United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NO 27711
Research and Development
EPA/600/SR-92/207 January 1993
EPA Project Summary
Radon Mitigation Studies: South
Central Florida Demonstration
Charles S. Fowler, Ashley D. Williamson, Bobby E. Pyle, Frank E. Belzer III,
and Raymond N. Coker
In this EPA radon mitigation demon-
stration project, 14 slab-on-grade
houses in Polk County, FL, having in-
door radon levels ranging from 8.7 1o
103 pCi/L,* were mitigated using sub-
slab depressurization (SSD) in a vari-
ety of applications. These applications
were employed to evaluate optimal de-
sign criteria to be recommended as
cost-effective and capable of reducing
indoor radon concentrations in houses
built over compacted soil fills. For all
houses, obvious accessible radon en-
try points were sealed, and 12-20 gal."
suction pits were dug into the fill mate-
rial. For all but one house, multiple
suction holes were necessary to re-
duce adequately the indoor radon con-
centrations. Two of the houses were
mitigated with exterior horizontal suc-
tion holes drilled through the stem
walls. In four of the houses, one or
more of the suction pipes was located
in the garage. All of the rest of the
interior suction holes were located in
closets or some other unobtrusive lo-
cation. Except for the two houses with
exterior systems, the other 12 had miti-
gation fans located in the attic.
In-line centrifugal fans were used to
mitigate each house, although a larger
radial blower was installed overnight
for experimental purposes in one
house, and a vacuum cleaner was used
to simulate a larger suction in another
house for pressure field measurements
only. Post-mitigation worst case radon
concentrations in these houses gener-
ally ranged from over 1 to about 8 pCi/
L. Some of these houses are still being
monitored quarterly with alpha-track
* 1 pCi/L = 37 Bq/m3
" 1 gal. = 3.8 L
detectors to assess long-term mitiga-
tion effectiveness.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle 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
Fourteen existing slab-on-grade houses
with initial indoor radon concentrations be-
tween 8.7 and 103 pCi/L in Polk County,
FL, were mitigated with sub-slab depres-
surization (SSD) systems from December
1987 through December 1989. All of the
slabs were poured on a compacted soil fill.
Most of the houses were on reclaimed
phosphate mining land, and the remaining
ones were on undisturbed mineralized soils.
These features produced a situation in
which very high sub-slab radon was usu-
ally present and the compacted soil me-
dium was resistant to evacuation of the
soil gas because of its low permeability.
The report summarizes the pre-existing
house and soil conditions of the 14 houses,
describes the design and installation of the
SSD systems that were used, evaluates
the systems' effectiveness, and records
conclusions and recommendations for the
design of successful SSD systems in
houses with such low-permeability fill ma-
terial.
Procedure
Candidate houses for participation the
first year's study were selected from earlier
surveys and measurements conducted by
the Polk County Health Department (PCHD)
and the Florida Department of Health and
Rehabilitation Services (DHRS). The sec-
ond year's houses were selected from ad-
Printed on Recycled Paper
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ditional measurements by PCHD or some
private measurement companies and from
volunteers who had heard of the project
from the first year's participants. The sec-
ond year's candidates were further
screened by telephone (for house informa-
tion) and radon measurements with char-
coal canisters. A full diagnostic visit was
made to the 22 candidate houses the first
year, a shortened diagnostic visit was made
to the 11 finalists of the second year, and a
full diagnostic was performed in the six
selected houses. The diagnostic testing
included 1) grab and/or "sniffer" measure-
ments of indoor and sub-slab radon con-
centrations, 2) "sniffer" samples to detect
soil gas entry points, 3) sub-slab communi-
cation tests to measure pressure field ex-
tensions from a suction hole, 4) infiltration
tests using fan doors to quantify the house
leakage area, 5) house differential pres-
sure measurements, 6) site and house
gamma radiation measurements, 7) sub-
slab pressure flow measurements, and 8)
some soil radium measurements.
Houses were selected based prima-
rily on homeowner willingness and coop-
eration, house construction features
(single slabs with minimal dropped floor
areas allowed), standard site and struc-
tural practices (nothing requiring solutions
unique to abnormal situations), minimal
indoor screening radon measurements of
8 pCi/L in the first year and 20 pCi/L in
the second, and adequate access to at
least three sides of the house. The houses
selected were therefore chosen to be simi-
lar in essential features but to have some
diversity in other features of interest. For
instance, all were required to be slab-on-
grade houses, but some monolithic slabs
were selected to compare with the slab-
in-stem-wall construction. A few houses
with frame exterior walls were selected to
compare with houses built with more com-
mon concrete block exterior walls. A few
L-shaped houses were included to com-
pare with rectangular ones. Houses of
moderate size were selected, attempting
to get a range of floor areas, but ex-
tremely small or very large houses were
avoided. Houses with a range of leakage
areas from relatively tight to fairly leaky
were selected.
One of the early goals of the project
was to develop a set of "generic" mitigation
strategies for use in slab-on-grade houses
and to install and evaluate such systems.
In the latter phase of the project, this ap-
proach developed into identifying the engi-
neering design criteria for planning and
installing SSD systems in slab-on-grade
houses built over compacted soil fills.
The mitigation systems selected varied
in approach and application between the
first and second years. Generally, the sys-
tems were installed in stages during the
first year so that effects of the various
components and steps could be studied,
analyzed, and evaluated separately. In the
second year's six houses, the full systems
were installed with specific research ques-
tions in mind, and the systems were acti-
vated in ways to answer these specific
questions.
While the houses selected forthis study
were fairly carefully controlled in many of
their structural and construction param-
eters, there was enough diversity so that
the mitigation systems installed represented
a wide range of features common to what
the commercial mitigator may_encounter_or
need. Mitigation suction holes were drilled
vertically through slabs in closets, garages,
utility rooms, and other spaces. Other suc-
tion holes were oriented horizontally
through stem walls from outside the house
and from the garage space. One suction
hole was drilled from the garage, at an
angle through the garage slab/house slab
interface to the house sub-slab fill material.
Mitigation systems varied in degree of com-
plexity from sealing entry points and in-
stalling one SSD suction hole to various
combinations of two to nine near-perimeter
and interior suction holes.
Results and Discussion
Since the purpose of this research was
to demonstrate and develop procedures
for reducing indoor radon concentrations
in this subset of the housing stock, the
ultimate description of the results will in-
volve to some degree the measurement of
the indoor radon values. Measurements of
other parameters that influence either the
introduction of radon into the structure or
the ability of a system to retard such con-
tamination 'are™also relevantrThis'section
briefly reviews some of the methods used
to measure the results, a synopsis of the
data collected, and an analysis of what
these data mean.
Methodology
The earliest measurements made in
the houses used in this study were the pre-
mitigation diagnostic measurements, which
have already been mentioned. For screen-
ing purposes, indoor radon measurements
by open-faced, 2-day charcoal canisters
were generally used. Sub-slab communi-
cation was measured using some type of
suction apparatus (vacuum cleaner or miti-
gation fan) evacuating a space below the
slab opening, and the pressure fields were
measured by a micromanometer placed at
each of the various smaller test holes drilled
in the slab in different directions and at
different distances from the suction hole.
The house differential pressures were mea-
sured with a micromanometer and various
combinations of house systems (air han-
dler, interior doors, etc.) in a range of
different modes. Potential radon entry
routes were tested using alpha scintillation
cell "sniffers" to check wall outlets, plumb-
ing penetrations, toilet bases, tub traps,
slab seams, obvious cracks, and any other
possible opening to the sub-slab space:
Pre-mitigation activity primarily con-
sisted of indoor radon measurements. Gen-
erally, long-term alpha track detectors were
deployed from the time the houses were
selected until the mitigation systems were
initially-activated, usually from _1--to'11
months; but most typically 2. A continuous
radon monitor that recorded integrated
hourly counts was usually deployed for,at
least 2 weeks in each house before the
mitigation system was activated. In the
second year's houses, the house air han-
dler was cycled between automatic and
continuous modes to measure air handler
effects that had been suspected from the
first year's data. Sometime during the pre-
mitigation data collection, usually when the
house was closed and operating as near
to normal as possible, including calm, stable
weather conditions, two openfaced (2-day)
charcoal canisters were exposed.
The methods of initiating the mitigation
process varied between the 2 years and
occasionally to some extent between
houses within a year. For the first six houses
in the first year's study, the mitigation sys-
tem was installed and activated one suc-
tion hole at a time. In the last two houses,
the two suction hole systems were installed
and activated without stages. In the sec-
ond year's houses, the multiple suction
hole mitigation systems were installed as
"Units'bill 'thefTactivated "according,'to"the
research objectives. In three houses, the
systems were activated with no pits dug
under the suction holes, and then com-
pared with the system with pits dug. In the
other three houses, near-perimeter suction
holes were compared with interior suction
holes. Later the full systems were run at
"optimum" settings in all six houses. In
both year's houses, where feasible, pres-
sure field extension was measured before
the suction pits were dug and again after-
wards. In all situations the indoor radon
concentrations were compared to the ear-
lier, contrasting, orpre-mitigation measure-
ments. Generally, the comparisons were
framed in terms of percent radon reduc-
tion, which usually took the form of (stan-
dard-modified)/standard x 100 where
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"standard" was the baseline condition (pre-
mitigation, normal operating conditions,
etc.) and "modified" was the average con-
centration after the feature being tested
was activated (suction hole activated, pit
dug, air handler placed in continuous mode,
etc.).
Evidence and Analysis
Generally, the pressure field extension
measurements improved when suction pits
were dug. Unfortunately, most of the im-
provement seemed to occur in the magni-
tude of the pressure field at fairly close test
points. Although there was some increase
in the measured pressure field radius, it
was usually not very great. Increasing the
number of suction points appeared to be
the more effective way of extending the
pressure field coverage.
Sealing slab openings identified as ra-
don entry points generally improved pres-
sure field extensions and reduced radon
entry; however, in some houses the direct
effects were hard to distinguish from other
variations. However, in two houses, seal-
ing tub trap areas appeared to have con-
tributed about a 40% reduction in indoor
radon concentrations, and in another, clos-
ing an open atrium produced about a 65%
reduction. Based on short-term (2 weeks
or longer) continuous radon monitor (CRM)
results, the one single suction hole system
produced from 20 to 70% reduction in
indoor radon, depending on which of two
sets of post-mitigation data was used for
comparisons in that house. In five of the
houses where two-hole systems were in-
stalled, 34-93% reductions were experi-
enced. (The lower percent reductions
occurred in the lower level houses.) In the
two initially high and "difficult" houses of
the first year's study, three-hole systems
produced 80-90% reductions.
In the second year's houses, several
additional features of the house and miti-
gation systems were evaluated. Generally,
continuous operation of the air handling
system tended to reduce indoor radon con-
centrations by 10- 65%. Roughly, the more
a house had been "pressurized" by the air
handler in the house differential pressure
diagnostic test, the greater was the radon
reduction effect of the air handler. This
seems reasonable since five of the six air
handlers were in the attic and the pressur-
ization indicates greater return leaks than
supply leaks. With the returns drawing rela-
tively radon-free air into the system and
any slight pressurization having the poten-
tial to impede some radon entry, one would
expect a radon reduction. At one house
with the air handler in a room closet, there
was less evidence of air handler impact on
radon concentrations. In a house with very
little pressurization caused by the air han-
dler, it still had a large effect on radon
reduction. It was later determined that the
reason for this phenomenon was that there
were about as. many supply leaks as return
leaks; so the radon reduction could be
ascribed to dilution by a leaky air handling
system.
As mentioned earlier, suction pits were
shown to improve pressure field exten-
sions. More importantly, digging a suction
pit in the three houses where this experi-
ment was conducted generally reduced
radon concentrations an additional 20%
over those measured with no pit dug. In
the three houses where the suction hole
placement was compared, one showed no
significant difference, between interior and
perimeter suction holes. However, interior
suction holes produced a 14-18% reduc-
tion of indoor radon concentrations in the
second and nearly 40% improvement in
the third. It was thought that the different
results in these three houses were prob-
ably caused by differences in the relative
leakiness of their respective stem walls,
with the lower effectiveness of the perim-
eter holes occurring in the houses with the
leakier stem walls. Overall, in these last six
houses, a three-suction hole system in
one house produced a radon reduction of
36-62% [depending on whether the normal
occupancy pattern (open-house) or the
closed-house levels were used as the ref-
erence], four-hole systems in four houses
produced 27-94% reductions, and a five-
hole system produced a 76% reduction in
the remaining house.
According to the (2 week or longer)
CRM measurements, three of the first eight
houses were reduced to less than the tar-
get 4 pCi/L level for indoor radon concen-
trations, three were between 4.6 and 5.0
pCi/L, and two were between 6.9 and 7.8
pCi/L. In the last six houses, four were
below 4 pCi/L, and the other two were
between 4.1 and 4.3 pCi/L However, if the
long-term (quarterly or longer) alpha track
detectors (ATDs) are used, then six of the
first eight houses had quarterly indoor con-
centrations less than 4 pCi/L, as did four of
the last six houses. The annual average
radon concentrations for these houses can
be approximated by the averages of the
four quarterly ATDs that were deployed.
Two of the first year's houses averaged
less than 4 pCi/L, with the other six aver-
aging from 4.7 to 9.3 pCi/L Three of the
second year's houses averaged less than
4 pCi/L, while the other three averaged
from 5.2 to 5.7 pCi/L What is not clear in
any of the higher averages is whether the
owners turned off the systems during the
year. It was the habit of some, to turn off
most or all appliances when they were out
of town; so the long-term ATDs may have
been exposed to higher concentrations in
this manner when the houses were unoc-
cupied.
Conclusions and
Recommendations
Radon levels of slab-on-grade houses
built over a compacted soil base are not
always easy to mitigate, especially if the
soil is a relatively strong source of radon,
as in some of the reclaimed phosphate
mining lands. SSD was demonstrated to
work in this type of environment, but the
importance of good diagnostic assessments
and carefully planned and well executed
installations is perhaps.greater underthese
conditions than with slabs built over gravel
fills. Based upon the results described
above, several conclusions can be drawn.
Diagnostic Methods Necessary
to Obtain Successful
Installations
In finished slab-on-grade houses, it was
often almost impossible to identify the ra-
don entry points, but because of the nature
of the sub-slab environment, neither strong
pressure fields nor adequate evacuation of
the radon-laden soil gas was possible.
Therefore, the greater the knowledge of
the source and the pathway of the radon,
the greater was the probability of diverting
or blocking its intrusion into the house.
The vacuum cleaner pressure field ex-
tension measurement was considered to
be crucial to take before planning a SSD
system. It gave the best approximation of
the recommended or effective distance
between suction holes, and thus, helped to
indicate how many suction holes would be
required. The sub-slab pressure-flow mea-
surements indicated a pripri the approxi-
mate flow that a SSD system would produce
with a given suction, thereby assisting in
planning for the optimum pipe size for use
in the mitigation system.
Installation Methods Applied to
Slab-on-grade Houses
While several alternative methods were
attempted to extend the pressure field and
to provide better coverage of the sub-slab
volume under the whole slab, few worked
very well. One that provided some im-
provement in all cases and much improve-
ment in almost all cases was digging
suction pits in the soil under the suction
holes through the slabs. The optimum prac-
tical pit size was determined to be 12-20
gal.
•U.S. Government Printing Office: 1993 — 750-071/60153
-------
The use of multiple suction holes at
well-chosen locations throughout the house
proved to bo the most successful strategy
to obtain an adequate pressure field ex-
tension under most of the slab area in
compacted soil fills. Generally, interior suc-
tion holes proved to be more effective at
extending pressure fields and reducing in-
door radon than did near-perimeter suction
holes. Geometrically this observation
seems reasonable if an approximately cir-
cular area of influence is assumed, and if
the suction hole near an exterior wall trun-
cates the circle, reducing the area. An-
other more significant feature that
influenced houses in this study was that, in
at least two cases, the stem walls were too
permeable to air movement. This led to the
fan's suction head's being lost to pulling in
outdoor air through the stem wall rather
than pulling as much radon-laden soil gas
from under the house. At times, near-pe-
rimeter suction holes were successful,
where the stem walls were less porous or
where the backfill adjacent to the stem wall
was less tightly compacted. For suction
holes that have to be placed near stem
walls, the pits should be dug toward the
house interior, exposing as little of the
stem wall as possible.
The SSD systems installed over tightly
packed soil fills generally produced low
flows through the pipes and fans. This
feature allowed for using smaller pipes
than would have been possible with gravel
fills and higher flows. The smaller pipes
produced less intrusive systems, more flex-
ibility in system placement, greater ease of
handling, and somewhat lower material
-costs.-- •- - -—- .L..~^..-—-~—~
Certain house features and homeowner
preferences necessitated a variety of suc-
tion hole placements and applications. The
report describes in greater detail the instal-
lation of horizontal suction holes through
stem walls and adaptations for placing suc-
tion holes in garages. Sealing radon entry
points and other openings where possible
was shown to improve SSD performance.
Several toilet bases and tub trap areas
were sealed, and some other cracks were
caulked. Generally, such actions helped in
at least one or two ways. If the opening
was far from a suction hole, then quite
possibly localized house depressurizations
could easily overwhelm the relatively slight
depressurization created by the distant suc-
tion hole, and radon would enter the house.
If the opening was nearer the suction hole,
then a significant portion of the mitigation
fan's suction could be "lost"to pulling house
or outdoor air into the exhaust piping rather
than radon-laden soil gas.
C. Fowler, A. Williamson, B. Pyle, F. Belzer, and R. Coker are with Southern
Research Institute, Birmingham, AL 35255-5305.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Radon Mitigation Studies: South Central Florida
Demonstration,"(OrderNo. PB93-122299/AS; Cost: $27.00; 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
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