Exterior walls of a home are its barriers to the outside world. Exterior walls are what keep us warm and dry. In today’s wall structures there are many components that help to keep the weather out. Let’s take a look at all the different materials that can make up an exterior wall from the wall sheathing all the way to the siding.
Building Papers and Air Barriers
There are many building papers and air barriers on the market. These papers are applied directly to the exterior wall sheathing under any exterior wall siding that may be applied. This building paper is the number one defense against the weather. It is what keeps the wall structures dry. This paper acts as an air/weather barrier and allows the walls to breath. The most common building paper and the one that has been around the longest is tarpaper, which is a tar impregnated felt paper. Tarpaper is waterproof and a wind and air barrier but cannot be left exposed to the weather because it is susceptible to wind and UV damage.
Other very common air and weather barriers are Tyvek and Typar building paper. Both are made from a high-density polyethylene. This family of building papers allows water vapor from inside the building to escape while eliminating exterior moisture and air infiltration from the exterior. It also cannot be left exposed to the weather because it is susceptible to UV damage. With hundreds of building papers and house wraps on the market today, the key is to find one that will keep the weather out of the interior structure of the wall.
Another key to keeping the moisture out of the structure is good flashing detail. Flashing directs any moisture that has made it through the building components on the exterior of the building back to the exterior of the structure. Most flashing details are above windows and at other wall and roof intersections.
Insulating the Exterior of a Home
In current practice it is common to add rigid insulation directly to the building paper and sheathing. There are some other variations to the insulation, such as spray-on foam, or a total rigid foam exterior insulation. I see exterior (and interior) spray-on foam as the future of insulation in the building industry. Spray-on foam leaves no voids and completely air seals all in one application. One of the benefits to exterior applied insulation is that the dew point of the water vapor on the interior of the building falls within the foam and nowhere near the organic components of the building structure. This is important because it helps to keep the wall cavity dry.
Rain Screen Siding
Rain screen siding is any siding applied on furring strips to allow drainage behind the siding. Since most exterior siding is not impervious to moisture and wind driven rain, a drainage plane behind the siding is necessary. This allows the siding to dry and allows any moisture driven through the siding to drain out the bottom and not into the wall cavity. The building papers can then perform the way they are supposed to and keep moisture out.
Insulation is fastened to the structure with the ½” x 3” pressure treated furring strips (see “rain screen siding”), again, fastening all the way into the studs in the wall cavity. When fastening 2” of rigid foam to the structure a minimum 5” screw must be used. A 5” screw will be driven through ½” of furring strip, 2” of rigid foam, ½” of sheathing, plus 2” of fastening to the studs. The siding can then be applied to the furring strips and again this creates the needed drainage behind the siding. I have also read that a cavity behind the siding can lessen the pressures directed on a building from the wind, thus creating a tighter more efficient home.
When siding is applied directly to the building paper or the insulation, it will sandwich the moisture between the siding and building paper or insulation causing it to hold moisture and not properly drain. Pressure treated furring strips ½ inch thick by 3” wide are applied to the insulation and fastened into the interior studs. The furring strips are also installed around windows and doors and flashed with an ice and water shield. Insect screens are applied to keep critters out of the 1/2” air cavity behind the siding at the top and bottom of each wall. Siding is then applied to the furring strips.
Some of the most common sidings today are vinyl, cedar shingles, cedar lap siding, cement board, stone, stucco, brick, T-1-11, and steel sidings. Steel probably creates the most weather or moisture resistance of the sidings and come in a wide variety of colors. The steel channels naturally create a drainage plane and allow the structure behind it to dry. Most vinyl siding, like steel, can be applied directly on the building paper or insulation. The weep holes in vinyl allow moisture to drain from behind the siding. Vinyl siding also comes in a variety of colors and textures and is also very durable. All other types of siding do not serve as weather or moisture barriers therefore I highly recommended they be applied on furring strips to create the rain screen siding. Brick, stucco, and stone have been applied to structures throughout the years with weep holes to allow proper drainage of moisture. Brick, stucco, and stone are very durable and also come in a variety of colors and textures. Cedar siding and cement board siding commonly come in lap siding or in a shingle variety. Both come in a variety of colors and are very durable when applied on rain screen.
Using variations of different siding textures on a structure can create a very aesthetically pleasing home. By using the right exterior components and flashing details on the exterior of your home, you can create a very durable structure.
Greg Stopher has over 16 years of experience in the construction field and earned a degree in Construction Technologies from the University of Alaska – Southeast. His company, Stopher Construction, LLC, is a general contracting company specializing in remodeling, custom finishes, additions and new home construction projects. He can be reached at 907-321-2350.