Building paper, also called house wrap, is a strong, fibrous paper that blocks water and moisture from entering from the outside but allows moist air to pass through from the inside, preventing a buildup of moisture inside walls that can lead to mold.
It's important that use the building paper that is recommended for your area. Many builders use standard 15-pound roofing felt. Felt labeled "ASTM 15" meets specific standards and is the norm in many areas.
Felt or building paper must be applied to walls properly to be effective. Install pieces in the correct order and make cuts carefully, so any moisture can travel downward at all points without slipping under the wrap. Install flashings (metal, self-adhesive, or both) as recommended by the window or door manufacturer or as required by local codes. Pull the wrap tight when you install it to prevent creases that could lead to slight waves in the siding.
Working with one or two helpers, expect to spend half a day applying 1,600 square feet or so of building wrap. It will take more time if extensive flashings are required. Before you begin, remove any old siding, nails, and building wrap from the sheathing.
Working with a helper, install the bottom piece of felt so it extends 1 or 2 inches below the sill plate. Roll out a length of 8 feet or so; check that it is level and aligned with the bottom of the house. Use a hammer stapler to tack it in place. At an inside corner, use a straight board to hold the felt tight to the corner as you staple.
Drive plenty of staples, especially if it is windy in your area. Position the successive panels so they overlap the lower panel by at least 6 inches. Use a utility knife to trim neatly around windows and doors.
If you will be attaching the building paper with cap nails, use a chalkline to mark the sheathing with the locations of studs. Working with a helper, unroll the paper so it is about 2 inches below the sill plate and pull it taut. Check that it is level and aligned and drive staples or cap nails.
Install succeeding courses of paper so they overlap the course below by about 6 inches. If you need to make a vertical joint, overlap the pieces by 12 inches. Use recommended contractor tape to seal all the seams as well as any tears in the paper. Tape around windows and doors as well.
If you have a rough opening where a new window or door will be installed, run the paper over the opening and secure it. Make two horizontal cuts along the opening at the top and bottom. Near the center of the opening, make a vertical cut running from the top to the bottom cut.
At each upper corner, make an 8-inch diagonal cut and temporarily fold and tape up the paper; this will expose some of the sheathing. Staple the side pieces so they are tautly wrapped.
Wrap the side pieces around the opening and up to the interior drywall (if it is installed) or onto the interior studs (if the drywall is not installed). With a hook-blade utility knife, trim away any excess building paper.
Cut a piece of self-stick flashing 12 inches longer than the bottom opening. Remove the bottom release sheet and apply it so it wraps up 6 inches onto the side sills. Drive two cap nails at each corner to keep the flashing in place where it has stretched.
If the window is installed after the siding, follow manufacturer's instructions; you may install drip-cap flashing over the top trim piece. If the window will be installed before the siding, set it into the opening and attach self-stick flashing pieces on the sides, then the top. Fold the upper flashing down; first install a drip-cap metal flashing if that is required.
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