Originally Posted By: dvalley
This post was automatically imported from our archived forum.
Venting crawlspaces is a bad idea. Code requires 1 sq. ft. of ventilation for every 150 sq. ft. of dirt floor. Using a vapor barrier over the dirt floor reduces the ventilation requirement to 1 ft. per 1500 sq. ft. of floor space. The intent is to vent out the humidity that the exposed earth lets in.
But, venting creates its own problems. In winter, there's an energy penalty: cold floors and higher heating bills. In summer, vents actually admit moisture in the form of warm, humid air. Warm air can hold more moisture than cool air. Warm air entering a cool crawlspace can reach its dew point and give up its moisture as condensation. That?s not good. Relative humidity, dew point and the stack effect combine to make crawlspace vents more likely to compound a moisture problem than to alleviate it.
For these reasons, I recommend to my clients to close the vents outside and seal them from the inside with 2-in. thick foam insulation and polyurethane caulk.
The following changes that were approved by ICC on September 2003
Delete all exceptions and add a new Section immediately following.
Unvented crawl space. Ventilation openings in under-floor spaces specified in Sections
R408.1 and R408.2 shall not be required where:
Exposed earth is covered with a continuous vapor retarder. All joints of the vapor
retarder shall overlap by 6 inches (153 mm) and shall be sealed or taped. The
edges of the vapor retarder shall extend at least 6 inches (153 mm) up the stem
wall and shall be attached and sealed to the stem wall,
And one of the following is provided for the under-floor space:
a. Continuously operated mechanical exhaust ventilation at a rate equal to 1
cfm for each 20 ft
of crawlspace floor area, including an air pathway to
the common area (such as a duct or transfer grille), and perimeter walls
insulated in accordance with Section N1102.2.8,, or
b. Conditioned air supply sized to deliver at a rate equal to 1 cfm for each 20
of under-floor area, including a return air pathway to the common area
(such as a duct or transfer grille), and perimeter walls insulated in
accordance with Section N1102.2.8, or
c. Plenum complying with M1601.4, if under floor spaces used as a plenum.
The new code has option for non-ventilated crawlspace that requires a polyethylene
ground cover and wall insulation to bring the crawlspace into the living space.
Basements and crawlspaces that are not within the insulated building envelope with
concrete surfaces such as walls and slabs have surface temperatures as low as 55 Deg F in
the summer. Ventilation with hot humid air in the summer with a dew point higher than
the surface temperature of the concrete results in condensation and mold growth.
Massachusetts Certified Home Inspections
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