Page 237 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
236 CHAPTER SIX

waterproofing and a protective wearing surface above it. Some types of
waterproofing can be placed on compacted subgrade fill and a single
structural slab cast on top of it.

6.6 Vapor Retarders

Where vapor migration from the soil is a potential problem, vapor
retarders are necessary to protect the structure from a continuous flow
of moisture. Where vapor-impermeable or moisture-sensitive floor-fin-
ishing materials are to be used, vapor retarders are particularly impor-
tant in preventing loss of adhesion, peeling, warping, bubbling, or
blistering of resilient flooring. Vapor retarders can also prevent buck-
ling of carpet and wood flooring as well as fungal growth and the
offensive odors and indoor air quality problems that accompany it.
In slabs-on-grade, polyethylene or reinforced polyethylene sheets
of 6-, 8-, or 10-mil thickness are most commonly used in these appli-
cations. For maximum effectiveness, the vapor retarder must lap over
and be sealed to the foundation; seams must be lapped 6 in. and sealed
with pressure-sensitive tape; and penetrations for plumbing, electri-
cal, or mechanical systems must be sealed. Vapor retarders under
slabs-on-grade are usually installed over a base layer of free-draining
gravel or crushed rock as a capillary break. Although vapor retarders
themselves will prevent capillary moisture movement, they are usu-
ally used in conjunction with a drainage layer to provide a margin of
safety in case of punctures or lap seam failures.
Figure 6-42 shows vapor retarder applications on basement slabs
and slabs-on-grade. The granular base should be a minimum of 3 in.
thick, and of compacted, mostly single-graded, coarse aggregate no
1
3
larger than 4 in. To protect the vapor retarder from puncture, a 2-in.
layer of fine, compactable sand fill may be rolled over the base. To
keep the sand from settling into the gravel layer, a geotextile fabric can
be placed over the coarse base material. Traditionally, a 2–4-in. layer of
sand fill is added on top of the vapor retarder, but there are two
schools of thought on whether this is necessary. In addition to provid-
ing a protection course on top of the vapor retarder, a layer of sand is
thought by some to provide a cushion for the concrete and to act as a
blotter to absorb excess moisture from the bottom of the slab. This sup-
posedly promotes more even curing of the concrete, prevents exces-

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.