United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati, OH 45268

EPA Project Summary

Research and Development EPA/600/SR-95/090 July 1995

The report documents analyses of

Phase 2 data from the Florida Radon

Research Program’s (FRRP’s) New

House Evaluation Project that were performed using a lumped-parameter

model. The houses evaluated in Phase

2 were monitored by the Florida Solar

Energy Center (FSEC) and the University of Florida (UF). Based on experience 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 focused primarily on empirically characterizing the radon resistance of the

house/soil interface for different foundation 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 Research Laboratory’s Air Pollution Prevention 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 cancer fatalities annually from chronic exposure. Indoor radon comes mainly from decay of naturally occurring radium (226Ra)

Lumped-Parameter Model

Analyses of Data from the 1992

New House Evaluation Project—

Florida Radon Research

Program

in underlying soils, although contributions

from water, building materials, and outdoor 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 competing rates of entry and of dilution by outdoor air. Indoor radon levels therefore vary

significantly with time due to pressurerelated 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, ventilating appliances, and fireplace or flue

openings). Since radon-related health risks

accumulate over years or even decades,

hourly or daily variations are relatively unimportant except as they affect the longterm average occupant exposure rate or

the results of short-term radon measurements. The U.S. Environmental Protection Agency (EPA) recommends remedial

action where long-term average radon levels are 4 pCi L-1 or higher. Indoor radon

levels in the U.S. average about 1.25 pCi

L

-1, and about 1% of all homes have levels that exceed 8 pCi L-1

.

The Florida Department of Community

Affairs (DCA) is developing radon-protective building standards to help reduce radon-related health risks. The standards

and their technical basis are being developed under the FRRP, which has studied

building designs, materials, dynamics, basic processes, and radon source potentials. The FRRP also has evaluated various radon-resistant construction features

by incorporating them into new houses

under the NHEP. Under this program, test

houses with radon-resistant features are

Kirk K. Nielson, Rodger B. Holt, and Vern C. Rogers
2

monitored to assess each feature’s effectiveness.

The effectiveness of radon-resistant construction features has been difficult to estimate 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 different sites, differences in house pressure

and ventilation characteristics, and differences in the coupling of radon potentials

with house dynamics. Soil radon potentials depend on soil radium concentration,

radon emanation coefficient, moisture, air

permeability, diffusion coefficient, and density. Indoor air pressures affect both radon 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 separated by sophisticated mathematical models, the models usually require more detailed data than are available from the

NHEP projects.

To deal with the complexity and variability of radon entry, a simplified, lumpedparameter 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 analyses with a detailed numerical model, and

from analyses of empirical data on house

ventilation rates and concrete slab properties. In its initial comparisons with NHEP

data, the lumped-parameter model suggested relatively large uncertainties in the

performance of the radon-resistant construction features.

Theoretical Basis and

Parameter Sensitivity

The lumped-parameter model was derived previously from sensitivity analyses

with the detailed Radon Emanation and

Transport into Dwellings (RAETRAD)

model and from empirical definitions of

typical house parameters. RAETRAD computes 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 foundation 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 concrete floor slab and advection and diffusion 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 properties, house sites, floor cracks, and indoor

air pressures. It explicitly represents radon entry by pressure-driven advective

flow through foundation cracks and by diffusive movement through both the cracks

and the intact concrete slab. The model

characterizes radon resistance by the ratio of net indoor radon concentration to

sub-slab radon concentration, Cnet/Csub.

This approach normalizes the different radon source strengths for soils under different houses to a common basis for comparison 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 monolithic poured-concrete slab and stem wall

construction. The UF houses similarly included nine houses with SSW construction and five with monolithic slabs. The

house areas, volumes, widths, and numbers 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 plasticizers. Slab reinforcement included wire

mesh, glass fibers, and (in some FSEC

houses) post-tensioning. Sub-slab ventilation (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 suction pits). All of the houses had SSV systems. 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 cappedSSV 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 radon concentration. The outdoor levels were

estimated from an empirical function of

the sub-slab radon concentrations.

Lumped-parameter model calculations

for comparison with measured Cnet/Csub ratios used house parameters and surrogate measurements. House ventilation

properties and air pressures were estimated from blower-door test data. Concrete slab water/cement ratios were estimated from reported values of the concrete slump. House dimensions were taken

from direct measurements, and soil water

saturation fractions were estimated from

soil moisture measurements. SSV effectiveness was estimated from changes in

sub-slab radon measurements under different SSV operating conditions.

Comparisons with the LumpedParameter Model

The comparisons of measured radon

concentrations with predictions from the

lumped-parameter model were made using Cnet/Csub ratios to normalize the different radon source strengths for each house

to a common basis. Parameters for determining the measured Cnet/Csub ratios were

estimated from measured sub-slab radon

concentrations. Parameters for use in the

lumped-parameter model were defined directly from measured values or were calculated from surrogate measurements in

certain cases. The resulting calculated values of Cnet/Csub were then compared with

measured Cnet/Csub ratios defined directly

from the indoor and sub-slab radon measurements.

Conclusions

The present analyses estimate more

precisely the effectiveness of radon-resistant building features than the previous

NHEP data. They also suggest that the

lumped-parameter model may accurately

predict Cnet/Csub ratios when houses are

built according to the FRRP construction

standard. The accuracy of the lumpedparameter 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, reduces indoor radon to about 9x10-4 of the

sub-slab concentration (with an uncertainty

of a factor of 2.2). Capping the SSV system does not significantly alter its radonresistance effectiveness compared to leaving 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 systems were activated. Future analyses

should include more data on active SSV

systems to better define their effectiveness.
3

K. Nielson, 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

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

Official Business

Penalty for Private Use $300

EPA/600/SR-95/090