United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/059 June 1992
EPA Project Summary
Natural Basement
Ventilation as a Radon
Mitigation Technique
A. Cavallo, K. Gadsby, and T.A. Reddy
Natural basement ventilation has al-
ways been recommended as a means
of reducing radon levels in houses.
However, its efficacy has never been
documented. It has generally been as-
sumed to be a very inefficient mitiga-
tion strategy since it was believed that
dilution was the mechanism by which
radon levels were reduced.
Natural ventilation has been studied
in two research houses during both
the summer cooling season and the
winter heating season. Ventilation rates,
environmental and house operating pa-
rameters, and radon levels have been
monitored; it can be definitively con-
cluded from radon entry rate calcula-
tions that natural ventilation can re-
duce radon levels two ways: (1) by
simple dilution, and (2) although less
obvious, by providing a pressure break
which reduces basement depressuriza-
tion and thus the amount of radon-
contaminated soil gas drawn into the
house.
Thus, basement ventilation can be a
much more effective ventilation strat-
egy than was previously believed. It
might be especially useful in houses
with low radon concentrations (of the
order of 10 pCi/L) or those with low
levels that cannot be mitigated cost-
effectively with conventional technol-
ogy.
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
Radon emanation from naturally occur-
ring soils, as distinguished from building
materials and mine tailings used as con-
struction fill, has been suspected of being
a significant source of indoor air pollution
in single family houses since the early
1980s. This concern grew out of studies
undertaken after the first energy crisis in
1973 to understand energy consumption
patterns in houses and to reduce energy
consumption, among other ways, by seal-
ing houses and reducing building air ex-
change rates. It was immediately realized
that reducing ventilation rates had the un-
desirable side effect of causing an in-
crease in trace gases such as volatile
organic compounds, oxides of carbon and
nitrogen, and moisture, decreasing both
comfort and safety.
It was initially believed that the effect of
ventilation on indoor radon concentration
was the same as for all other indoor air
pollutants; i.e., that ventilation reduced in-
door radon levels by dilution. This is based
on a very simple model: if radon entry rate
SRn is assumed to be constant and equal
to the removal rate, SRn = \C , where \
is the air exchange rate ana CRn is the
radon concentration.
Results from initial experiments (in which
it was found that basement radon concen-
trations were inversely proportional to the
ventilation rate), as predicted by the above
equation, seemed to confirm this hypoth-
esis. Thus, to reduce radon levels by a
factor of 10 would require an increase in
the air exchange rate by that same factor,
Printed on Recycled Paper
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which in most cases is neither practical
nor desirable. The experiments used an
air-to-air heat exchanger to control the
basement ventilation rate. An air-to-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 different
from natural ventilation in which a base-
ment window is opened, providing a pres-
sure break; nevertheless, it resulted in
ventilation's being thoroughly discredited
as a means to control indoor radon.
However, the mechanisms which bring
radon into a house are completely differ-
ent from those causing high levels of many
other indoor air pollutants. Most often, the
source of undesirable indoor chemicals is
within the house itself; e.g., poorly sealed
paint cans and cleanser containers, or rug
pads and foam stuffing in furniture. Radon
entry into a house is dominated by pres-
sure-driven flow of soil gas rather than by
omissions from building materials. The
subsoil pressure field of the house is
caused by: wind-generated depressuriza-
tion of the house, basement depressur-
Ization caused by air handler operation,
and (most importantly) by basement de-
pressurizatfon induced by the tempera-
ture difference between the outdoors and
the house interior (stack effect).
The above discussion indicates that ra-
don entry rate S^ cannot be a constant
but must be a function of the basement-
to-subsoil pressure differential. Thus, base-
ment ventilation can theoretically reduce
indoor radon levels both by dilution and
by providing a pressure break which re-
duces the basement-to-subsoil pressure
differential which in turn reduces the ra-
don entry rate.
Experiments
The effect of natural basement ventila-
tion (i.e., opening basement windows) on
indoor radon levels has been examined in
two Princeton University research houses:
PU31 during the winter heating season
and the summer cooling season, and in
PU21 during the winter heating season.
This Summary reviews only the results
from research house PU21.
Instrumentation
The houses are instrumented to mea-
sure:
1. Pressure differentials across the build-
ing shell and between the basement
and the upstairs (PU21 only), using
differential pressure transducers.
2. Basement, living area (PU21 only),
and outdoor temperatures, using ther-
mistors.
3. Basement, living area, and subslab
and in-the-block radon levels (PU21
only), using a CRM (Lawrence Berke-
ley Continuous Radon Monitor) or a
PRO (Pylon passive radon detector).
4. Basement relative humidity, using a
CS 207 relative humidity probe.
5. Heating and air-conditioning system
usage, using a sail switch.
6. Building air exchange rate and
interzonal flows, using a PFT
(perfluorocarbon tracer) system. As
many as four gases may be used in
this system, but for these experiments
only two were needed. Emitters (four
to eight per zone) were placed in
temperature regulated holders in the
basement and living area.
In addition, a weather station at
Princeton University monitored tempera-
ture, rainfall, relative humidity, barometric
pressure, and wind speed and direction.
The weather station data as well as
house dynamics data were read every 6
seconds and averaged over 30 minutes,
while the air infiltration and interzonal flow
measurements were averaged over a mini-
mum of 2 days.
Experiments in Research
House PU21
Natural ventilation experiments have
been carried out in research house PU21
during the winter heating season; the re-
sults of these experiments are summa-
rized here.
The research house has the following
characteristics:
SIZE:
1970 ft2* living area, 525 ft2 base-
ment.
TYPE:
Modified ranch. The living room/din-
ing room has a cathedral ceiling with
a large window area facing almost
due south. A cinderblock basement
underlies about 30% of the house,
with the remainder built on a slab.
There is a cinderblock chimney stack
in the center of the house.
FIREPLACE:
Large fireplace in the living room.
HEATING SYSTEM:
Central, gas, forced-air heat furnace
in basement.
COOLING SYSTEM:
Central air conditioning.
HOT WATER: Gas hot water heater in
basement.
RADON LEVEL:
~120 pCi/L in basement.
The house had been mitigated with a
subslab mitigation system which was
turned off during the ventilation experi-
ment. The perimeter floor/wall shrinkage
crack had also been sealed and Dranjer©
basement drain seals installed as part of
the mitigation. Figure 1 is a basement
floor plan of research house PU21; loca-
tions of the basement window, radon in-
strumentation, and capillary adsorption
tubes (CATS) are indicated. Figure 2 is
the upstairs (living area) floor plan for
PU21; locations of the CATS and radon
instrumentation are indicated.
The effect of opening a basement win-
dow on indoor radon levels and the base-
ment/outdoor pressure differential in PU21
is illustrated using continuous radon and
pressure data in Figures 3 and 4. Data
points are 30-minute averages of the pa-
rameters; the experiment was carried out
between Julian Date (JD) 47,1990 (90047)
and JD90050.5. Shown in Figure 3 are
basement radon levels as measured with
a pumped CRM, which has a response
time of less than 30 minutes, and upstairs
radon levels as measured with a Pylon
(PRO), which has a response time of about
3 hours. Plotted in Figure 4 is the pres-
sure differential across the south wall of
the basement (positive values indicate that
the basement is depressurized relative to
the outdoors). A normally closed base-
ment window was opened at JD90048.4
and 90049.45, and closed at JD90048.83
and 90049.8.
The basement/outdoors pressure differ-
ential responds immediately to the closing
or opening of the window with a ~1.5-Pa
change in this parameter. (Note that, even
with the window open, the basement still
remains depressurized relative to the out-
doors.) This is a strong indication that the
radon entry rate into the basement must
change; this is in fact the case, as verified
by measurements in other experiments of
building air change rates and interzonal
flows, radon levels, and radon entry rates.
Radon levels respond over a longer pe-
riod of time to a window opening or clos-
ing. This is to be expected since the total
basement air exchange rate (defined as
the flow of outdoor air plus the flow from
the living area into the basement) is ap-
proximately 1 air change per hour (ACH),
and the building air exchange rate is about
0.3-0.6 ACH. Thus, the time necessary to
* 1 ft2 = 0.0929 nf-
-------
23
25.75
14.25
Window
X - CATs
R - Radon Sampler (Dimensions in Feet: 1 ft = 0.3 m)
Figure 1. Basement floor plan of PU21 showing CA Ts, radon sampler, and window.
achieve a new steady state must be of
the order of 2 to 3 hours. In addition, the
response time of the upstairs radon de-
tector is itself of the order of 3 hours,
which is why there is such a difference in
the time response of the upstairs and base-
ment radon levels.
It is also of some importance to note
that natural variations in the building be-
havior are of the same order of magnitude
as those caused by opening a basement
window. An example of this occurs around
time JD 90048. The decrease in indoor
radon and basement depressurization in
this time period was caused by an un-
usual midwinter temperature spike in which
the outdoor temperature rose and fell by
8°C in a 12-hour period, changing the
indoor/outdoor temperature differential and
the magnitude of the stack effect. It is
essential that an experiment be of suffi-
cient duration to be able to average over
such excursions.
The natural ventilation experiment in
PU21 was conducted over a 17-day pe-
riod in February; two periods of 2 and 3
days each were used to determine the
baseline building conditions (windows
closed), and three 4-day periods were used
to determine the building operating pa-
rameters with a single basement window
(~2.2 ft2 window area) open. In Figures 5
through 7, described below: in experiments
1 and 5, the basement window was closed;
and in experiments 2,3, and 4, the base-
ment window was open.
The effect of basement ventilation on
basement and upstairs radon levels is
shown in Figure 5. With the windows
closed, basement radon levels were about
120 pCi/L, while upstairs levels were about
a factor of 2 or less lower (80 pCi/L). This
is a fairly typical result, and a consequence
of the basement's being isolated from the
living area. With one basement window
open, the upstairs levels were about a
factor of 2 higher than the basement lev-
els. This is quite unusual and indicates a
radon entry route into the living area which
bypasses the basement. This result was
checked by making two simultaneous con-
tinuous measurements of the upstairs ra-
don levels. A similar result was noted in
the measurements made in the summer
of 1989 on PU31; this indicates one way
that basement ventilation, while certainly
reducing indoor radon levels, might not be
as effective in reducing living area radon
levels as in reducing basement levels.
Another consequence of a reduction in
basement radon entry rate is an increase
in subslab and basement radon levels.
This is observed, as shown in Figure 6, in
which basement and subslab radon levels
are plotted for the different experiment
. periods. The strong decrease in basement
radon levels with the window open and
the simultaneous increase in subslab ra-
don levels are clear. The reason for the
magnitude of the increase in subslab ra-
don levels is not obvious, since it would
depend on the amplitude and spatial dis-
tribution of subslab soil permeability, mois-
ture, and radium content. Qualitatively, the
effect is certainly present.
A critical factor in this experiment is to
quantify the effect that basement ventila-
tion has on the building air exchange rate,
since the observed reduction in radon lev-
els could be caused by a large increase in
the ventilation rate. This has been done
using the perfluorocarbon tracer (PFT) sys-
tem: results are illustrated in Figure 7, in
which building air exchange rate and base-
ment radon levels are plotted. The build-
ing air exchange rate increases by a fac-
tor of 2, from 0.3 to 0.6 ACH, when the
basement window is opened. Note that
the basement radon levels decrease by a
much larger factor (~6-8), again indicating
that dilution cannot account for the entire
decrease in radon levels. Doubling the air
exchange rate corresponds to a ventila-
tion rate of 115 cfm,* roughly comparable
to that achieved by a subslab depressur-
ization system, which for this house re-
duces radon to much lower levels than
basement ventilation. However, the main
application of natural ventilation is ex-
pected to be in lower radon level houses
where installation of a subslab system
might not be justified.
Using the interzonal flows and tracer
gas concentrations measured by the PFT
system, the basement and living area ra-
* 1 cfm = 0.0004719 m3s.
-------
r/.s
7.9
74.8
I
t
external .;:,,
Deck >
t
Shed
;'*''i'|Oa(?>Brt; ..... H'Vv, '
Dining
Room
Living Room
Bath
Bedroom
Study
Kitchen
23.8
Bath
Bedroom
15
^^
10
-^«-
15
X - CATs
R - Radon Sampler
Flgura2. Upstairs floor plan ofPU21 showing CATs and radon sampler.
(Dimensions in Feet)
N
200
Figure 3. Basement, upstairs radon level vs.
Mlandate;sBquenceofwindowopen
andwindovtclosed, PU21O-open;
C - closed; T= temperature spike.
90047.0 90048.0 90049.0
Julian Date
90050.0
UpRn
BsmtRn
-------
don entry rates can be calculated. The
two-zone system of flows and tracer con-
centrations is illustrated in Figure 8. Ra-
don entry rates SIR) (i=1 ,2) can be calcu-
lated two ways. The first method is to use
the flow rates deduced from tracer gas
measurements but assume that C,, and
C are the radon concentrations in zones
, 1 (basement) and 2 (living area), respec-
tively:
s,nn = (R,o + R,2)C1l-R21c12 d)
(2)
The second method is to assume that
the tracer gas and radon behave in the
same fashion once they enter the house
and that the interzonal flow from the living
area to the basement (R21) is very small
compared to the basement infiltration plus
interzonal flow from the basement to the
living area (R10 + R12). In this case the
ratio of the tracer gas emission rate in
zone 1, S,,, to the concentration of tracer
gas in zone 1, C,,, is the same as the
ratio of the radon entry rate in zone 1 to
the radon concentration in zone 1 :
:S1Rn/C1Hn
(3)
Results of the entry rate calculation us-
ing Eq. 3 are shown in Figure 9. There is
a factor of 3 decrease in the entry rate
with natural basement ventilation com-
pared to that without ventilation, and this
difference is substantially outside the er-
ror bars of the individual data points.
The two methods for calculating the en-
try rate are compared in Figure 10. Using
the computed interzonal flow rates (Eq. 1)
results in substantially more uncertainty
than when Eq. 3 is used; this is a conse-
quence of the errors inherent in the
interzonal flow calculations using tracer
gas measurements. There is, nonetheless,
general agreement between the two meth-
ods. The computation using the interzonal
flows always yields a lower entry rate than
the second method: this is consistent with
the presence of an entry route into the
living area which bypasses the basement.
The entry rate of radon into the living
area can be calculated from Eq. 2 using
the interzonal flow data from those peri-
ods when the basement window was open
and upstairs radon levels were approxi-
mately twice as large as the basement
levels. The radon entry rates in both zones
are about equal in this case, about 5 u.Ci/
h. With the basement window closed, the
basement radon entry rate (about 20 jxCi/
h) predominates. This adds an extra com-
plication to the use of natural ventilation
as a mitigation strategy. It remains to be
seen how widely this effect is observed.
90047.0 90048.0 90049.0
Julian Date
90050.0
Figure 4. Outdoor/basement pressure differential vs. Julian date; sequence of window open and
window closed, PU21O = open; C = closed; T = temperature spike.
200
100-
_-*SI
v-- BsmtRn
*UpRn
12345
Experiment
Figure S. Basement, upstairs radon, PU21: experiments 1,5, window closed; and experiments 2,3,
4, window open.
-------
FJgumS. Basement, subslab radon, PU21:
experiments 1,5, window closed; and
experiments 2,3,4, window open.
200
I
I
100-
I
< \ -L
J
234
Experiment
1200
-400
H BsmtRn
SbsIRn
200
Figure 7. Building ACH, basement radon,
PU21: experiments 1, 5, window
closed; experiments 2.3,4, window
open.
roo-
1 2 3 4
Experiment
0.8
r 0.7 _^
I
- 0.6 !aT
-0.5 |
.fc
7.4 .§
I
-0.3
0.2
BsmtRn
Bid ACH
Flgut* 8. Flows and tracer concentrations for
two zones.
Cf Zone 2
<
-Hf
fl,7
Cf Zone 1
Ct= Concentration of Tracer i in Zone J
Rf - Flow from Zone i to Zonej
ZoneO
-------
Therefore, measurements in PU21
clearly demonstrate the mechanisms by
which natural ventilation acts to lower ra-
don levels. Both dilution and reduction of
the basement/outdoor pressure differen-
tial and the concomitant reduction in ra-
don entry rate are factors, with the sec-
ond effect being the more important.
Conclusions
Natural ventilation experiments con-
ducted during the summer cooling season
and the winter heating season in research
house PU31 and during the winter heat-
ing season in research house PU21 have
demonstrated that basement ventilation
can reduce indoor radon both by reducing
the radon entry rate and by dilution. Cal-
culations based on measurements using
the PFT system allow the effects of dilu-
tion and entry rate reduction to be delin-
eated and quantified: a decrease in the
basement radon entry rate of a factor of
2-5 and an increase in the building air
exchange rate of about a factor of 2 have
been documented. These results contra-
dict earlier assumptions of the efficacy of
(and mechanisms by which) natural venti-
lation can reduce indoor radon levels, and
indicate that natural ventilation can re-
duce indoor radon levels by much larger
factors than was previously believed.
A rough cost estimate for natural base-
ment ventilation in research house PU21
can be made with the following "assump-
tions: 1) 4911 degree days for the
Princeton area, 2) 115 cfm constant in-
crease in the winter ventilation rate,
3) furnace efficiency of 0.7, and 4) a
heating oil cost of $1/gal.* With these*
assumptions, the additional heating cost
would be $225/yr. This compares sur-
prisingly favorably with the running cost
of a subslab depressurization system
($0.12/kWh, 90 W fan, $50-$100 for ex-
haust of conditioned air) of $140-$190/
yr. Thus, in certain circumstances, base-
ment ventilation could indeed be a rea-
sonable mitigation strategy.
300'
200-
!
700-
30
-20
- 10
Experiment
--.Q-- BsmtRn
«Entry Rte
Figure 9. Basement radon entry rate, basement radon, PU21: experiments 1, 5, windows closed;
experiments 2, 3,4, windows open.
30-
20-
10-
Eq.3
12345
Experiment
Figure 10. Entry rate calculations compared, PU21: experiments 1,5, window closed; experiments
2, 3,4, window open.
1 gal. = 3.785 L.
U.S. Government Printing Office: 1992 648-080/60021
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Recommendations
Experiment results suggest that:
1. Further experiments on natural ven-
tilation should be undertaken in:
a. Low radon level houses (basement
radon concentrations of 10 pCi/L or
less) to verify that low radon levels
can be adequately reduced by this
method.
b. Houses of different construction
styles (to document the magnitude
of reduction in radon concentration
attainable).
2. Other natural ventilation strategies,
such as living area ventilation in-
stead of or in conjunction with base-
ment ventilation, should be exam-
ined.
3. Forced ventilation using air-to-air
heat exchangers should be care-
fully compared to natural ventila-
tion.
A Cavalto, K. Gadsby, and T.A. Reddy are with Princeton University, Princeton, NJ
03544.
Honald B. Moslay is the EPA Project Officer (see below).
The complete report, entitled "Natural Basement Ventilation as a Radon Mitigation
Technique,"(Order No. PB92-166958/AS; Cost: $17.00, 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
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