Bonneville Power Administration
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Vapor Barrier and Ventilation Control
Effective vapor barriers and good ventilation are important for a well insulated home. The normal activities of cooking, cleaning, and breathing all produce moisture in a heated house. Good vapor barriers will assure that most of the moisture from the heated house doesn't pass through the walls, ceiling, or floor of the building. Adequate ventilation, particularly in attics and crawl spaces, will prevent moisture from condensing on the insulation or the building structure.
Moisture can saturate insulation, drastically reducing its ability to resist heat loss. It can also cause mold, mildew, and other decay in building materials and insulation. Wet insulation is much heavier than dry insulation and can overload your ceiling, causing sagging or even more serious damage.
Some insulation, such as batt or blanket or rigid board, is available with an attached vapor barrier. If you use loose fill insulation (as you might in the attic), sheets of 6-mil polyethylene may be stapled between ceiling joists throughout the attic as a vapor barrier. Special low perm paint is also available for this purpose. If you choose not to install a vapor barrier, you may have to increase ventilation to the area.
Walls cannot be ventilated in the way that attics and crawlspaces are, so you must provide a good vapor barrier. Use batt-type insulation with an attached vapor barrier facing or 6-mil polyethylene stapled across the studs. You may also apply a vapor barrier paint to the wall if it faces the heated space.
Vent openings in attics and crawl spaces must be placed so that air can flow in one opening, across the insulated area, and out the other (Fig. 9.1). This is usually made possible through cross-ventilation.
Cross-ventilation can be achieved in crawl spaces by placing vents at direct opposite sides of the space. The Uniform Building Code recommends a venting area of 1 square-foot for each 150 square feet of crawl space/floor area.
When perimeter insulation is installed, special ventilation is required. This type of insulation is much less effective in cold weather if the crawl space is vented. Vents must be closed during cold weather and opened again when the heating season is over.
Attics are best ventilated by taking advantage of convection, the natural tendency of warm air to rise. In an attic, this is called the "chimney effect." Half of the vents are placed at the eaves (the lower part of the attic) and half at the gables or ridges above. The heat of the sun and the force of wind naturally provide attic cross-ventilation with this system.
Cross-ventilated attics must provide 1 square foot of net free area (NFA-see Glossary of Terms) for each 300 square feet of ceiling area (with or without a vapor barrier).
If the attic is not cross-ventilated (i.e., all vents are at the gables or at the soffits, or if there is not natural ventilation), increased ventilation may be recommended. If no vapor barrier is installed, 1 square foot NFA should be provided for each 150 square feet of ceiling area. If your attic does not provide this much ventilation, it may be easier to install a vapor barrier. Attics insulated with a vapor barrier must provide 1 square foot of NFA for each 300 square feet of ceiling area when not cross-ventilated.
Mesh screens and/or rain louvers can reduce the net free area of vents by as much as one-third. Unless the louvers have "free air" stamped on them, use Table C to find the net free area of vents when covered with various vent materials.
If passive ventilation isn't practical, you may install power ventilation. Ask your utility for help in locating and sizing the type appropriate for your situation. Air turbines are not acceptable.
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