United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-95/149 April 1996
EPA Project Summary
Design and Testing of Sub-Slab
Depressurization for Radon
Mitigation in North Florida Houses:
Part I - Performance and Durability
C. E. Roessler, R. Morato, R. Richards, H. Mohammed, D. E. Hintenlang,
and R. A. Furman
A demonstration/research project
was conducted to evaluate sub-slab
depressurization (SSD) techniques for
radon mitigation in North-central Florida
where the housing stock is primarily
slab-on-grade and the sub-slab medium
typically consists of native soil and
sand. Objectives included developing
and testing the use of a soil depressur-
ization computer model as a design
tool, optimization of SSD design for
North Florida houses, and observation
of the performance and durability of
the installed systems.
Between May 1989 and August 1990,
SSD systems were designed and in-
stalled in nine houses—seven with
simple rectangular floor plans and two
with more complex L-shaped designs.
Installations included a single-suction-
point system in one house and two-
suction-point/single-fan systems in
eight houses. The installation in one of
the larger L-shaped houses consisted
of a single-suction-point system in ad-
dition to a two-suction-point/single-fan
system. All systems used small diam-
eter, nominal 50-mm (2-in.) piping.
All houses were equipped with con-
tinuous radon monitors and integrat-
ing radon monitors were also deployed.
All houses were visited on a regular
schedule for measurements and obser-
vations.
The mitigation successfully reduced
indoor radon concentrations originally
on the order of 10 to 30 pCi/L to post-
mitigation values of <4 pCi/L in all nine
houses. Levels were reduced to values
on the order of 2 pCi/L or less in three
houses.
Installation experiences demon-
strated the importance of avoiding
"short-circuit" air flow leakage near
suction points, providing drainage for
moisture that condenses in the system
during cooler weather (even in Florida),
and sealing around discharge ducts at
roof penetrations to prevent re-entry of
exhausted sub-slab gases.
System manipulations indicated that
a single suction point was sufficient on
two houses with 160 to 170 m2 (1700 to
1800 ft2) slabs, but that passive ventila-
tion is not likely to be effective for this
type of sub-slab medium.
During the limited time available for
durability observations (3 to 18
months), the systems retained effec-
tiveness in maintaining reduced indoor
radon concentrations, no fans failed,
and no structural effects were ob-
served.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Air Pollution
Prevention and Control Division, Re-
search 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).
Background
This work was conducted in North
Florida because of the presence of el-
evated indoor radon1 levels and the need
to investigate mitigation techniques sue-
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cessful for the housing stock and condi-
tions represented.
In late 1986, results from a statewide
indoor radon survey identified two focal
areas of elevated indoor radon in Florida:
one in the Bone Valley phosphate mining
region of West Central Florida (Polk,
Hillsborough, and surrounding counties);
and the other in the North Florida Haw-
thorn formation region—with the greatest
affected populations in the Gainesville-
Ocala area (Alachua and Marion coun-
ties). Several other studies have confirmed
the presence of indoor radon levels rang-
ing from <1 to about 200 pCi/L in this
area.
In Florida, the housing stock is primarily
of slab-on-grade construction with several
variations of floor-wall joining. There is a
small percentage of crawl-space and slab/
crawl-space combination houses (both
open and enclosed crawl spaces), and
there are very few houses with basements.
The U.S. Environmental Protection
Agency (EPA) has suggested that soil de-
pressurization is the most successful
method of limiting indoor radon; thus sub-
slab depressurization (SSD) appeared to
be a promising mitigation method for
Florida houses. However, at the time this
project was initiated, most mitigation ex-
perience in the U.S. had been with base-
ment houses. Furthermore, the sub-slab
materials commonly used in Florida con-
struction consist of native soil and sand.
These would be expected to have lower
air permeabilities than the coarse gravels
commonly used under basement slabs in
regions of the U.S. where SSD has been
highly successful. This suggested that
more complex and more robust systems
might be required to successfully control
radon in construction typical of Florida.
Radon mitigation demonstration work in
Florida was begun with the 1987 initiation
of the EPA-sponsored Florida Radon Miti-
gation Project - Phase I in Central Florida
(Polk county). In late 1987, EPA also spon-
sored a University of Florida (UF) project
to identify elevated radon houses that
might serve as candidates for a parallel
North Florida mitigation project. During the
1987-88 winter, screening measurements
(charcoal collector method) were made in
nearly 400 Gainesville and Ocala vicinity
slab-on-grade houses in neighborhoods
designated on the basis of geological po-
tential for elevated radon. In these screen-
ing measurements on this selected group
of houses, about 70% of the indoor radon
concentrations exceeded 4 pCi/L and
about 20% of the total exceeded 20 pCi/L.
The North Florida effort continued with
the August 1988 initiation of the research
and demonstration project, Florida Radon
Mitigation Project Phase II - North Florida.
Objectives
This project had a demonstration objec-
tive and a series of research objectives.
The demonstration objective was to dem-
onstrate mitigation methods that are ef-
fective for the substrate and construction
type characteristic of the North Florida
region. Initial emphasis was on sub-slab
depressurization (SSD). The project had
three research objectives:
1. Develop tools for design of SSD
systems. This includes testing the
use of a soil depressurization
computer model2.
2. Optimize SSD design for North
Florida houses.
3. Observe the short- and long-term
performance and durability of the
installed SSD system in this en-
vironment
This project involved the following work
areas:
1. Select and characterize a candi-
date pool of houses.
2. Mitigate a subset of these houses
—select houses, design mitigation
systems, and install mitigation
systems.
3. Monitor:
3a. Collect baseline data prior
to mitigation.
3b. Monitor initial post-mitigation
performance.
3c. Conduct special studies on
installed systems for the pur-
pose of system optimization.
3d. Evaluate durability of in-
stalled systems-continue
monitoring and observations
for the duration of the
project.
Diagnostic Methods
From the data obtained in a house iden-
tification study, 12 elevated radon houses
were selected as potential candidates for
1The terms "radon" and "Rn" are used to designate the
radon isotope, radon-222; and "radium" is used to
designate radium-226.
2 Further development and testing of a computer model
previously developed at UF for simulating sub-slab
pressures and flows during the operation of soil de-
pressurization systems was authorized. The work,
which included expanding the model, developing it as
an SSD design tool, and validation, is presented in Part
II of this report.
the mitigation demonstration. These
houses were visited, and the EPA diag-
nostic measurements were performed.
These diagnostic observations included
descriptive information, sub-slab measure-
ments, radon measurements, and house
dynamics observations.
Sub-slab measurements included de-
termination of soil gas radon by "sniff and
"grab" sampling, sub-slab communication
testing, calculation of effective permeabil-
ity, and sampling of the sub-slab material.
Indoor radon measurements included
short-term measurements of concentra-
tions in the living space and also mea-
surements of radon in the building shell.
House dynamics measurements included
indoor/outdoor and indoor/sub-slab pres-
sure differential measurements under vari-
ous conditions and blower door pressur-
ization/depressurization tests.
Mitigation System Design and
Installation
The design procedures are described in
the Part II report. Briefly, potential suction
points were located on the basis of acces-
sible, unobtrusive locations—usually in in-
terior closets. The UF soil depressuriza-
tion computer model was then used as a
design tool. For an initial set of five houses
selected for mitigation in 1989, the model
was used to simulate pressure fields un-
der proposed designs. Suction system
pressures and flows were predicted by
superimposing the sub-slab "system curve"
on the respective fan performance curves
of candidate fans. For each house, the
number of suction points, their locations,
and the fan size were selected from the
combination giving a pressure field cover-
age believed to be adequate to overcome
inflow of radon-bearing soil gas. Subse-
quently, the computer model was used to
develop soil depressurization system
guidelines for the Florida radon-resistant
building code. For a second set (four
houses) selected for mitigation in 1990,
system designs were specified using the
evolving code guidelines.
To save cost, reduce space require-
ments, and facilitate installation, nominal
50-mm (2-in.) polyvinyl chloride (PVC) pip-
ing was specified for the major runs of the
SSD systems rather than the nominal 100-
mm (4-in.) piping reported in the literature
for previous mitigation projects. It was an-
ticipated that, because of the low flows
associated with the low permeability
Florida sub-slab medium, flow-related pres-
sure losses in the smaller piping would
not be large enough to compromise the
effectiveness of the system.
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At each suction point, sub-slab fill and/
or soil was removed to form a roughly
hemispherical pit, approximately 0.5 to 0.9
m (20 to 36 in.) in diameter. Nominal 100-
mm (4-in.) PVC piping with a cleanout
branch to serve as an access port was
installed through the slabs. The remain-
der of the suction system consisted of
nominal 50-mm (2-in.) PVC piping. Suc-
tion piping was run vertically from the pit
to the attic. Fans were located in the attic.
For the systems with two suction points,
lateral piping was run from the vertical
risers to a tee located under the suction
fan. For the single-suction-point systems,
the vertical piping was run directly up to
the fan.
Systems were installed by the research
team. Electrical hookup was provided by
licensed electrical contractors.
Monitoring
Approach, Parameters, and
Measurements
At each house, monitoring was con-
ducted during three time periods: 1) the
baseline data collection period, 2) the sys-
tem installation and tuning period, and 3)
the post-installation performance/durabil-
ity monitoring period. Data collection con-
sisted of a combination of 1) continuous
multi-parameter data acquisition (in a sub-
set of four houses), 2) continuous radon
monitoring, 3) integrated radon monitor-
ing, and 4) point measurements and ob-
servations in conjunction with site visits.
The continuous recording data-acquisi-
tion systems which were installed in the
subset of four houses (referred to as "in-
strumented houses") consisted of data log-
gers with sensors for pressure differential
(outdoor vs. indoor and sub-slab vs. in-
door), indoor radon, temperature (indoor
and outdoor), rainfall, and wind speed and
direction. Data were sampled every 30
seconds and summed or averaged, and
hourly sums or averages were stored in
memory.
Indoor radon was monitored continu-
ously in all houses, either as part of the
data logging system (hourly averages) in
the instrumented houses or by a stand-
alone continuous radon monitor (4-hour
averages) in the other houses. Integrating
radon monitors (electret ionization cham-
bers) were also used.
During site visits to the houses, pres-
sures and flows were measured in the
suction lines near the suction point, and
"sniff and "grab" sample measurements
were made of radon concentrations in the
sub-slab and/or exhaust air. Qualitative
observations were made of the system
and house condition.
Baseline Measurements
Baseline data collection was targeted
for at least a month-long period prior to
installation of the SSD system. Measure-
ments included indoor radon concentra-
tion by integrating detectors, indoor radon
concentration by continuous monitoring,
pressure differentials (in some instru-
mented houses), and weather data (in
some instrumented houses).
Post-Installation Performance/
Durability Monitoring
Following installation and tuning of the
mitigation system, continuous data acqui-
sition systems (instrumented houses) or
continuous radon monitors (non-instru-
mented houses) were operated, integrat-
ing radon monitors were deployed, and
periodic house visits were performed.
Post-installation monitoring was con-
ducted according to the following general
three-stage schedule:
• Stage 1 Monitoring (in service <6
months)-Continuous and/or integrat-
ing indoor radon monitoring was per-
formed and houses were visited bi-
weekly to observe system operation,
measure pressures and flows, and
service radon monitoring equipment.
• Stage 2 Monitoring (in service 6 to 12
months)—Houses without data loggers
were visited monthly. For houses with
data loggers, data acquisition was
continued, data were reviewed, and
visits were performed as necessary.
• Stage 3 Monitoring (in service >12
months)—As a longer-term follow-up,
visits were conducted approximately
every 6 months to inspect the sys-
tems, measure pressures and flows,
and deploy radon monitors for a week-
long measurement.
Performance and durability were evalu-
ated in terms of:
• System Performance and Interaction
with the Sub-slab Medium—System
pressures and flows, noise and vibra-
tion, and requirements for adjustments
and maintenance.
• Condition of the Sub-slab Environ-
ment-Effective permeability calculated
from pressures and flows, and ex-
haust air and/or sub-slab radon con-
centrations.
• Effectiveness-Indoor radon concen-
trations.
• Structural Effects-Observations for
evidence of subsidence, heaving,
cracking, separation of joints, etc.
In addition, responses were made to
homeowner questions or homeowner-iden-
tified problems.
Results and Discussion
House Characterization
Diagnostics were performed on 12
Gainesville and Ocala vicinity slab-on-
grade houses during the last week of No-
vember 1988.
Installation of Demonstration
SSD Mitigation Systems
SSD systems were installed in nine
houses: six in Gainesville and three in
Ocala (Table 1). House floor plans in-
clude seven rectangular and two more
complex, L-shaped designs. The installa-
tions include one house with a single-
suction-point system, seven with two-suc-
tion-point/single-fan systems, and a house
with both a two-suction-point/single-fan
system and a single-suction-point/single-
fan system. Four houses were instru-
mented for continuous data acquisition.
Five of the systems were installed be-
tween May and November 1989, three in
Gainesville and two in Ocala. These
houses were all of simple, single rectan-
gular slab configuration with slab areas
ranging from 158 to 195 m2 (1700 to 2100
ft2).The system at the smallest house con-
sists of a single suction point and a single
fan; all of the others are two-suction-point/
single-fan systems. The Gainesville houses
were equipped with continuous data ac-
quisition systems.
During the summer of 1990, systems
were installed in two additional houses
with simple rectangular slabs (149 to 181
m2 or 1700 to 2100 ft2) and in two larger
(195 to 203 m2 or 2100 to 2200 ft2 )
houses with L-shaped floor plans. All of
these systems had two suction points con-
nected to a single fan. The system in the
largest house also had a third suction
point with a second fan. A continuous
data acquisition system was installed in
one of the rectangular houses.
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Table 1.
House
Summary of Mitigation Installations
North Florida Project
Slab,
m2(ff)
Operation
Date
Indoor Rn,
pCi/L
Unmitigated
System on
Rectangular Slabs (7 houses):
Ocala-1
Ocala-2
Gainesville-1*
Gainesville-2*
Gainesville-3*+
Gainesville-4*
Ocala-3
167 (1800)
164(1760)
164(1760)
194 (2087)
158 (1700)
181 (1950)
149 (1608)
L-Shaped Slabs (2 houses):
Gainesville-5#
Gainesville-6
195 (2100)
203 (2188)
May 1989
May 1989
Jul 1989
Nov 1989
Oct 1989
May 1990
Aug 1990
Jul 1990f
Jul 1990f
16
10
11
25
9
11
30
25
26
2.5
2.0
3.5
2.5
2.0
2.6
2.0
2.5
2.5
* Continuous data acquisition systems (4 houses).
f Although Gainesville-5 & -6 were turned on July 1990, they required further adjustment and
became successful Oct 1990.
System Types:
+ Gainesville-3: Single-suction-point system
# Gainesville-5: Dual installation
(Two-suction-point/single-fan system
plus single-suction-point/single-fan system)
All others: two-suction-point/single-fan systems
1 house
1
7
9 houses
Mitigation Results
The mitigation successfully reduced in-
door radon concentrations originally on
the order of 10 to 30 pCi/L to post-mitiga-
tion values of < 4 pCi/L in all nine houses.
Levels were reduced to values on the
order of 2 pCi/L or less in three of the
houses.
Design and Installation
Experiences
Mitigation Design
As indicated above, the UF soil depres-
surization model was used as a design
tool in placing suction points and sizing
system components. The results of this
work are presented in the Part II report.
Moisture Condensation
The early installations had some
undrained low points in the horizontal pip-
ing runs in the attics; with the advent of
cool weather in November 1989, water
condensation from the moist exhausted
air essentially blocked these systems and
compromised their effectiveness. This
problem was overcome by installing drain
lines from the moisture traps.
Sub-Slab Leakage
It was observed that air leakage near
the suction point can compromise the sys-
tem effectiveness. For example, in one
case, "short-circuit" flows from a leakage
crack near one suction point of a two-
point, single-fan system resulted in exces-
sive flows at that suction point, an imbal-
ance of the system, a compromised pres-
sure field, and unsatisfactory effectiveness.
Caulking the crack resulted in satisfactory
performance. Subsequent failure of the
silicone caulking resulted in degraded per-
formance; this was remediated by
recaulking with urethane elastomer. Other
experimental work and simulation with the
computer model indicated that leakage at
points more remote from the suction point
has much less influence on effectiveness.
Re-entrainment
An adventitious experience indicated the
potential for re-entrainment problems. Fol-
lowing the initial installations in two houses,
indoor radon levels were >:10 pCi/L when
the systems were operating. Attic levels
of 10's of pCi/L were found in subsequent
radon monitoring. Investigation revealed
that the roof penetration was not sealed
around the vent pipe, apparently provid-
ing the opportunity for discharged sub-
slab gases to enter the attic and be drawn
into the house ventilation system. Sealing
the roof penetrations reduced radon con-
centrations in the attics and indoors to <4
pCi/L (Table 1).
Optimization Studies
Pipe Sizing
Nominal 50-mm (2-in.) suction piping
was installed as planned. For most of the
cases (61% of the suction holes), flows
were sufficiently low that calculated pres-
sure losses due to flow were <15 Pa/10
m, and for 90% of the holes losses were
calculated to be <100 Pa/10 m. In the two
cases of the highest flows where calcu-
lated losses were >100 Pa/10 m, actual
pressures on the order of -300 Pa (-277
to -328 Pa) were observed. The systems,
involving these suction points in combina-
tion with a second suction point, were
effective in reducing indoor radon levels
by factors of 3 to 10, resulting in indoor
radon levels of 3.5 pCi/l or less for these
houses. The use of the smaller piping
permitted savings in cost, space, and in-
stallation effort.
Suction Points
The effectiveness of single-suction-point
operation was tested in several of houses
with two-hole systems by operating these
systems for a period of time with one or
the other suction line valved off. These
experiments indicated that:
1. Two suction points successfully
maintained levels below 4 pCi/L for
slab areas up to 2100 ft2.
2. A single suction point was sufficient
on three houses with 1700 to 1800
ft2 slabs.
Passive Ventilation
The potential effectiveness of passive
sub-slab ventilation was tested in several
of the houses by monitoring indoor radon
with the fans off and the suction lines
open. These experiments indicated that
passive venting (fan off, vent line open)
was not effective for a packed sand/soil
sub-slab medium.
Durability
Special questions were posed concern-
ing durability for systems operating under
Florida conditions. Would continued op-
eration impact the sub-slab environment
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in a manner that affects the continued
effectiveness of the system? If there were
effects on the sub-slab environment, would
these have structural effects on the build-
ing? Would continued performance of the
fans be compromised by the low flow and
high temperature in Florida installations?
As of the end of 1990, the 1989 instal-
lations had been monitored for post-miti-
gation periods on the order of 13 to 19
months. Insufficient time had elapsed for
significant durability monitoring on the 1990
systems which had been installed during
the period May through August. During
the limited observation period (3 to 18
months), the following were observed:
1. With the transient exceptions noted
below, the systems exhibited rela-
tively constant performance and re-
tained their effectiveness in main-
taining reduced indoor radon con-
centrations.
2. In one case, failure of silicone caulk-
ing of a leakage crack near a suc-
tion point resulted in increased
"short-circuit" flows. This was
remediated by re-caulking with ure-
thane elastomer, a more durable
material.
3. With the advent of cold weather,
condensation formed in horizontal
attic runs that were not self-drain-
ing. This resulted in an audible gur-
gling noise, reduced flow, and in-
creased fluctuations in indoor ra-
don concentrations. This condition
was remediated by installing traps
and drains.
4. No fan failures were observed—any
effect of low flow on fan life was
not expressed during the available
observation period.
5. No structural effects were observed.
6. With the exception of the "gurgling"
associated with the water conden-
sation before the installation of traps
and drains, there were no
homeowner complaints of noise or
other annoyances.
Conclusions and
Recommendations
Design Considerations
1. SSD was effective for North-central
Florida slab-on-grade houses of
both simple rectangle and L-shaped
floor plans.
2. For the sub-slab media found in
this region, low flows permitted use
of smaller diameter, nominal 50 mm
(2 in.) piping.
3. Two suction points were success-
ful for slab areas up to 200 m2
(2100ft2).
4. A single suction point was suffi-
cient on three houses with single-
level, rectangular slabs with areas
on the order of 160 to 170 m2 (1700
to 1800ft2).
5. Experiments with installed active
systems (fan off, vent line open)
indicated that passive ventilation is
not likely to be effective for this
type of sub-slab medium.
Installation Considerations
1. Air leakage near the suction point
can compromise system effective-
ness; leakage at points farther from
the suction point has much less
influence on effectiveness.
2. Even in Florida, moisture can con-
dense in the system during cooler
weather; it is important to avoid low
points in horizontal attic runs and
to install traps and drains if water
trapping points cannot be avoided.
3. It is important to seal around the
discharge duct at the roof penetra-
tion to prevent re-entry of the ex-
hausted sub-slab gases. Examina-
tion for other sources of re-entrain-
ment is also warranted.
Performance and Durability
The following conclusions are limited by
being based on short observation times-
3 to 18 months:
1. Pressure and flow values in SSD
systems may exhibit some tempo-
ral variability; documentation of per-
formance from point measurements
should be based on averages from
a series of measurements taken on
different days.
2. On a near-term basis, SSD sys-
tems as installed in this project re-
tain effectiveness in maintaining re-
duced indoor radon concentrations.
3. Continued integrity of sealing of po-
tential short-circuit air flow sources
near suction points is essential to
continued effectiveness. System
maintenance should include inspec-
tion of such sealing.
4. During cooler weather, unintended
trapping of moisture condensation
in horizontal attic runs can compro-
mise system performance. Mainte-
nance should include inspection for
such inadvertent effects.
5. Fan failures have not been identi-
fied as a problem in the short term
(based on observing a small num-
ber of systems).
6. Structural effects have not been
identified in the short term.
7. Other than for the noises associ-
ated with the water condensation
before correction, these systems
have not generated homeowner
complaints.
Short-term durability information would
be enhanced by following all houses for at
least a year, and long-term durability in-
formation would be gained by following all
the houses even longer.
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C. Roessler, R. Morato, R. Richards, H. Mohammed, D. Hintenlang, and R. Furman
are with the University of Florida, Gainesville, FL 32611.
David C. Sanchez is the EPA Project Officer (see below).
The complete report consists of two volumes entitled "Design and Testing of Sub-
Slab Depressurization for Radon Mitigation in North Florida Houses: Part I.
Performance and Durability."
"Volume I. Technical Report" (Order No. PB96 -103 585; Cost: $21.50, subject to
change)
"Volume II. Data Appendices," (Order No. PB96-103 593; Cost: $35.00, subject to
change)
The above reports 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
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
EPA/600/SR-95/149
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