United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-95/159 December 1995
EPA Project Summary
Demonstration of Radon
Resistant Construction
Techniques - Phase II
James L. Tyson and Charles R. Withers
The Florida Radon Research Program
(FRRP), sponsored by the Environmen-
tal Protection Agency and the Florida
Department of Community Affairs
(DCA), is developing the technical ba-
sis for a radon-control construction
standard. The full report summarizes a
project that examined indoor radon in
houses that were constructed accord-
ing to the draft residential construction
code.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The data presented here are from work
performed by the Florida Solar Energy
Center designed to demonstrate radon re-
sistant construction techniques in new
Florida houses. Fifteen houses are in-
cluded in the Phase II study. Some analy-
sis, however, includes the 13 houses in
the 1991 study, to expand the data pool
from which conclusions can be drawn.
Three types of data were collected:
1) Soils were tested to determine type,
permeability, radon and radium con-
tent, radium emanation coefficient,
and moisture.
2) The house slab was examined to
characterize the crack size and
number, and the pressure field ex-
tension under the slab created by
the sub-slab depressurization sys-
tem was measured.
3) The house itself underwent exten-
sive testing, including blower door,
tracer gas, and duct leak measure-
ments.
The full report discusses and provides
a compliance checklist of the adherence
of the subject houses to the radon codes
and standards. It then explains the objec-
tives, methods used, results expected, and
problems encountered in each part of the
project. Then the results of testing are
presented, followed by an analysis of the
data for correlations to radon intrusion.
Soil
Fill soil with high radium content contin-
ues to be used in the area of this study. A
total of 70% (16 of 23) of the houses for
which both radium measurements are
available had higher radium values in the
fill soil than in the native soil. Native soil
radium seems to have a greater effect on
final indoor radon levels, probably due to
the thin fill soil layer under most houses.
Neither native nor fill soil radium levels
directly correlate with indoor radon, how-
ever. High levels of radium in fill soils can
still import a radon problem onto a site
that otherwise would not have one, based
on native soil gas readings.
Sub-slab measurements of soil-gas ra-
don taken at the same point on different
days can vary by as much as 100%, and
measurements taken at different points
on a slab on the same day can also vary
by 100% or more. This variability matches
that of measurements taken in different
seasons on the same slab, and leads to
the conclusion that a number of measure-
ments taken on the same day at different
locations on a site are necessary to ad-
equately characterize radon potential.
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Slab
All sub-slab mitigation systems had ad-
equate pressure field extension, although
pressure fields for both monolithic and
stem-wall slabs dropped to zero just in-
side or at the slab edge. The pressure
field can be short-circuited if the ventila-
tion mat or suction pit is located closer
than 6 ft (1.8 m) from the slab edge. The
6 ft distance mentioned in the code should
be taken as a minimum, as distances up
to 40 ft (12.2 m), measured from the end
of a ventilation mat to the slab edge, have
been depressurized.
Post-tensioned slabs performed best at
preventing cracks, containing an average
crack length of 13 ft (4 m) in four slabs.
Stem-wall systems had an average of 36
ft (11 m) of crack in 10 slabs, and mono-
lithic slabs an average of 100 ft (30.5 m)
in 14 slabs. Average crack length in slabs
using plasticizer in the concrete was 63 ft
(19.2 m) compared with 76 ft (23.2 m) in
slabs without plasticizer. Plasticizer use
seems to have the desired effect of re-
ducing total crack length when all slabs
are lumped together, but conflicting re-
sults appear when slabs are separated by
type. Average crack length in two mono-
lithic slabs with plasticizer was 35 ft (10.7
m), and was 114 ft (34.7 m) in 10 mono-
lithic slabs without plasticizer. Stem-wall
slabs, however, had an average of 75 ft
(22.9 m) of cracks in three slabs with
plasticizer and an average of only 16.5 ft
(5 m) of cracks in eight slabs without
plasticizer. Also, all three slabs in these
groups with no cracks were among those
without plasticizer. Clearly, more data are
required before a definitive conclusion can
be reached.
Placing reinforcement in the top por-
tions of slab inside corners helps to pre-
vent cracks from starting in these corners.
Houses with corner reinforcement had 50%
less total cracking than those without cor-
ner reinforcement. In at least one instance,
however, the corner reinforcement merely
forced the crack to start beyond the ex-
tent of the reinforcing bars.
Total crack length does not correlate
with indoor radon levels. In some in-
stances, high levels of radon were drawn
through cracks during testing, but these
tests used much higher levels of depres-
surization than those found in the house
environment, and most cracks are pro-
tected by the intact vapor barriers under
new homes. No data are available, how-
ever, on how long these vapor barriers
will remain intact, and it is possible that
radon levels in these houses will rise if
vapor barriers deteriorate over time.
Pipe penetrations through slabs are an-
other avenue for radon intrusion; testing
would help determine the extent to which
this occurs. Radon levels in houses with-
out tar protecting pipes in slab penetra-
tions had an average indoor radon level
33% higher than those with tar on the
pipes. Monitoring in house 7 has also
shown that slab penetrations for plumbing
pipes not built to code standards can con-
tribute to indoor radon levels.
High ambient radon levels found on the
house 7 site were not duplicated on other
reclaimed mine sites tested during the
project. This site does present the oppor-
tunity, however, to study the effects of
weather and atmospheric conditions on
the exhalation rate of radon from the soil.
The best measure of slab leakiness is
the amount of conditioned air being pulled
through the slab during mitigation fan op-
eration. Dividing by the house volume
gives slab air changes per hour (ach),
which takes into account all slab openings
including cracks and pipe penetrations.
House
Houses needing mitigation system acti-
vation in the project were on average
smaller and tighter, with a higher value for
slab ach. Activated houses also had higher
levels of depressurization than unactivated
houses.
The radon stress test did not give mean-
ingful results and was discarded during
the project. No calculated values based
on its data showed any correlation with
indoor radon levels. The stress test should
be relegated to research houses, where
much longer time periods for testing are
possible.
No direct correlations with indoor radon
were found in this set of houses, due to
the complex nature of the house as a
system, it is usually impossible to isolate
one building component from another dur-
ing testing, and each component is likely
to have an opposite effect on the results
of a test. The factors seeming to have the
closest relationship with indoor radon are
the sub-slab radon level as the source
term, the differential pressure across the
slab as the driving force, and the slab ach
as the medium of radon intrusion. House
leakiness affects the dilution of the radon
level once it has entered the house and
may be the deciding factor as to whether
or not a house's mitigation system should
be activated.
There continue to be problems relating
to energy efficiency, especially relating to
house shell design and heating, ventila-
tion, and air-conditioning (HVAC) system
installation procedures. The lack of test-
ing of these systems and the house enve-
lope itself have led many builders to ig-
nore certain building components and in-
stallation procedures that are causing en-
ergy inefficiency to be built into new
homes.
Recommendations
Recommendations for the continuation
of the new house evaluation project in-
clude shifting testing emphasis from slab
cracks to slab pipe penetrations. The test-
ing apparatus is available to use the same
protocol on pipe penetrations as has been
used on slab cracks. Number and types
of slab penetrations should be catalogued
as slab cracks have been. Sealing of sub-
slab vapor barriers to the pipe penetra-
tions and the use of tar on pipes where
they contact the concrete should be con-
sidered as mandatory additions to the ra-
don code.
Different types of pipe penetration pro-
tection should be tested in a laboratory
setting to isolate the pipe penetration and
determine the best way to protect against
radon intrusion. Obtaining direct correla-
tions between different types of protection
for pipe penetrations and radon intrusion
through the slab will be much easier when
all other conditions can be controlled. Mul-
tiple penetrations can also be poured in
the same test bed to allow more accurate
results to be achieved through averaging.
Older homes should be investigated to
determine if vapor barriers remain intact
over long periods of time. Ignoring the
repair of slab cracks while emphasis shifts
to the testing of other slab penetrations
could create a radon problem if vapor
barriers do not last long. Older houses
could be surveyed for cracks while new
carpet is being installed. Vapor barrier in-
tegrity could easily be determined by us-
ing the same testing protocol now in place.
Slab cores could also be cut to examine
firsthand the condition of older vapor bar-
riers.
Requirements for concrete slump should
be relaxed from 4 to 5 in. (10 to 12.7 cm).
Requiring a 4-in. slump would be unen-
forceable: it would require an inspector to
be on hand for every concrete truck deliv-
ery on every slab built in the code area. It
would also be an economic hardship to
builders: 4-in. slump concrete cannot be
pulled across a slab by hand and would
require a pump truck to place the con-
crete on any slab with access problems.
Requirements for specific spacing of
slab control joints should be dropped from
the code. Average total crack length for
the 28 houses over the 2-year project was
71 ft (21.6 m), with an average drop to 42
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ft (12.8 m) after subtracting the three
houses with the most cracks. Contrast
this with an average control joint length
requirement of 442 ft (134.7 m) based on
the average project house footprint. The
control joint spacing requirement clearly
requires more work from the builder than
can be justified, since slab cracks have
not been shown to be directly correlated
to indoor radon levels.
Testing of new houses should continue,
to collect data on how sub-slab depres-
surization (SSD) works in real houses,
and how the houses themselves perform
as barriers to radon. Determining correla-
tions between indoor radon levels and in-
dividual parts of the radon code will be
difficult, however, due to the complex na-
ture of real-world houses. Investigations
of code sections that can be isolated in a
laboratory setting will yield better results
due to an enhanced ability to control the
experiment.
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James L Tyson and Charles R. Withers are with the Florida Solar Energy Center, Cape
Canaveral, FL 32920.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Demonstration of Radon Resistant Construction Tech-
niques - Phase II," (Order No. PB96-121512; Cost: $44.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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EPA/600/SR-95/159
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