Completing the building skin
Once the exterior wall and roof framing are in place, the next phase of construction involves applying the home’s skin to strengthen the structure and make it watertight. The strength is supplied by installing bracing and sheathing, and the weather tightness comes from applying layers of insulation and other barriers. It’s important to complete this phase quickly and pay attention to details that protect the framing from moisture and assure the home’s energy efficiency.
In many older homes, the wind literally blew through cracks in the walls. It carried away indoor moisture and pollutants along with heated or cooled air. Energy efficient homes, by contrast, depend on precise engineering of exterior walls to create a predictable, controlled indoor environment.
Building the wall system best suited to your home will require a marriage of diverse materials: vapor barriers, air barriers, insulation, sheathing and in some cases, insulating panels. These products work together to control the passage of air, heat, and moisture between the inside and outside of your home, while also bracing the frame.
Vapor Barriers- A tight barrier inside the exterior wall prevents indoor moisture from moving into stud cavities where it can condense and rot your home from the inside out. Unlike air barriers, which resist the flow of air but allow water vapor to pass, vapor barriers limit the diffusion of water molecules. Builders often use polyethylene (plastic sheeting) as an inexpensive vapor barrier although other materials including foam, foil and specially formulated paints also can serve as vapor retarders.
“The function of a vapor barrier is to retard the migration of water vapor. Furthermore, vapor barriers are not typically intended to retard the migration of air. That is the function of air barriers.”[1]
Vapor barriers are installed along, in, or around walls, ceilings, and floors.
A true vapor barrier is one that completely prevents moisture from passing through its material, as measured by the “moisture vapor transmission rate.” If the material has any porousness, but the barrier still provides protection from moisture, it is called a vapor diffusion retarder.[2] Furthermore, vapor retarders also are commonly referred to simply as vapor barriers.
Look for these details when polyethylene is installed: All seams in the material should overlap. Reinforced tape should cover not only the seams, but also any exposed nails or staples. A bead of flexible caulking will create an airtight union between vapor barrier and sole plate at the base of the wall. To keep electrical outlets and other mechanical s from breaking the continuous membrane, the barrier must be carefully cited, adjusted, and caulked to fit around awkward intrusions.
Insulation- Unintended gaps or seams in any insulation, even rigid foam, dramatically lower the product’s stated R-value, which is a measurement of resistance to the transfer of heat. When you inspect the work of insulation crews, look for the tell tail voids.
Fiberglass batts- Economical and widely available, this type of insulation resembles blankets made from glass fibers attached to a Kraft-paper or foil backing; it has a proven performance record. In recent years, new types of glass fibers have been developed to reduce the characteristic skin irritation caused by the material. Fiberglass batts are now sometimes wrapped in a plastic material to reduce skin contact during installation.
To attain its nominal R-value, however, fiberglass batts must fill walls or ceiling cavities completely. For this to happen, the batts must be sized to fit stud cavities precisely before being stapled in place. The glass should be notched to fit snugly around ducts, pipes, and wires.
Cotton batts- Now available in several southern states, itch free cotton batts made from recycled denim boast the same insulating qualities as fiberglass. Cotton batts are dense cell structures of rigid foam panels that create a thermal break, which minimizes the impact of framing details on a wall’s R-value. Foam can be applied directly to framing as sheathing, provided the contractor first installs diagonal wind bracing at the wall corner. Research has shown that taping the seam between foam panes may eliminate the need not only for air barriers, but interior vapor barriers as well.
Spray in foam- The latest CFC free spray in place foam is blasted directly into wall cavities, where it expands to 100 times its original volume. Expansion results in an excellent air infiltration barrier. The foam fills cavities completely, leaving few gaps, even in cavities that contain complex mechanical s.
Sheathing- Not every wall needs sheathing. Two of the primary reasons for using it, however, are to provide lateral bracing and create a wind barrier. Without it, those issues need to be addressed with other materials.
Structural sheathing has enough strength to stiffen a wall so that it withstands any reasonable stress caused by wind or seismic wood produces a wall with lateral resistance six times greater than horizontal tongue and groove boards. Add glue to the panels, and that figure jumps by a factor of 5.
Covering an entire home with plywood can get expensive. If you’re building in a relatively benign climate, you may choose a less costly material.
Oriented strand board (OSB) offers many of the advantages of plywood at about half the price. If kept dry, it has good bracing strength and makes a suitable nailing base. Created from resin-soaked stands of small, fast growing trees, the product resists moisture but lacks plywood’s ability to dry out quickly. Consequently, OSVB requires flawless application of interior vapor barriers, and watertight siding, particularly in warm, moist climates.
Fiber panels, an economical, light weight product made from wood scrap, meet most requirements for racking resistance. The best fiber sheathings have waterproof asphalt coatings on both sides and along their edges. To ensure the panels function properly, the contractor must nail or staple fiber panels at close intervals, caulk all seams and apply additional sealant where panels meet rough window and door openings and protrusions through the home’s envelope.
Air barriers- Exterior house wraps consist of a woven polyethylene-based fabric. The wrap allows moisture to escape from inside the walls of your house while preventing wind from blowing through gaps in sheathing. In addition to their function as air barriers, house wraps enable builders to get vulnerable sheathing material under cover quickly. Although the wrap isn’t waterproof, it has enough water resistance to fend off occasional rain. It also filters ultraviolet light, protecting sensitive rigid foam panels.
More important, because house wraps allow water vapor to escape, plywood sheathing that gets wet can dry out through the house wrap. This feature can buy your builder extra time to finish roofing and close in the shell. Brooke Gilbertson of Gilbertson construction, Lake Stevens, Washington, says, “we’re using house wraps on every size project, even little additions, and dormers. There’s no job that’s too small. A mistake I have seen is where the builder hacks and slashes the wrap and nails up a bunch of pieces. That’s no good. You need to leave sheets intact and make sure it penetrates all rough openings at least 1 inch on all sides.”
Many of the most popular wraps come in 9-foot rolls, making them very easy to install. Builders simply tack the material to the sheathing as it unrolls, then go back, and cut an X over every rough opening. To keep drafts from sneaking under walls or around windows and doors, the wrap should overlap all openings by a couple of inches.
Like seams in vapor barriers, seams in air barriers should be taped. To protect against floor level drafts, the fabric should be attached securely to the home’s foundation.
[1] Building Science (2011). BSD-106: Understanding Vapor Barriers. Retrieved from buildingscience.com.
[2] Energy.gov (2019). Vapor Barriers or Vapor Diffusion Retarders. Retrieved from energy.gov.
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