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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.
Vapor Barriers
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
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.
Ventilation
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.
Fig. 9.2 shows the different types of structural vents. Owners of homes
which are very "tight" may also want to install exhaust fans in
high moisture areas such as kitchens and bathrooms to use while cooking or
bathing.
In special cases, dehumidifiers may need to be installed. These are very
effective in removing excess moisture from inside air. Your utility will
help decide whether you need a dehumidifier.
Crawl Spaces
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
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.
TABLE C
Net Free Area Required for Different Types of Vent Material |
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| Type of covering |
NFA |
| ¼" hardware cloth |
*Gross opening ÷ 1 |
| ¼" hardware cloth with rain louvers |
Gross opening ÷ 2 |
| 8 mesh screen |
Gross opening ÷ 1¼ |
| 8 mesh screen with rain louvers |
Gross opening ÷ 2¼ |
| 16 mesh screen |
Gross opening ÷ 2¼ |
| 16 mesh screen with rain louvers |
Gross opening ÷ 3 |
| No screen with rain louvers |
Gross opening ÷ 2 |
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| *The size of the vent-hole in a vent assembly. |
Next: Glossary of Heating and Insulation Terms
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