Page 246 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
245
FOOTINGS, FOUNDATION WALLS, BASEMENTS, AND SLABS

requirements for crawl space ventilation. A minimum of four openings
should be provided (one at each corner), placed as high in the founda-
tion wall as possible. The CABO One and Two Family Dwelling Code
requires a minimum net area of ventilation openings of 1 sq. ft. for
each 150 sq. ft. of crawl space area, with one opening located within 3
ft. of each corner. Ventilation openings must be provided with corro-
sion-resistant wire mesh with the least dimension of the mesh being
1 8 in. Net and gross ventilator areas for different types of screens and
louvers are given in Figure 6-49. After calculating the required net
area, multiply by the coefficient shown to determine the overall size or
gross area of ventilators needed. Ventilation will dissipate soil mois-
ture vapor, but it also will cool the underside of the floor sufficiently
to require insulation to prevent winter heat loss. An alternative control
measure is to cover the exposed soil. With a vapor retarder of polyeth-
ylene film, heavy roll roofing (55 lb.), or a proprietary membrane, the
required net area of ventilation may be reduced to 1 sq. ft. for each
1,500 sq. ft. of crawl space area. Vents still should be placed within 3
ft. of each corner but may be omitted entirely from one side of the
building.
In concrete foundation walls, blockouts can be provided for crawl
space ventilation openings using either plywood or lumber to frame a
penetration of the correct size through the formwork. Make sure the
concrete flows around and fills in underneath the blockout by
mechanical vibration or hammering against the forms. In concrete

Ventilator covering Coefficient
1
/4″ mesh hardware cloth 1
Screening, 8 mesh.in. 1.25
Insect screen, 16 mesh/in. 2
1
Louvers plus /4″ mesh hardware cloth 2
Louvers plus screening, 8 mesh/in. 2.25
Louvers plus insect screening, 16 mesh/in. 3
*Gross ventilator area required net area coefficient
FIGURE 6-49

Net and gross ventilator area.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.