ASHRAE Journal - July 2020 - 8

LETTERS

Building Sciences:
Concrete Solutions
Thank you to Joseph Lstiburek
for his May 2020 Building Sciences
column on stem wall construction,
"Concrete Solutions."
Here in Climate Zones 5 and 6, I
recommend an ICF stem wall. Also,
we normally place the polyethylene
vapor barrier under the XPS or EPS
insulation (minimum R-12) to avoid
the damage and penetrations that
typically occur during placement of
reinforcing bars/mesh and radiant
floor heat tubing. The downside to
this approach has been water (e.g.,
rain) that is more difficult to remove
before the concrete placement. Do
you see any other problems with this
placement of the vapor barrier?

Don Gibbs, P.Eng., Associate Member ASHRAE,
Vernon, BC, Canada

The Author Responds
The polyethylene should always be
in direct contact with the concrete
slab, except as noted below. The
second law of thermodynamics and
Fick's law come into play. The second
law points out that moisture flow
is from more to less (concentration
gradient) and from warm to cold
(thermal gradient). Fick's law points
out that vapor diffusion is a linear
function of surface area.
The XPS or EPS will not be able to
dry downward when the polyethylene is located under the XPS or
EPS, and it is very difficult for the
XPS or EPS to dry upward through
the concrete. Folks get away with it
in some applications if radiant heat
is installed in the slab, as the heat
can provide enough energy to cause
drying upward. In essence the radiant heat needs to make the XPS or
8

ASHRAE JOURNAL

ashrae.org

EPS significantly warmer than the
ground so moisture flow is from the
XPS and EPS upward. Bottom line-
don't do it.
So what about the rips and tears
in the polyethylene? Relax. Minor
damage to the polyethylene vapor
barrier does not affect its vapor diffusion control, as vapor diffusion
is a linear function of surface area
(Fick's law), but it does affect its air
control. What is the surface area of
the punctures in the polyethylene
to the total surface area of coverage?
Minor-who cares. But what about
airflow? Ah, Grasshopper, the air
control is provided by the concrete
slab, and the vapor control is being
provided by the polyethylene.
The only time the polyethylene
should be under the insulation
is if you have an ice rink above it.
Canadians know this. Hockey is in
our genes.

Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE,
Westford, Mass.

Research Update
On Lower GWP
Flammable
Refrigerants
It is very nice to note that ASHRAE
has committed to developing three
initial research projects on safer use
of flammable clean refrigerants as
reported in February's "Research
Update on Lower GWP Flammable
Refrigerants." Providing such useful
technical knowledge would facilitate
and accelerate the safe use of the
low-toxicity refrigerants with low
burning velocity.
It is good to see planning for phasing out flammable refrigerants.
However, the explosion hazards of

J U LY 2 0 2 0

clean refrigerants already in use
should be watched in this transition
period. (See ASHRAE Journal Letter to
the Editor, "Flammability and New
Refrigerant Options," December
2017.)
For example, leakage of flammable
clean refrigerants in air-conditioning units and refrigerators installed
in boxes or cabinets (such as in small
apartments) can give a high mixing
ratio of flammable gases. A small
ignition source can initiate an explosion. Air pressure would increase
rapidly because of the confined space
and obstacles.
The following points should be
watched or studied in future sponsored projects:
* Difficulty to know how much
and what flammable clean refrigerants are used in many places without tight control;
* Missing or illegible labels
identifying the flammable nature of
the refrigerants; this would result in
facility management being unaware
of the flammable refrigerant and not
taking appropriate precautions.
* Improper or illegal recruitment
of untrained personnel;
* System compatibility upon
changing to lower-GWP refrigerant;
* Study of flame propagation rate
and turbulent burning velocity; and
* Study of destructive damage
due to high transient explosion
pressure in boxes.
Research with experimental studies and numerical simulations on
explosions involving lower GWP
flammable refrigerants with some
identified hazardous scenarios are
recommended for safer facility
management and firefighting.
W.K. Chow, Ph.D., Fellow ASHRAE;
Philip C.H. Yu, Ph.D., Fellow ASHRAE; Y.W. Ng, Ph.D.,
Hong Kong
https://www.ashrae.org/
ASHRAE Journal - July 2020

Table of Contents for the Digital Edition of ASHRAE Journal - July 2020
