What if I use a vapor closed cavity insulation? Well, we now have a problem. Ah-hah – a problem! Yes, closed cell high-density foam cavity insulation applied to the inside of OSB sheathings that are in turn covered on the exterior with impermeable foam sheathings is risky. Unless you provide a small gap between the exterior face of the OSB and the back surface of the foam sheathing to provide for some hygric redistribution. Or if you are “perfect” with your rainwater control such as when you use a fully adhered membrane – think roof membrane standing up applied to a wall. Otherwise, go with a gap. What works? Grooved foam, “bumpy” OSB, “crinkled” building wrap, 1/8-inch polypropylene mesh, dimpled polypropylene sheets. Lots of stuff (Photograph 9 and Photograph 10). But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap. With “crinkly” stuff you loose next to nothing.7 Is it worth it? Yes, in my opinion, the loss in thermal performance is trivial compared to the reduced risk and improved durability. The 5 percent thermal loss is also easily offset by the improved thermal performance of the closed cell high-density foam cavity insulation. Even if you think you are perfect with a water control membrane, go with a gap for at least the relief of hydrostatic pressure. Most of think we are perfect when we really are not. Remember EIFS. Perfection is hard (Photograph 11).
OSB Thickness for Wall Sheathing
In some locations, OSB may be used in place of plywood for wall sheathing as long as the panel dimensions are those specified by the architect. Where plywood is specified in the plans, though, California code prohibits substitution of OSB. Before performing such a substitution, check with a building inspector to determine whether the codes in your area permit it.
The most inexpensive sheathing material is oriented strand board (OSB). This manufactured wood product is composed of small wood chips that are arranged in layers and glued together. OSB is sold in 4-by-8-foot panels, and the standard panel thickness for OSB sheathing is 7/16 inch. Studies have demonstrated that OSB is an acceptable sheathing material that is comparable, and in some cases superior, to plywood. If, however, your garden shed is frequently exposed to rain and humidity, then OSB may not be the best choice because it is more susceptible to moisture damage than some other sheathing.
What is OSB Sheathing?
Used for sub floors, exterior walls, and roof decking, OSB sheathing is a structurally-engineered board that is made up of compressed wood strands. These strands of wood are arranged in layers that are bonded together with resin, which helps produce a strong product. OSB, or oriented strand board, is unlike traditional plywood in that it has no laps, gaps, knots or voids. In most applications, this sheathing is dimensionally stronger and stiffer than comparable dimensional plywood boards. Many builders gravitate toward it not only for the increased strength of the finished product, but for the cost effectiveness as well. Usually, the price difference between OSB and plywood is significant enough to warrant common usage.
How to Replace Roof Sheathing
If you’re not experienced and comfortable working on a roof, this is a project that is best left to the pros. If you do choose to tackle the project, determine if the existing sheathing is sturdy enough to walk on as the tear-off method starts at the ridge and progresses downward. This allows you to stand on the lower panels as you tear off the upper ones. If the sheathing is not structurally safe, however, construct scaffolding at the roof’s edge to stand on while you tear off the lower panels and replace them with new panels before tearing off and replacing the next higher row of panels. A flat bar, or cat’s paw, is standard for prying up the panels.
OSB board edges that are unsealed may swell when they are unprotected. Boards exposed to heavy moisture can swell and warp. These unprotected edges are vulnerable to water absorption and can expand as much as 15 percent. However, OSB boards and panels tend to shrink less rapidly than those made of plywood. Because of the absorption properties of OSB boards, contractors often prefer to use plywood on edges such as those found on roof decking that is exposed to weather conditions. For boards and decking installed away from exposed edges, the OSB material is used on these same roofs because of its lower cost.
How to Waterproof OSB Behind Vinyl Siding
The exterior walls of your vinyl siding-clad home consists of layers of different materials designed to keep your home dry and warm in the winter (or cool in the summer). The framing is covered on the exterior by plywood or oriented strand board (OSB). OSB consists of resin-coated strands of wood placed together in the same direction and pressed together into boards in a thermal press. Before the vinyl siding is installed on the house, a waterproof underlayment called house wrap must be placed on the OSB to prevent water from seeping through the siding and reaching the OSB, where it can damage the wood.
What to Consider when Buying OSB Sheathing
Understanding the composition of the OSB sheathing is the key to understanding the effectiveness of using the material. To understand it better, it is best to compare it with traditional plywood. Although the sheathing is also made from wood, it makes use of strands or flakes and bonded together using resin. Unlike plywood, OSB sheathing makes use of wood from fast growing trees, therefore making it a more environmental-friendly option. Typical types of wood used to build the material include southern yellow pine, poplar, aspen, and a mixture of other types of soft hardwood. These hardwoods are considered and termed soft because they are physically soft; nonetheless, these soft hardwoods are still part of the hardwood family.