United States	National Risk Management

Environmental Protection	Research Laboratory

Agency	Cincinnati, OH 45268

Research and Development	EPA/600/SR-96/044 May 1996

EPA Project Summary

Effectiveness of Radon Control
Features in New House
Construction, South Central
Florida

Charles S. Fowler, Susan E. McDonough and Ashley D. Williamson

The State of Florida has a radon stan-
dard for new construction. This study
was conducted to evaluate the effec-
tiveness of two slab types (monolithic
and slab-in-stem wall) in retarding ra-
don entry in new houses built in accor-
dance with the proposed standard over
high radon potential soils. Fourteen
houses were monitored during their
construction on sites whose soil gas
radon concentrations were screened to
be over 1,000 pCi/L, Some of the house
sites had concentrations over 12,000
pCi/L. Slab integrity was monitored over
time, and post-construction ventilation
and radon entry were measured in all
the houses. The houses with slab-in-
stem wall foundations exhibited more
slab cracking than did those with mono-
lithic slabs. Those houses also had
higher average radon entry rates, ra-
don entry velocities, and concentration
ratios than the monolithic slab houses.
Both slab types proved to be effective
in retarding radon entry, especially
when penetrations were properly
sealed. Six of the houses had post-
construction average indoor radon con-
centrations of less than 2 pCi/L; six
had average concentrations of 2 - 4
pCi/L; and two had average concentra-
tions exceeding 4 pCi/L. One of the two
houses with elevated indoor radon con-
centrations was on the site with the
highest soil radium content (averaging
13.9 pCi/g), radon flux measurements
through the compacted fill soil (6.1 pCi/
m*s), and sub-slab radon concentra-
tions (12,000 pCi/L), The other house
was suspected of having an inad-
equately sealed tub trap.

This Project Summary was developed
by the National Risk Management Re-
search Laboratory's Air Pollution Pre-
vention Control Division, 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

Background

The Florida Radon Research Program
(FRRP) was implemented to provide ra-
don research related to the detection, con-
trol, and abatement of radon in new house
construction and in existing buildings. The
purpose of this research effort was the
development of construction standards for
radon resistant buildings and correspond-
ing standards for mitigation of radon in
existing buildings. From the fundamental
studies in the first years of the program
came a draft standard for radon-resistant
building construction. The FRRP then
shifted emphasis to field evaluation or vali-
dation of specific areas of the proposed
standards. The majority of these demon-
stration studies have been evaluations of
new houses constructed either in Alachua
and Marion Counties or in Polk and
Hillsborough Counties. In these studies it
was found that implementing the standard
recommendations resulted in low indoor
radon concentrations in most cases. How-
ever, many of the houses were built on
sites that had soil gas radon concentra-
tions of less than 1000 pCi/L. In many of
the houses built on more elevated radon
potential soils, the passive construction

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features alone did not seem to control
radon entry sufficiently to keep indoor ra-
don concentrations below 4 pCi/L. One
possible contributing factor to some of the
failures was the fact that the builders did
not always communicate schedule
changes reliably to the investigators, who
then were not able to inspect all of the
slab sealing features as they were sup-
posed to be accomplished. Finally, in the
course of these studies, it was determined
that some measurement or experimental
protocols were not as effective as others
in determining certain critical parameters,
and that the frequency or timing of collect-
ing other useful data could be improved.

Project Objectives

Part of the Florida approach has been
to map different levels of radon potential
within the state. The results of some of
the previous studies have indicated that,
in the lower and medium potential areas
of the state, application of requirements of
the standards seems to be effective in
controlling indoor radon concentrations.
However, results from the few houses from
previous studies that were built in the
higher radon potential areas have been
inconclusive. The overall purpose of this
work was to evaluate the performance
and effectiveness of the radon resistant
features of the "passive" barrier floor sys-
tem in 14 new houses built over relatively
high radon potential (>1000 pCi/L) soils in
South Central Florida. Areas where houses
were being constructed on reclaimed phos-
phate mining lands and mineralized former
groves where the soil gas radon was closer
to 10,000 pCi/L or higher were actively
sought in order to test the radon resistant
features in close to "worst case scenarios."

Within the context of this overall pur-
pose, two objectives influenced the ap-
proach to the research:

1.	Evaluate the relative effectiveness
of two slab edge details, monolithic
slab (MS) and slab-in-stem wall
(SSW), in providing resistance to ra-
don entry, and

2.	Evaluate the effect of sealing slab
penetrations on radon entry into
houses.

Technical Approach

Sites were sought in areas of the region
that were known or projected from past
experience to have high probability of el-
evated soil gas radon concentrations. A
package of information on the project, the
standard, and the requirements for partici-
pation in the study was prepared and pre-
sented to builders and/or prospective
homeowners in these areas, and their in-

volvement was solicited. The construction
of each selected house was monitored
with the aid of a construction check list,
and diagnostic measurements were made
of the site, the slab, and the completed
house according to predetermined proto-
cols. All of the houses used in this re-
search were of slab-on-grade (SOG) con-
struction, and efforts were made to have
a balanced number of MS and SSW
houses. An active sub-slab depressuriza-
tion (ASD) system using ventilation mat-
ting was installed in each house selected
and constructed as part of the study. Data
were collected of: the site; the fill soil (if
used); the concrete placement, curing, and
cracking; the installation of the air han-
dling (AH) system; and the completed
house radon entry characteristics.
Throughout the data collection and analy-
sis, predetermined checks were made of
data quality indicators, including calibra-
tions of the measurement devices, repli-
cation of certain measurements, and the
ongoing adherence to good measurement
practices.

Materials and Methods

Site selection

Once a candidate site was identified
and permission was obtained from either
the builder or the owner, at least one soil
gas radon grab sample was taken in ac-
cordance with the FRRP Standard Mea-
surement Protocols. The sample was usu-
ally extracted from a 1.2 m (4 ft) depth
and from as near the center of the pro-
jected slab footprint as could be estimated.
If the results of this radon grab sample
estimated the soil gas radon concentra-
tion to be greater than the target value of
1000 pCi/L, then the house site was se-
lected for the project. No houses with any
portion of the main floor underlain by ei-
ther a basement or a crawl space were
considered for inclusion. Eight builders
were used, who constructed from one to
four of the houses each. The only two-
story house in the study had only 81 m2 of
conditioned slab area in contact with the
soil; while five of the smaller single-story
houses ranged from 150 to 200 m2, six
others ranged from 215 to 285 m2, and
the two largest houses had conditioned
areas of over 330 m2. Eight of the houses
had MS foundations (one a post-tensioned
MS), and six were of SSW construction.

Pre-Slab Activities

When the site was prepared for the
slab placement, site characterization mea-
surements were made of the compacted
fill and native soils. These measurements
consisted of soil gas permeability and ra-

don measurements, soil core extractions,
and the placement of soil radon flux can-
isters. Permeability was usually measured
at two locations near the center of the
slab and at two to four others within the
slab footprint. These measurements were
made at depths of 0.3, 0.6, 0.9, and 1.2
m. Radon grab samples were taken at the
1.2 m depth. Between the two center per-
meability probe locations, a soil core was
extracted of the fill and native soils, usu-
ally to at least 1.2 m. These samples
were boxed and shipped to the University
of Florida (UF) Environmental Radiation
Laboratory where they were analyzed for
soil radium content and soil radon emana-
tion coefficient. If weather and scheduling
permitted, the compacted fill radon flux
canisters were placed within the footprint,
left overnight, and collected for shipment
to the analysis laboratory the next morn-
ing. If rainfall was predicted or if a week-
end interfered with a shipment, then the
data quality was reduced to the point that
deployment and shipment were not rea-
sonable. The ventilation matting was in-
stalled for the ASD system, with careful
attention to ensure that the exhaust riser
would be placed in a wall with other plumb-
ing risers or in a chase if one was avail-
able. The riser was connected to the mat-
ting with a toilet flange. The mat and flange
were recessed into the fill soil so that the
slab thickness was not reduced around
the flange. Sub-slab sampling lines were
placed, usually with one under each quad-
rant of the house and one in the installed
ventilation matting near the house center.
After all of these features and the termite
treatment were placed, the proper place-
ment of the vapor barrier was monitored.
The primary areas of attention were to
ensure that an adequate barrier quality
was used, correct overlaps were main-
tained, penetrations and tears were sealed
and repaired, and the edge was treated
correctly. While these activities were be-
ing performed on the vapor barrier, slab
reinforcement was placed at reentrant cor-
ners and at large rectangular openings or
penetrations (such as tub or shower traps)
as required by the standard.

Slab Placement

Once the pre-slab installations were
complete, the site was ready for the slab
placement. This activity was closely moni-
tored at all sites. The project sponsor, the
Florida Department of Community Affairs
(DCA), required that all concrete was to
have high range water reducing admix-
ture (superplasticizer) incorporated in the
mix design. The standard required that
the added water be kept below a fixed
minimum. To ensure that the mix design

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was formulated properly, a quality control
specialist from the corporate office of one
of the local batch plants was called in to
aid the plant in formulating and mixing the
concrete properly. Most of the contractors
used wood grade stakes not conforming
with the standard, but no clear alternative
to this common practice was found that
they would accept and use. The curing
and loading practices specified in the stan-
dard were monitored carefully.

Post-Slab Activities

Metallic penetrations through the slab
were treated with tar, plastic sleeves, or
some other interface to separate the metal
from the concrete. The use of tar both
insulated the pipe from corrosion by the
concrete and bonded the pipe to the slab.
Sleeves, however, protected against cor-
rosion while often leaving an air gap be-
tween the pipe and the slab. Penetrations
made of plastic, such as polyvinyl chloride
(PVC), unless treated with tar, also left a
sub-slab soil gas entry route. Extra mea-
sures for sealing these gaps were em-
ployed. This penetration sealing was moni-
tored for durability during the curing and
loading visits and afterwards when slab
cracking was checked. After the slab cur-
ing process was completed, the slab was
inspected periodically for evidence of slab
crack formation. Before the floor cover-
ings were placed, the cracks were mapped
on a floor plan of the house, usually with
the lengths and widths of the cracks re-
corded, and portions of one or more of
the cracks were measured. The measure-
ment protocol followed was similar to that
used in earlier studies, except a for few
modifications to improve the seal of the
test chamber to the slab and to ensure a
more reproducible grab sample of the
chamber gas. The measurements were
analyzed to determine the crack leakage
area and to measure the radon concen-
tration of the gas pulled through it.

Air Handling System and Other
Post-framing Installations

The continuation of the ASD piping up
a wall or chase into the attic and out the
roof required supervision because leaks
in this system would be extremely coun-
terproductive to the radon resistance of
the house. Once the framing and roofing
were complete, the AH system was in-
stalled. Specifications of the sealing and
placement of plenums, ducts, grills, and
boxes were monitored during the installa-
tions. The wiring and operation of exhaust
fans in bathrooms, kitchens, attics, etc.,
were also monitored to ensure standard
compliance. Certain features were com-
mon to all the houses including the use of

ventilation matting for ASD soil gas col-
lection, a 152 n.m (6 mil) vapor barrier,
superplasticizer in the concrete mix, and
acceptable sealing of slab penetrations
and AH ducts in accordance with Florida
energy code requirements.

Post-Construction Ventilation
and Radon Entry
Characteristics

After the house shell was completed
and the AH system was installed, tested,
and powered, the radon entry characteris-
tics of the house were measured. The
basic protocol followed was that used by
UF in their Alachua and Marion County
study, with minor adjustments in some of
the houses. Indoor radon concentrations
in one or more rooms, sub-slab concen-
trations from one of the sub-slab sampling
lines, and outdoor (ambient) concentra-
tions were measured hourly for at least
six days. Simultaneously, half-hourly in-
door/outdoor, indoor/sub-slab, and room-
to-room pressure differential averages and
indoor, outdoor, and other relevant tem-
peratures were recorded. Sub-slab grab
samples were usually taken before, be-
tween, and after the house ventilation ad-
justments were made. These house con-
ditions were AH off/interior doors opened,
AH on/doors open, AH on/doors shut, op-
erated for about two days at a time. Gen-
erally this testing was attempted after the
house was completed and before the oc-
cupants moved in. However, a few of the
houses were completed when there were
breakdowns in the measurements system,
and some of the houses were finished
within the same week as another, making
for situations in which the houses were
already occupied before the equipment
was available for testing. In those situa-
tions, the testing had to be done with the
owners' cooperation.

Results

In each of the 14 houses, three sets of
diagnostic measurements were taken: site
characterization (including site selection
measurements), slab crack, and post-con-
struction ventilation and radon entry. When
the site characterization measurements
were being made at house F-04, the
permeameter probe was leaking at the
weld of the head, and ultimately broke. A
replacement could not be found before
the slab was placed; so the characteriza-
tion soil gas radon and permeability mea-
surements were not usable. Sites F-05
and F-09 had more clay in the native
soils, and the permeability measurements
were lower there than at the other sites,
except for F-14. The resulting flows
through the radon grab scintillation cells

were too little for adequate sampling, lead-
ing to low radon concentration measure-
ments during the site characterization vis-
its. Site F-14 had drainage problems;
therefore, the permeability was very low,
and the site characterization soil gas ra-
don concentrations were taken at depths
just above the apparent water table. Rain-
fall or scheduling problems prevented
placement of the radon flux canisters at
sites F-02, F-08, F-09, F-10, F-13, and F-
14.

The screening measurements (one or
two probes) at sites F-01, F-02, and F-03
were within the standard error of the char-
acterization measurements (average of
four to six probes). Those at site F-06
also agreed reasonably well (within 20%).
The screening samples at sites F-05 and
F-09 were taken in very clayey layers,
whose radon concentrations were higher
than expected, and the characterization
measurements were artificially low be-
cause of low gas flows..However, the dis-
crepancies between the selection and
characterization measurements at sites F-
07, F-08, F-10, F-11, F-12, F-13, and F-
14 were more difficult to explain. They
may reflect the wide range of variability
inherent in reclaimed soils; they may be
the result of soil mixing that occurred be-
tween the two measurement times; or they
may have been influenced by changes in
the soil condition, such as moisture con-
tent. The recorded radon fluxes did not
show any correlation with the soil gas
grab radon concentrations, but they were
measuring different spaces. The grab
samples were usually from 1.2 m depths-
well into the native soil in all cases. The
flux canisters were placed on top of the
compacted fill. The average soil gas
permeabilities were basically within an or-
der of magnitude of one another, except
for sites F-05, F-09, and F-14. House F-
02 had excessive slab cracking, some of
which was caused by having to move
some plumbing after the slab had been
placed because the plans had been mis-
read. The slab quality overall improved as
the project progressed. None of the three
major concrete suppliers were familiar with
the use of superplasticizer in the concrete
mix design. Many of the early mixes were
not formulated properly, which necessi-
tated calling in a quality assurance officer
from one of the home offices to assist in
developing the mix design and training
the operators in mixing it.

The soil radium content of the surface
soils on these sites was well in excess of
the recommended radium concentrations
for foundation backfill material of 0.8 to
1.0 pCi/g, with the lone exception of site
F-02. High radium fill may have been im-

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ported to site F-11, or it may have been
below the recommended concentration. At
six other sites, the fill or top horizon of the
prepared base was higher in radium con-
tent than the lower horizons of the native
soil. At sites F-01 and F-07 the fill was
tested and found to be higher than any of
the lower horizons, and at sites F-03, F-
08, F-10, and F-14 it is suspected that
imported fill may have contributed to higher
radium contents in the uppermost layer of
prepared soil. Site F-02 was the only one
where low concentration fill was known to
have been imported, but it appears that
lower concentration fill than the native soil
may have been used at F-05, F-12, and
F-13. At site F-06 the fill had 7 pCi/g
radium concentration, about what the na-
tive soil had. At site F-04 the soil radium
concentration was very high from the sur-
face down to 1.5 m. The radon flux through
the compacted base corresponded very
well with the soil radium concentrations in
either the fill soil or in the uppermost hori-
zon, which was a reasonable correspon-
dence.

After the house shells were completed,
the house radon entry was measured.
House radon was measured in 13 of the
houses under the three house conditions:
AH off/interior doors open, AH on/doors
open, and AH on/doors shut. The data
were collected in house F-09 with the
house shut (most of the time) and the AH
in its normal operating mode. In 10 of the
13 houses for which most of the data are
available, the indoor concentrations ex-
hibit the following pattern: AH off > AH on,
doors opened > AH on, doors shut. In the
two of the houses in which this pattern
was not observed, there were some pos-
sible explanations that may have contrib-
uted to the deviation from the norm. In
house F-04, there was some evidence
that the house may have been entered
when the measurements were being made,
especially during the AH off condition. In
house F-06, there appeared to have been
a heavy rain that led to elevated indoor
(and sub-slab) radon concentrations dur-
ing the AH on, doors open condition. It
also rained during the whole week of data
collection in house F-10, which may ac-
count for its high indoor concentrations.
Further measurements were made there
later, confirming elevated radon concen-
trations, but not as high as with the rain.

House F-04 had the highest sub-slab ra-
don concentrations (and radon flux and
soil radium content) and also had the sec-
ond highest indoor concentrations, while
house F-02 had the lowest sub-slab con-
centrations and some of the lowest indoor
concentrations. The other houses had in-
termediate sub-slab and indoor concen-
trations with no clear pattern of correla-
tion.

Discussion

From the hourly or half-hourly indoor
and outdoor radon measurements, the net
radon concentration (C^,) was calculated
by subtraction. The radon entry rate (RER)
was then calculated by:

RER = c/VV3.6

where is the rate of house ventilation
by outdoor air and Vh is the interior house
volume. The radon entry velocity (or con-
ductance) (REV) was calculated by lump-
ing several velocity terms into one vari-
able to produce:

REV = RER / (Ah * Ce)

where Ah is the house area and Cs is
the sub-slab radon concentration. The ra-
don concentration ratio (CR) was calcu-
lated by taking the ratio Cnet/C . The means
of these various measures o? slab barrier
effectiveness (Cn0t, RER, REV, and CR)
for the two types of slab edge details
were compared statistically. The net in-
door radon concentrations in the two
groups of houses showed no significant
differences. The RER, which takes into
account the house ventilation rates and
house volumes, showed definite differ-
ences between the two slab types, but the
variability within and between the groups
was so great that these differences were
not significant at a 5% significance level.
The REV, taking into account the house
slab area and the sub-slab radon concen-
trations, and the CR, taking into account
the sub-slab radon concentrations, pro-
duce significant differences for the AH off
and AH on/doors shut conditions. With
this small sample size and the high vari-
ability in the measured and calculated pa-
rameters, it was difficult to show signifi-
cance in all the analyses. It is expected
that an increased sample size would re-

duce the variability in some of these pa-
rameters, increase the power of the com-
parisons, and indicate more significance
in the results.

In earlier work, others had collected the
measured radon and house data from two
years of studies based on houses from
the same general area as those reported
here, and calculated CRs based both on
the measurements and on lumped param-
eter model calculations. For MS houses in
those studies, the overall geometric means
of their measured and calculated CRs for
houses with the ASD system installed but
either capped or passive were 5 to 6x10-4.
For SSW houses the corresponding geo-
metric means ranged from 7 to 9x10"4.
The respective CRs from this study were
* 2 to 3x10" and 4 to 6x104. These reduc-
tions were assumed to be attributable to
the improved sealing procedures enforced
in this year's study. One other compari-
son between the slab types showed a
noticeable difference: the amount of slab
cracking. The MS foundations averaged
less than 6 m of slab crack length per
slab, while the SSW houses had over 20
m of cracks per slab. However, house F-
02, which had the alterations in the plan
design after the original slab was placed,
was a SSW foundation. The moving of
some of the plumbing penetrations re-
quired breaking the slab, which caused
some additional cracking. This activity bi-
ased the slab cracking data in favor of the
MS foundations, but it did not account for
all of the difference.

Conclusions

The results from this study demonstrated
that houses built over MS foundations
show less slab cracking and greater resis-
tance to radon entry than did those built
over SSW foundations, in accordance with
previous findings. But both types of slabs
were shown to be effective at retarding
radon entry, even in houses built over
relatively high radon potential soils (1000-
5000 pCi/L or higher). The performance
of these slabs was evaluated using mea-
sures such as REV and CR. When com-
pared with CRs from previous studies in
the same area of the state, those from
this year's houses were lower by about
half. Most of the improvement is attributed
to stricter enforcement of the sealing of
slab penetrations.

4

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C.S. Fowler, S.E. McDonough, and A.D. Williamson are with Southern Research

Institute, Birmingham, AL 35255-5305.

David C. Sanchez is the EPA Project Officer (see below).

The complete report, entitled "Effectiveness of Radon Control Features in New
House Construction, South Central Florida," (Order No. PB96-177761; Cost:
$44.00, subject to change) will be available only from:

National Technical Information Sen/ice
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:

Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268

United States

Environmental Protection Agency

National Risk Management Research Laboratory (G-72)

Cincinnati, OH 45268

Official Business

Penalty for Private Use $300

BULK RATE
POSTAGE & FEES PAID
EPA

PERMIT No. G-35

EPA/600/SR-96/044

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