United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/102 July 1992
Project Summary
The Effects of Natural and
Forced Basement Ventilation on
Radon Levels in Single Family
Dwellings
A. Cavallo, K. Gadsby, and T.A. Reddy
For the first time, the effect of venti-
lation on radon concentrations and ra-
don entry rate in a single-family dwell-
ing has been extensively studied and
documented. Measurements of radon
concentrations, building dynamics, and
environmental parameters made in
Princeton University research houses
over several seasons and under differ-
ent building operating conditions have
demonstrated the functional depen-
dence of radon entry rate on basement
depressurization.
This work clarifies the role of natural
ventilation in reducing indoor radon
concentrations. Although natural ven-
tilation has always been recommended
as a way to reduce indoor radon levels,
its erratic behavior has been noted and
its efficacy has never been docu-
mented. This work shows conclusively
that natural ventilation can decrease
radon levels two ways: (1) by simple
dilution, and (2) although less obvi-
ous, by providing a pressure break
(defined as any opening in the building
shell which reduces the outdoor/indoor
differential pressure). This reduces
building depressurization and thus the
amount of radon contaminated soil gas
that is drawn into the building.
The most important results of these
experiments show the linear depen-
dence of radon entry rate on basement
depressurization and the precise, quan-
titative comparison between radon en-
try rates possible when, for example,
radon mitigation is attempted by seal-
ing off the basement sump. This is the
first time such a scientific approach
has been taken to quantify the results
of this mitigation strategy.
The experiments also examine the
role of basement forced pressurization
and depressurization in determining
radon concentration in the basement
and living area of a house.
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
This systematic investigation of natural
and forced ventilation in Princeton Univer-
sity research houses, instrumented to mea-
sure house dynamic and environmental
parameters, has filled an important gap in
understanding the role of natural ventila-
tion in reducing radon levels in single-
family dwellings. It is noteworthy that,
although natural ventilation is often men-
tioned as a simple way to reduce indoor
radon levels, experiments have never been
conducted to quantify the magnitude of
reduction achieved. The lack of under-
standing of this element of radon entry
into houses was the motivation for this
work, which is the first program to investi-
gate these effects in detail.
A consequence of this lack of experi-
mental work has been considerable con-
fusion in the size of the reduction of radon
concentration possible as well as the rela-
tive importance of each of the mecha-
nisms (dilution and the reduction in base-
Printed on Recycled Paper
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ment deprassurization) by which natural
ventilation effects these reductions. Al-
though the flow of radon-contaminated soil
gas due to basement depressurization has
long been known as the most important
cause of high radon levels in houses, the
critical role of introducing a pressure break
in the building shell to reduce radon levels
has never before been quantified.
A 1988 EPA mitigation manual empha-
sizes the importance of the pressure break
and dilution mechanisms in achieving high
reductions through natural ventilation, but
has only anecdotal data on the reductions
achievable with natural ventilation and no
data to separate out the relative effects of
the two mechanisms. Another detailed
discussion of natural ventilation in 1988,
white more complete, lacks a theoretical
background and experimental verification,
and tends to be somewhat anecdotal. This
serves to emphasize the need for this
series of experiments to clarify these is-
sues.
One set of ventilation experiments ex-
plored the following simple model: if the
radon entry rate S^ is assumed to be
constant and set equal to the removal
rate, we have: S^ = RC^, where R is
the air exchange rate and C^ is the radon
concentration.
Results from these experiments, in
whfch it was found that basement radon
concentrations were inversely proportional
to the ventilation rate when S^ is con-
stant, as predicted by the above equation,
confirmed this model. Thus, to reduce
radon levels by a factor of 10 when SR[l is
constant (i.e., when only the dilution
mechanism comes into play), would re-
quire an increase in the air exchange rate
by that same factor. In most cases, such
a large exchange rate is neither practical
nor desirable. The experiments were done
using an air/air heat exchanger to control
the basement ventilation rate. An air/air
heat exchanger operates in a balanced
mode with inflow and outflow equal and
would neither pressurize nor depressurize
the basement. This is actually very differ-
ent from natural ventilation in which a
basement window is opened, providing a
pressure break.
It is widely recognized that the mecha-
nisms which bring radon into a structure
are completely different from those caus-
ing high levels of many other indoor air
pollutants. Most often the source of un-
desirable indoor chemicals is found within
the structure itself, such as poorly sealed
paint cans and cleanser containers, or rug
pads and foam stuffing in furniture. In
contrast, radon entry into a building is
dominated by the pressure-driven flow of
contaminated soil gas rather than by emis-
sions from building materials. The subsoil
pressure field of the building is caused by
three factors: '.
(1) wind-generated depressurization of
the structure,
(2) basement depressurization caused
by the operation of the air handler
and ventilation equipment, and most
importantly,
(3) by the stack effect, that is base-
ment depressurization induced by
the temperature difference between
the outdoor environment and the
building Interior.
To understand the relative importance
of the competing effects of ventilation and
radon entry begin with the simplest case:
a single-zone system such as a slab-on-
grade-house. In a steady state condition
the radon entry rate (SRn) must be equal
to the removal rate by ventilation. The
mass balance equation is:
SRn ~
(1)
where Rv is the ventilation flow rate and
CR is the radon concentration.
From the above discussion, the radon
entry rate must be a function of the de-
pressurization of the structure:
(2)
where 0.5
-------
«r
^.•s
C TO
5
4
3
2
1 "
^
k(AP)
0 0
AP (Pa)
Figure 1a. Radon entry rate vs. differential pressure for soil underneath basement slab.
Soil gas laminar flow S =k(AP).
-------
1
1 _
2 3
AP (Pa)
4
FlgunSa. Radon entry rate vs. differential pressure for gravel underneath basement slab.
Soil gas flow turbulent S*k(APexp0.5). ':
1 .
I
0 12 3 4 5 t
AP (Pa)
Figure 2b. Ventilation flowrata vs. differential pressure for gravel underneath basement slab.
Flow turbulent/laminar n^c(APexp0.65).
1.5
I
7,0.
0.5 .
0.0
(k/c)(APexp-0.15)
1
5
AP (Pa)
Figure 2c. Radon concentration vs. differential pressure for gravel underneath basement slab.
Soil gas flow turbulent C^-fk/c) (APexp-0.15). :
basement and the upstairs are mea-
sured with differential pressure
transducers.
2. Basement, living area, and outdoor
temperatures are monitored using
thermistors.
3. Basement, living area, and subslab,
and in-the-block radon levels are
monitored with a CRM (Lawrence
Berkeley Continuous Radon Moni-
tor) or a PRO (Pylon passive radon
detector).
4. Basement relative humidity is moni-
tored with a relative humidity probe.
5. Heating and air conditioning sys-
tem air handler use is monitored
using a sail switch.
6. A PFT (perfluorocarbon tracer) sys-
tem is used to measure building air
exchange rate and interzonal flows.
Up to four gases may be used in
this system, but for these experi-
ments only two were needed. Emit-
ters (four to eight per zone) are
placed in temperature regulated
holders in the basement and living
area.
In addition, a weather station at Prince-
ton University monitors temperature, rain-
fall, relative humidity, barometric pressure,
and wind speed and direction.
The weather station data as well as
house dynamics data are read every 6
seconds and averaged over 30 minutes,
while the air infiltration and interzonal flow
measurements are averaged over a mini-
mum of 2 days.
The effect of opening two basement
windows on basement radon levels and
the outdoor/basement pressure differen-
tial is shown in Figures 3 and 4. Base-
ment radon levels are shown in Figure 3;
there is clearly a significant drop in this
parameter, from an average of about 90
pCi/L to about 10 pCi/L when the win-
dows were opened on JD89220.6. The
magnitude of this drop was completely
unexpected. The large diurnal variation in
basement radon levels is due to the op-
eration of the attic fans which depressur-
izes the entire house, increasing the ven-
tilation rate as well as the radon levels.
Measurements of a typical differential pres-
sure transducer are illustrated in Figure 4
(positive pressure indicates that the out-
door pressure is above that of the base-
ment). The large peaks (~3 Pa) in out-
door/basement pressure differential are
due to the operation of the attic fans.
There is an abrupt pressure drop when
the windows are opened, indicating that
the pressure field of the building has been
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Table 1. Ratio of Winter to Summer Radon Levels in Houses
(Assuming APmm =4 Pa, AP Mn=0.25Pa)
Soil Gas Ventilation Radon Level as
Exponent Exponent _ Function of AP
, , Winter/ C^ .Summer
Laminar
0=1
Turbulent
a=Q.S
0.65
0.65
: (AP) '
2.6
0.66
200
8
I
roo .
89216
89218
89220
Mian Date
89222
Figure 3. Basement radon vs. Mian date, PU31.
Two basement windows were opened (O) atJD89220.6.
Q_
"c
6
5
4
3
2
1 _
vui
k
Attic fan AP spike
89216
89218 89220
Julian Date
89222
Figure 4. Outdoor/basement pressure differential vs. Julian date, PU31.
Basement windows opened (O) atJD89220.6;
Note effect of attic fans.
modified. It is clear that, for this house
only, a very small pressure differential
(-0.5 Pa) is needed to drive the radon
level to 10 pCi/L. This result again strongly
suggests that a modification of the base-
ment/soil pressure differential is important
in reducing the basement radon level; how-
ever, the measurement of the building air
exchange rate and interzonal flows and a
calculation of the radon entry rate are
essential for a definitive evaluation of this
problem.
Radon entry rate can be calculated us-
ing:
S1Hn = (R10 + R12)C11 " R21C12
(5)
where C,, and C1Z are basement and liv-
ing area radon concentrations, R10 is the
exfiltration from zone 1 (basement), and
R12 and RZ1 the interzonal flows from the
basement to the living area and the living
area to the basement, respectively. The
interzonal flows and exfiltration are mea-
sured with the PFT system.
The central role of basement depres-
surization in driving radon entry in houses
is shown in Figure 5, where basement
radon entry rate (S1R) calculated using
Eq. 5, is plotted as a function of outdoor-
to-basement pressure differential mea-
sured at the north band joist. These data
are the result of measurements made over
18 months with basement windows closed,
natural ventilation (basement windows
open) and forced basement ventilation.
The duration of each experimental period
was between 2 and 7 days; each data
point used values averaged over the ap-
propriate period.
The radon entry rate is clearly a linear
function of basement depressurization for
AP < 4 Pa, implying that the flow of soil
gas into the basement is laminar. This is
to be expected since the basement slab
for PU31 was poured directly onto the soil
(that is, there is no gravel layer beneath
the slab); as mentioned previously air flow
through most soils is expected to be lami-
nar. At the highest basement aspressur-
ization (AP = 5 Pa) it appear that the
radon entry rate does not increase rela-
tive to 4 Pa; it may be limited by the flow
of radon through the soil. This data point
was obtained in an experiment in which
the attic fans were on continuously to de-
pressurize the house. It must be empha-
sized that the natural operating regime of
most houses, and the range over which
vjrtually all of these data was taken, is for
ah outdoor-to-basement pressure differ-
ential of less than 4 Pa.
Basement radon concentration as a
function of the outdoor/basement pressure
differential for closed-house conditions is
-------
I
j
i
30
20.
10 .
1.5
- 1.2
- 0.9
- 0.6
-0.3
c
I
I
0.0
•a Bsmt Rn
_, BldgACH
0123456
Experiment
FIgura 7. Basement radon, Building ACH.
Experiments 1,5, windows closed; Experiment2, window? open; Experiments 3,4, basement pressurized.
A Cavalo, K. Gadsby, and T. Reddy are with Princeton University, Princeton, NJ
08544. \
Timothy M. Dyess Is the EPA Project Officer (see below).
Ths complete report, entitled "The Effects of Natural and Forced Basement Venti-
lation on Radon Levels In Single Family Dwellings," (Order No. PB92-192194/AS;
Cost: $19.00, 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
Center for Environmental
Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
EPA/600/SR-92/102
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