problems, would appreciate feedback.
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Brian,
Two issues:
With a SIP, the OSB is structural. If the amount of rotten OSB is minor, this isn't an issue. However, if there is a lot of rotten OSB, make sure to consult an engineer before proceeding.
Second, your plan will probably work, as long as your flashing details are impeccable. Personally, though, I would never install siding over OSB-faced SIPs without a rainscreen gap, because of the risk of OSB rot.
+1 on the rainscreen bit.
With polyurethane the foam itself is far more structural than with EPS core SIP and stress-skins (are you sure it' s a SIP, and not a stress-skin?) There was a period in the late '80s into the early '90s where stress-skin post & beams were popular. A co-worker of mine lives in one of those, circa 1989, and is constantly fighting ants due to similar flashing issues keeping the OSB damper than it should be, but at least he doesn't have the water-intrusion factors you get with vertical ship-lap. But with an EPS core the ants find digging and tunneling nests into the foam easier than it would be with a polyurethane core. DO keep an eye out for insect infestation though- urethane cores are not immune, by any means.
It may be worth adding an inch of foam and a half-inch OSB nailbase to the exterior to get the drain-plane well proud of the foundation, if you can't otherwise flash it properly at the bottom.
Thanks for the feedback .The panels are definitely urethane OSB both sides.
Going to install water table at foundation and flash properly to avoid problems there. The rotted OSB is minimal so far, checking one other section this week before we decide to go this route.
Dana's right - you need a drainage plane under the siding. I've installed a lot of flashing only to find out later what I did wrong.
Remove the cedar and the Tyvek - Tyvek of that era can (and will) trap water and cause rot when the chemicals in Tyvek mix with the tannins in cedar, so you want to get it off the building. If you are doing vertical siding, install a rainscreen fabric, horizontal 1x3's or 4's, then your siding.