A building wrap or air retarder cuts drafts and seepage that creep through exteriors, especially at corners and around windows and doors, but allows moisture to pass through, allowing the house to breathe (similar to Gore-Tex). Building wraps do not insulate much, but they block conditioned interior air from escaping, and block unconditioned outdoor air from creeping in, especially on windy days. Thus, they reduce heat transfer, keep the house more comfortable, and save energy.
Building wrap is sometimes confused with vapor barriers.
Building wraps have permeability ratings of 5.0 or higher, to block air, but allow moisture to escape from the wall cavities. The higher the perm rating, the more moisture can pass through. Do not use asphalt felt paper or tar paper as building wrap. Asphalt felt paper has a permeability rating of approximately 1.0, so it traps moisture against the sheathing (not very well, but well enough to cause problems).
Materials used as building wrap:
- Tyvek, 6 mil or greater in thickness.
How and where to install building wrap:
- On exterior of house, between sheathing and siding (including brick and stone). Staple to sheathing with 2-inch overlap on top, and 6-inch overlap on sides.
- Cut house wrap into framed openings for doors and windows, and tuck back and staple excess against side of framing. You can also use rigid closed-cell foam under the windowsill when installing a window.
- You don’t need to put building wrap against exposed parts of the foundation wall, because concrete does not allow drafts to pass through (as long as there are no bypasses), concrete does not rot, and concrete is porous, so any trapped moisture will eventually find its way out.
When replacing siding, you should add building wrap and insulation if they do not already exist. You install the building wrap onto the sheathing first, then install the insulation over the building wrap.
When installing or replacing siding:
- Don't bury problems under new siding. If sheathing is rotted in one area, find out why, and fix the sheathing as well as the cause of the rot.
- Always overlap siding around obstructions, so that water will not find its way in.
- Use corner boards to protect the corners of the house where siding tends to leak.
- Be aware that some vinyl siding can be insulated from behind with molded panels called backer boards.
- If you choose wood siding, apply primer to the backside of wood siding before you install it to the sheathing. Use paint, stain, or clear sealant to protect wood siding once it is installed. Stain is preferable, because moisture entering the wall from the inside can push paint off the wall, but can seep through stain causing only fading and weathering.
- If they do not already exist, you should add building wrap and rigid closed-cell foam panels between the sheathing and the siding. You attach the building wrap to the sheathing, then install the insulation over the building wrap. You may need to build out window casings to match the new thickness of the insulation and siding.
Advantages of adding rigid closed-cell foam between sheathing and siding:
- Thermally insulates wall. If you install foil-faced rigid foam panels, you will also block radiative heat transfer. If you install enough rigid panels, you may not need to put any insulation in the wall cavity.
- Resists air infiltration (and therefore, convective heat transfer) between interior and outdoors.
- Prevents condensation from forming in the wall cavity by keeping the temperature and humidity in the wall cavity similar to that in the interior of the house.
Do not put a vapor barrier in between the sheathing and siding. Vapor barriers belong on the interior side of walls.
Siding drain planeEdit
You should install a drain plane on all walls. A drain plane is a gap that you create between the insulation, building wrap, and sheathing and the exterior siding. You create this gap by installing furring strips or plastic mesh to hold the siding off the wall.
Functions of drain plane:
- Prevents water from entering the wall from the outside.
- Helps prevent mold from growing in cellulose insulation.
- Allows any stray water or condensation to drain out.
- Provides air circulation to allow the back of the siding to dry (especially important with wood siding).
- Extends the life of the siding by keeping it cool and dry.
- Increases paint retention, since moisture will no longer force its way out through the paint.
See Building insulation.