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House Wrap

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Modern house wrap has been popular in residential construction since the mid-1980's. Before house wrap was developed, building felt was used extensively to protect the structure. The job of house wrap is to keep moisture from soaking into the structure but to allow water vapor to exit. They also serve a second purpose of keeping wind from infiltrating the wall cavities. Even a well-built home will have gaps where air and water can infiltrate. Preventing air infiltration is one of the keys to effective insulation.
Since it is virtually impossible to keep water and air from penetrating almost any exterior surface, some type of barrier is required.

House wraps are installed over the exterior sheathing. Generally it is installed soon after the sheathing to protect the home during construction. In order for house wrap or felt paper to be effective, seams must be overlapped and sealed according to the manufacturers instructions. The overlap must be installed so that water will run on top of the lower piece and not behind (similar to roof shingles).

Insulation is shown to be effective when it's dry and the air is still. When it's wet or when air is allowed to circulate through the insulation, it loses much of it's effectiveness. Tests have show that insulation can lose more than 50% of it's effectiveness under adverse conditions.

Cost and Effectiveness
According to a nationwide DOE study energy savings of between 16%-29% were realized using house wrap. It typically costs between $300-$400 to wrap a 2000 square foot home. According to the study, the house wrap paid for itself after an average of 14 months. Heating costs varied depending on house size, fuel type and heating system.
A modern 2000 sq. ft. house has over ½ mile of construction joints. Each of these joints has the potential for air penetration. According to the study house wraps were effective in reducing energy cost in all climates.

The Code
The Code requires a moisture barrier behind porous materials such as brick, stucco and stone. However, there is generally no requirement for a barrier behind siding attached to structural rated sheathing. Traditionally, 15 lb. building felt has been used behind porous masonry. Many experts believe that building felt is still the best moisture and water barrier to use under any exterior surface. However, it has some disadvantages: It is much more labor intensive to install; it's more difficult to seal the edges and seams; and it doesn't hold up well to sun and wind if left exposed during the construction process.

Another benefit of house wrap is the protection that it offers during the construction of the home. Most house wraps are UV protected so exposure during the construction won't harm them. House wrap that resist water infiltration will protect the structure during construction.

Sealing penetrations
Leaks around windows and doors are common even in new construction. Most sidings are not waterproof, and water will penetrate them in extreme weather conditions. If your windows and doors are not sealed, they will eventually leak. Tar based adhesive paper wrapped around the window opening will keep the window or door from leaking. Make sure that the window installer (framer) follows this method:

  1. Install a strip of paper across the bottom of the rough opening and fold the top of this strip inward
  2. Install strips on the sides of the opening, making sure that the bottom of the side pieces overlap the sill strip completely.
  3. Install the window
  4. Overlap the top edge of the window with the paper.

Notes to the owner-builder
Your local code may not require house wrap, but unless you live in a very dry windless climate, you need house wrap. For more information about house wrap click on the link below:


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