It often is misconstrued that siding is the primary barrier protecting us from inclement weather. In fact, it only is an aesthetic covering for the primary protection, which is the house wrap or rain screen under the siding. On grandma’s house, this function was provided by felt paper (asphalt-impregnated paper), which commonly is used in roofing as the primary waterproofing membrane. We have lost touch with the importance of this layer. In houses 40 to 50 years old or older, this layer may have degraded such that it is not doing its job any more. If a project requires removing old siding, it is a prudent contractor’s responsibility to check the integrity of this membrane on the house.
Today, there are many products that protect the envelope from moisture. One of the latest additions is the rain screen, which provides a space between the siding and wall to allow moisture that gets behind the siding to run down the exterior surface of the wall and drain at the bottom. Rain screens may be corrugated or 1/8- to 5/16-inch thick, depending on climate and application. I consider a rain screen to be cheap insurance against water damage, which is the cause of 90 percent of building failures.
In today’s world, contractors can no longer be so focused on one trade that they lose sight of the fact that homes are systems. There are two elements to this thought. First, all contractors must be aware of opportunities to upgrade the energy performance of a home. Second, there are opportunities that may not occur again during the current ownership of a home. For example, once new siding is up, it is highly unlikely an owner will install exterior insulation.
Although 2- by 4-inch walls with R-11 insulation met code when the house was built, it is no longer sufficient in most climates to address current and future energy requirements.
The perfect time to install exterior rigid foam insulation is before re-siding. This can take several forms, depending on what type of siding is being installed. Typically closed-cell polystyrene blue board is used directly over the existing sheathing. In most climates, 1 inch (R-5) is sufficient to reduce heat loss through the walls and thermal bridging through the studs. In colder climates, 2 inch is better.
Depending on how the house was built, most siding can be applied directly over foam that is attached to the existing sheathing. Typically the foam is glued and nailed to the sheathing. Mark the existing location of studs so siding can be nailed with 2-inch nails through the foam into the studs. House wrap is installed over the foam and under the sheathing.
If cementitious or wood siding is used, install brick mould around doors and windows. The dimension typically works out just right to capture the edge of the foam and siding. If necessary, 7/16-inch-thick OSB can be used to furr out the brick mould.
When applying 2-inch foam, use half of a structural insulated panel, so the final dimension is 2 1/2 inches. This is 2 inches of foam laminated onto an OSB surface. Either polystyrene or urethane foam may be used for insulation. Urethane foam has about twice the R-value as polystyrene. Again, mark the studs, and glue and screw the “outsulation” into the studs. Install the house wrap. Siding then can be nailed into the OSB. Doors and windows can be trimmed with brick mould with a 1- by 4-inch nailed perpendicular to the wall surface into the brick mould to catch the end of the outsulation plus siding.
By paying attention to the significant role walls play in providing comfort in homes a siding contractor can be a major part of energy upgrades to homes. Understanding how moisture flows in and around siding leads to better protection of walls. In addition, tax credits and rebates may be available to help the homeowner cover increased costs. What is most important is how much better the house will look and feel the rest of the time the current owners live there.