ASHRAE Journal - November 2014 - 48

COLUMN BUILDING SCIENCES

FIGURE 7 What if Your Slab is Wet? If you don't have a low w/c concrete and you
don't have time and if you don't have an effective vapor barrier and capillary break,
you need to install a topside vapor barrier. A fluid-applied vapor barrier such as
epoxy works well. The dumb way is to use a plastic sheet. Don't use a plastic sheet.

Wood Floor

FIGURE 8 Insulating the Slab. Install the insulation under the plastic vapor barrier
and over the top of a granular capillary break. Do not, and I repeat, do not, install
the insulation over the top of the plastic vapor barrier and under the slab. The
insulation will get wet and stay wet forever. Under the plastic it can dry into the
granular layer. Memo to everyone. With insulation, you absolutely need a granular
layer with no fines to act as a capillary break, and further note that the only insulation that will work is an extruded polystyrene.

Topside Fluid-Applied
Vapor Barrier

Wood Floor
Concrete Slab

Concrete Slab
Polyethylene
Vapor Barrier
Granular
Base

ASHRAE JOURNAL

ashrae.org

N OVEM BER 2014

FIGURE 9 Conditioning the Wood Flooring: Wood expands and contracts with relative humidity changes, as it is a hygroscopic material. In most places the interior
winter relative humidity is low (typically 30%) and high during the summer (typically 65%). That means the wood flooring is going to float between 6% moisture
content by weight and 12% moisture content by weight-a 6% seasonal swing. If
you install wood flooring at 6%, it will buckle when it goes to 12%. If you install
wood flooring at 12% it will shrink and leave big gaps when it goes to 6%. Old
guys knew this and conditioned wood to the mid-range moisture content it would
see during service. For most of the lower 48 states that would be 9% moisture
content by weight.

Moisture Content vs. Relative Humidity

Equilibrium Moisture Content (%)

The smart way is a fluid-applied vapor barrier (Figure
7). Epoxies work well. The dumb way is to use a plastic
sheet.
Why is using a plastic sheet a dumb idea? Folks on
the Internet love it. It has got to be the right way to go
if it is on the Internet. If the vapor barrier layer is not
completely fully adhered, condensation will occur in
air pockets. If these air pockets communicate with the
interior, you will grow mold and algae. And the floor
will smell. What are the odds that a thin sheet of plastic
will be installed in an absolutely airtight manner on
the top of a concrete slab where the perimeter of the
plastic layer is sealed in a continuous airtight manner to the concrete and where the thin sheet of plastic
will survive the construction process and not have
any holes in it such as holes necessary to fasten the
flooring?
That the polyethylene plastic sheet recommendation
still comes from many in the wood flooring industry,
including wood floor trade associations, is flabbergasting and disappointing. You would think they would
know better. The old guys knew better with the bitumen
layer. But apparently they all died off and didn't tell the
new guys. Come on guys, put the plastic sheet under the
concrete not on the top of the concrete.
What if I want to insulate the floor? Easy, place the
insulation under the plastic vapor barrier and over the
top of a granular capillary break (Figure 8). Do not, and I
repeat, do not, install the insulation over the top of the
plastic vapor barrier and under the slab. The insulation

Polyethylene
Vapor Barrier
Extruded Polystyrene
(XPS) Rigid Insulation
Granular
Base

32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Relative Humidity (%)

ASHRAE Journal - November 2014

Table of Contents for the Digital Edition of ASHRAE Journal - November 2014