ASHRAE Journal - November 2014 - 45

COLUMN BUILDING SCIENCES

Photo 1). The bitumen would be both a
FIGURES 2A, 2B, 2C, 2D Clever Stuff. The backside of wood flooring was machined so that it would not warp due to difvapor barrier and a capillary break. It
ferential drying between the top and bottom surfaces during seasonal moisture content changes.
was sticky, icky, gooey and smelly and
otherwise really good. It needed to be
A
B
continuous. To keep it from getting
over everything it was covered with
felt paper. Sometimes it was installed
Hollow Back
Plain
in two layers. This was nothing more
C
D
than a single ply or double ply built up
roof installed on a concrete slab. This
was an inside roof. A roof. Roofers did
Hollow or Scratch Back
Scratch Back
the install. Roofing technology.
Over the top of this felt-covered
bitumen layer (or two bitumen
layers) wood sleepers and wood plank flooring were
FIGURE 3 Monolithic Slab. A continuous polyethylene vapor barrier and capillary
break needs to wrap the entire concrete surface that contacts the ground-espeinstalled. It worked fabulously. Old guys also got very
cially the perimeter. It is also a good idea to paint the exposed edge of the slab
clever at machining the backside of the wood floorabove grade to keep it from getting wet from "splash back" (rainwater hitting the
ing so that it would not warp due to differential drying
ground and bouncing back up) and enthusiastic over irrigation by occupants. The
paint needs to be "liquid water" closed and "vapor water" open-hygrophobic and
between the top and bottom surfaces during seasonal
vapor permeable-that would be typical acrylic latex paint formulated for concrete
moisture content changes (Figure 2a, 2b, 2c and 2d).
substrates.
The only improvement over a century was to replace
Wood Baseboard
the wood sleepers with plywood (Photo 1) that was covWood Flooring
ered with a slip surface to keep the wood flooring from
Concrete
Slab
squeaking as it changed moisture content seasonally.
Perimeter of
Then brain damage set in. It was the '60s. Could have
Slab Protected
been drugs. Maybe Elvis. Maybe the Beatles. Folks began
Above Grade
to leave off the bitumen and just install the felt. How
Sub-Slab Vapor
Barrier
Perimeter
dumb is that? By this time felts were typically impregEdge of Slab
nated with some bitumen. But impregnated felt is not a
Protected
Below Grade
vapor barrier or a capillary break. The whole point of the
Ground Saturated
felt was to be a cover sheet for the bitumen. It was never
With Water
intended to be a replacement for the bitumen. Well
it didn't work. No surprise. But a lot of times folks got
lucky, as it became common practice to install a sheet
of plastic under the concrete to separate the concrete
So how do you separate the concrete from the ground
from the ground. And if the concrete was allowed to dry
underneath? Well, you have to first understand that
before the wood flooring was installed, things would
concrete slab foundations have a bottom and sides. Most
work out-as long as the concrete did not get wet from
people forget the sides. The sides are a really, really big
the ground underneath or from the side.
deal. We plant things on the sides of buildings. We water
Today, if we are smart, we don't need to separate the
the things that we plant on the sides of buildings. The
wood flooring from the concrete as long as we separate
ground is very wet on the sides of buildings.
the concrete from the ground and we allow the concrete
When we have a monolithic slab, a continuous polyto dry before we install the wood flooring. Easy to say,
ethylene vapor barrier and capillary break needs to wrap
but not always easy to do. And like most easy things,
the entire concrete surface that contacts the ground-
there are "tricks" that you need to know. Sometimes
especially the perimeter (Figure 3). This means attaching
folks don't share the tricks. Many times folks don't know
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford,
the tricks in the first place.
Mass. Visit www.buildingscience.com.
N OVEM BER 2014

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