United States
Environmental Protection
Agency
National Kisk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/090 July 1995
& EPA Project Summary
Lumped-Parameter Model
Analyses of Data from the 1992
New House Evaluation Project—
Florida Radon Research
Program
Kirk K. Nielson, Rodger B. Holt, and Vern C. Rogers
The report documents analyses of
Phase 2 data from the Florida Radon
Research Program's (FRRP's) New
House Evaluation Project that were per-
formed using a lumped-parameter
model. The houses evaluated in Phase
2 were monitored by the Florida Solar
Energy Center (FSEC) and the Univer-
sity of Florida (UF). Based on experi-
ence from the Phase 1 of the NHEP,
the Phase 2 monitoring was aimed at
better isolating the effects of specified
radon-resistant construction features.
The FSEC data included 15 houses,
and the UF data included 14 houses.
The lumped-parameter analyses fo-
cused primarily on empirically charac-
terizing the radon resistance of the
house/soil interface for different foun-
dation designs. The analyses were also
aimed at comparing the effectiveness
of active and passive radon protection
features.
This Project Summary was developed
by the National Risk Management Re-
search Laboratory's Air Pollution Pre-
vention and Control Division, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Inhalation of indoor radon (222Rn) and
its decay products dominates exposures
to natural radiation in the U.S. population.
Radon causes 7,000 to 30,000 lung can-
cer fatalities annually from chronic expo-
sure. Indoor radon comes mainly from de-
cay of naturally occurring radium (226Ra)
in underlying soils, although contributions
from water, building materials, and out-
door air may also be important. Radon
enters buildings through cracks and pores
in their floors and foundations, and its rate
of accumulation depends on the compet-
ing rates of entry and of dilution by out-
door air. Indoor radon levels therefore vary
significantly with time due to pressure-
related changes in entry rate (from wind,
temperature, and air-handler changes) and
pressure- and occupant-related changes
in dilution rates (e.g., from the pressure
changes, door and window openings, ven-
tilating appliances, and fireplace or flue
openings). Since radon-related health risks
accumulate over years or even decades,
hourly or daily variations are relatively un-
important except as they affect the long-
term average occupant exposure rate or
the results of short-term radon measure-
ments. The U.S. Environmental Protec-
tion Agency (EPA) recommends remedial
action where long-term average radon lev-
els are 4 pCi L~1 or higher. Indoor radon
levels in the U.S. average about 1.25 pCi
L1, and about 1% of all homes have lev-
els that exceed 8 pCi L1.
The Florida Department of Community
Affairs (DCA) is developing radon-protec-
tive building standards to help reduce ra-
don-related health risks. The standards
and their technical basis are being devel-
oped under the FRRP, which has studied
building designs, materials, dynamics, ba-
sic processes, and radon source poten-
tials. The FRRP also has evaluated vari-
ous radon-resistant construction features
by incorporating them into new houses
under the NHEP. Under this program, test
houses with radon-resistant features are
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monitored to assess each feature's effec-
tiveness.
The effectiveness of radon-resistant con-
struction features has been difficult to es-
timate because of the complexity of radon
entry and accumulation processes, and
because of uncontrolled differences among
the houses. These differences include
varying soil radon potentials at the differ-
ent sites, differences in house pressure
and ventilation characteristics, and differ-
ences in the coupling of radon potentials
with house dynamics. Soil radon poten-
tials depend on soil radium concentration,
radon emanation coefficient, moisture, air
permeability, diffusion coefficient, and den-
sity. Indoor air pressures affect both ra-
don entry rates and house ventilation.
House floor and foundation properties also
affect radon entry rates for a given soil
radon potential. Although the effects of
these parameters can potentially be sepa-
rated by sophisticated mathematical mod-
els, the models usually require more de-
tailed data than are available from the
NHEP projects.
To deal with the complexity and vari-
ability of radon entry, a simplified, lumped-
parameter model was developed to help
interpret the NHEP data by accounting for
the uncontrolled differences among the
houses. The lumped-parameter model was
developed from numerous sensitivity analy-
ses with a detailed numerical model, and
from analyses of empirical data on house
ventilation rates and concrete slab prop-
erties. In its initial comparisons with NHEP
data, the lumped-parameter model sug-
gested relatively large uncertainties in the
performance of the radon-resistant con-
struction features.
Theoretical Basis and
Parameter Sensitivity
The lumped-parameter model was de-
rived previously from sensitivity analyses
with the detailed Radon Emanation and
Transport into Dwellings (RAETRAD)
model and from empirical definitions of
typical house parameters. RAETRAD com-
putes radon entry into a house using an
elliptical-cylindrical form of two-dimensional
gradient operators. With this computationally
efficient approach, two-dimensional arrays
of properties represent the house founda-
tion and its vicinity soils for use in finite
difference calculations. The detailed
RAETRAD numerical model has shown
that the primary radon entry routes and
mechanisms are diffusion through the con-
crete floor slab and advection and diffu-
sion through cracks in the concrete floor.
The lumped-parameter model is based
on simplified empirical approximations of
the RAETRAD sensitivity analyses that
allow it to represent various soil proper-
ties, house sites, floor cracks, and indoor
air pressures. It explicitly represents ra-
don entry by pressure-driven advective
flow through foundation cracks and by dif-
fusive movement through both the cracks
and the intact concrete slab. The model
characterizes radon resistance by the ra-
tio of net indoor radon concentration to
sub-slab radon concentration, Cnet/Csub.
This approach normalizes the different ra-
don source strengths for soils under dif-
ferent houses to a common basis for com-
parison of the house radon resistance.
House Parameters and Radon
Measurements
The FSEC houses included eight houses
with floor slabs poured into hollow-block
stem walls (SSW) and seven with mono-
lithic poured-concrete slab and stem wall
construction. The UF houses similarly in-
cluded nine houses with SSW construc-
tion and five with monolithic slabs. The
house areas, volumes, widths, and num-
bers of stories averaged higher for the
SSW houses than for the monolithic-slab
houses in both data sets, suggesting the
tendency to use SSW designs for larger
houses. The concrete slumps for the floor
slabs were higher for the UF houses due
to the more frequent use of super plasti-
cizers. Slab reinforcement included wire
mesh, glass fibers, and (in some FSEC
houses) post-tensioning. Sub-slab ventila-
tion (SSV) systems included both suction
pits and ventilation mat, with well-point
pipe being used in some of the FSEC
houses (generally in connection with suc-
tion pits). All of the houses had SSV sys-
tems. The houses were monitored with
the SSV systems in capped, passive, and
(in some cases) active (fan-ventilated)
modes.
Indoor radon levels measured for each
SSV mode were compared to capped-
SSV sub-slab radon levels for consistent
comparisons of radon resistance. Indoor
radon data were reduced by estimated
outdoor radon levels to obtain Cnet, the net
soil-related component of the indoor ra-
don concentration. The outdoor levels were
estimated from an empirical function of
the sub-slab radon concentrations.
Lumped-parameter model calculations
for comparison with measured Cne/Csub ra-
tios used house parameters and surro-
gate measurements. House ventilation
properties and air pressures were esti-
mated from blower-door test data. Con-
crete slab water/cement ratios were esti-
mated from reported values of the con-
crete slump. House dimensions were taken
from direct measurements, and soil water
saturation fractions were estimated from
soil moisture measurements. SSV effec-
tiveness was estimated from changes in
sub-slab radon measurements under dif-
ferent SSV operating conditions.
Comparisons with the Lumped-
Parameter Model
The comparisons of measured radon
concentrations with predictions from the
lumped-parameter model were made us-
ing Cne/Csub ratios to normalize the differ-
ent radon source strengths for each house
to a common basis. Parameters for deter-
mining the measured Cnet/Csub ratios were
estimated from measured sub-slab radon
concentrations. Parameters for use in the
lumped-parameter model were defined di-
rectly from measured values or were cal-
culated from surrogate measurements in
certain cases. The resulting calculated val-
ues of Cnet/Csub were then compared with
measured Cnet/Csub ratios defined directly
from the indoor and sub-slab radon mea-
surements.
Conclusions
The present analyses estimate more
precisely the effectiveness of radon-resis-
tant building features than the previous
NHEP data. They also suggest that the
lumped-parameter model may accurately
predict Cne/Csub ratios when houses are
built according to the FRRP construction
standard. The accuracy of the lumped-
parameter model is suggested by a ratio
of 1.01±0.16 for the calculated/measured
geometric means of the Cnet/Csub ratios.
Several other important conclusions
about radon resistance are suggested by
the data analyses. SSW construction, in
accordance with the FRRP standard, re-
duces indoor radon to about 9x10'4 of the
sub-slab concentration (with an uncertainty
of a factor of 2.2). Capping the SSV sys-
tem does not significantly alter its radon-
resistance effectiveness compared to leav-
ing it in the passive mode. Monolithic slab
construction may improve radon resistance
by approximately 33%, reducing indoor
radon levels by a factor of 0.67 compared
to SSW construction. Activation of SSV
systems with exhaust fans may improve
radon resistance by approximately 70%,
reducing indoor radon levels to about 0.3
times the levels that occur when the SSV
system is in the passive or capped mode.
The present data on active SSV systems
are sparse and uncertain, however, due
to the few houses where the SSV sys-
tems were activated. Future analyses
should include more data on active SSV
systems to better define their effective-
ness.
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K. Me/son, R. Holt, and V. Rogers are with Rogers and Associates Engineering
Corp., Salt Lake City, UT 84110-0330.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Lumped-Parameter Model Analyses of Data from the
1992 New House Evaluation Project—Florida Radon Research Program," (Order
No. PB95-243077; Cost: $17.50, 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 Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Technology Transfer and Support Division (CERI)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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EPA/600/SR-95/090
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