Huber ZIP System & SIPs

150x0_ZIPSystem-Structural1-SiennaHuber Engineered Woods has released a housewrap-less sheathing system called ZIP System. This is basically OSB sheathing (7/16” thick for walls and 1/2” & 5/8” thick for roofs) with a liquid water resistive/water vapor permeable coating on the outside. Their goal is to eliminate the material and labor cost of applying a house wrap.

We’ve been asked a few times if we could make SIPs with ZIP System sheathing and/or use ZIP System tape on SIP joints.

The simple answer is no. While the ZIP System may work fine on conventional stick framed structures, we’ve learned that having a liquid water resistant material on the cold side of a high performance building system greatly increases the risks of sheathing rot. This includes, not only SIPs, but other building systems that use wood sheathing but have very little in-cavity air movement such as spray foam insulation in stud cavities.

Hilltop 140402_6There are other issues as well like size availability (4’x8′ is one of our least common SIP sizes) and cost.  We could produce 4’x 8′ panels, and we might be willing to do so if the project is built in a warm climate.  However, the durability of a ZIP SIP in the Northeast is highly limited and we choose not to put our name on a product we do not think will last.

Foard Panel recommends using old school tar paper as a housewrap on SIP envelops. It allows water vapor to pass through and allows liquid water to diffuse through. These two functions allow sheathing to dry to the outside most easily. Combine tar paper with a capillary break (1/4” min) behind the siding or other finish and you have the most durable water drainage system we’ve seen since Bo started installing SIPs in 1983.

Paul Malko
Mechanical Engineer

Posted in Technical Bulletin