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Sealing a Crawlspace

Sealing a Crawlspace

New Crawlspace Encapsulation Research

Wood is a hygroscopic material, which means its moisture content changes with

the surrounding environment. Elevated humidity levels in the crawlspace, caused

by general crawlspace humidity issues and flooding, resulted in increased moisture

content levels in the wood framing and subflooring, which increase the moisture

content at the underside of the hardwood flooring, causing cupping and buckling.

General Crawlspace Humidity/Condensation Information

The ventilation of enclosed crawlspaces is under close scrutiny by the building

the science community and research has shown that ventilation can actually 

a significant contributor to excessive moisture in crawlspaces.

During the warmer, more humid months of the year, humid air from outside of the

crawlspace enters through crawlspace vents and vapor pressure drives (or

pushes) water vapor into the crawlspace. The air movement is perpetuated by the

“stack effect” of warm air inside the house rising and escaping through the attic,

creating a vacuum that pulls air into the house through windows and doors.

This, in turn, creates a vacuum in the crawl space that draws

air in through the crawlspace vents and other openings. The humidity level is

increased when moisture from the exposed earth evaporates.

Typically, cross-ventilation provided by through-wall crawlspace vents is intended

to “dry-out” elevated moisture content levels caused by high humidity.

The North Carolina Residential Code (SCRC) requirements for ventilation of the

crawlspace are as follows:

R408.1 Ventilation. The under-floor space between the bottom of

the floor joists and the earth under any building (except space

occupied by a basement) shall have ventilation openings through

foundation walls or exterior walls. The minimum net area of

ventilation openings shall not be less than 1 square foot (0.0929 m2


for each 150 square feet (14 m2

) of under-floor space area, unless

the ground surface is covered by a Class 1 vapor retarder material.

When a Class 1 vapor retarder material is used, the minimum net

area of ventilation openings shall not be less than 1 square foot

(0.0929 m2

) for each 1,500 square feet (140 m2

) of under-floor space

area. One such ventilating opening shall be within 3 feet (914 mm)

of each corner of the building.

The current total net free area of ventilation is unknown, but it is apparent that the

cross-ventilation in the area where the majority of the damage occurred is

inadequate to “dry out” the condensation that accumulates on framing and HVAC


Several options might be suggested to reduce moisture levels in the crawlspace.

These might include:

1) Unvented (Encapsulated) Crawlspace: According to more recent studies

conducted by the building science community, the most effective means of

controlling moisture levels is to encapsulate (completely seal) the

crawlspace. The SCRC, which can be found for free online, provides

standards for unvented crawlspaces and many termite companies now

provide encapsulation services. While encapsulation is the most effective

means of controlling moisture, it is also typically the most expensive option.

2) In a vented crawlspace, a combination of moisture control techniques might

be recommended. These might include:

  1. Installing a code-compliant continuous vapor barrier over exposed

earth floors.

  1. Ensuring that all HVAC ductwork is properly sealed to prevent the

cooling of wood framing in the crawlspace, which will increase


  1. Relocating HVAC under-cabinet registers, which concentrate cool air

in the summer and discharges in a localized area across the floor,

resulting in a cooler floor surface that causes increased

condensation damage to subfloor and floor framing in the vicinity of

the cabinet.

  1. Providing gutters at all roof eaves and ensuring that all downspouts

are equipped with extensions that promote roof runoff being directed

away from the house.

  1. Installing crawlspace vent wells around at- or below-grade vents to

prevent surface and roof runoff from flowing into the crawlspace.

  1. Installing crawlspace vent fans to promote air circulation and the

desired drying effect.

  1. Increasing the net free area of crawlspace ventilation by replacing

the existing vents/grates with new vents/grates having a larger net

free area than the existing vents.

  1. Installing new crawlspace vents in the perimeter wall where practical.

Call Structural Pros for a free crawlspace inspection