United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S8-90/063 Jan. 1991
&EPA Project Summary
Engineering Design Criteria for
Sub-Slab Depressurization
Systems in Low-Permeability
Soils
C. S. Fowler, A. D. Williamson, B. E. Pyle, F. E. Belzer, and R. N. Coker
Engineering design criteria for the
successful design, installation, and
operation of sub-slab depressuriza-
tion systems have been developed
based on radon (Rn) mitigation
experience on fourteen slab-on-grade
houses in south-central Florida. The
Florida houses are characterized as
hard to mitigate houses because of
low sub-slab permeabilities. Pre-
mitigation indoor concentrations
ranged from 10 to 100 pCi/L.
Mitigation experience and results
have been combined into tables and
graphs that can be used to determine
recommended numbers and
placement criteria for suction holes.
Fan and exhaust pipe size selection
is assisted by other tabulated and
derived information. Guidance for
installation of the sub-slab system to
enhance the system's operation and
effectiveness is also provided. This
guidance is being reported in the
form of a design manual for use by
mitigators when they are dealing with
houses similar to these.
This Pro/ect Summary was
developed by EPA's Air and Energy
Engineering Research Laboratory,
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
Sub-slab depressurization (SSD) is
generally the most common and most
effective radon (Rn) mitigation strategy
employed in basement and slab-on-grade
houses. In many areas of the country, the
standard building practice is to place a
layer (often 4 in. [100 mm] or so) of
coarse gravel directly beneath a vapor
barrier before pouring the slab. When this
has been done, an SSD system is usually
quite effective because of the good
permeability and communications
afforded by the gravel layer. However,
many older houses were built before
using gravel became a common practice,
and in some areas of the country gravel
is not readily available. In these houses
the slabs are poured over either the
native soil or a fill soil that has been
compacted to some degree to prevent
settling away from the slab once the
concrete has hardened. Most of the time
such a soil fill has much lower
permeability to air flow than does gravel.
In such instances an SSD system will not
operate as effectively as it would over a
coarse aggregate bed. Since much of the
literature about SSD systems addresses
slabs poured over gravel, guidance in the
installation of SSD systems over low
permeability soils has generally been
lacking. Some researchers have reported
cases of low permeability beneath the
slabs and have either made somegeneric
observations about the average slab area
affected by given suction holes or offered
unique remedies found to work in specific
houses. However, no uniform guidance
document exists that uniquely addresses
design and installation strategies for
solving this problem.
In 1987, the Radon Mitigation Branch
(RMB) of the U.S. Environmental
Protection Agency's Air and Energy
Engineering Research Laboratory
(AEERL), Research Triangle Park, North
Printed on Recycled Paper
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Carolina, initiated a regional
demonstration of radon mitigation in slab-
on-grade houses in the phosphate mining
area of Polk County, Florida. South
Central Florida is one area in the U.S.
where coarse gravel is not readily
available. The customary building
practice is to prepare a base of
compacted fill soil, overlay it with a vapor
barrier, and then pour the slab.
From December 1987 to September
1989, 14 single-story slab-on-grade
houses with living areas of about 1300-
2600 ft2 (120-240 m2) and initial indoor
radon concentrations of 10-100 pCi/L
(400-4,000 Bq/m3) were mitigated with
SSD systems. The systems ranged from
central- and perimeter-located single
suction hole systems to up to four central
and/or five perimeter suction holes, with a
variety of combinations. Suction pits
ranged from no pits, to pits up to 12-15
gal. (0.05-0.06 m3) in size. Different sizes
of fans and pipes were installed. Suction
holes were drilled through the slab from
inside the house, and horizontally through
stem walls under the slab from outside
the living shell. Fans were located in
attics and outside the houses.
This design guide is an outgrowth of
the results that have been measured in
these houses over the last two years.
This document has several purposes. It
may be used by mitigators to aid them in
the design and installation of SSD radon
mitigation systems. Since radon
mitigation is a relatively new industry, in
some areas where this document may be
used it may also provide a reference to
supplies, equipment, and sources useful
in the mitigation field. Because this
document reports some lessons learned
during the demonstration and research
conducted in these 14 houses, another
purpose is to alert mitigators to potential
pitfalls and problems in installations,
often discovered too late by experience.
Scope
Every house is a unique structure.
There are many variables, from
geological or physical characteristics, to
construction features, to operational
house dynamics, to seasonal
environmental factors, to home owner
inputs that may affect the potential for
radon's entry into that structure. Fourteen
houses is not an adequate sampling to
predict all possible problems or
situations. It is hoped, however, that the
guidance offered here helps the mitigator
get started in the right direction and helps
the user structure the planning and
installing process in a proper framework.
Situations will occur where the
information provided in this document will
not be applicable or adequate. For some
houses, SSD is not the preferred, or even
a recommended, mitigation option. For
instance, if there are major unsealed
openings in the slab or extensive
cracking whereby the sub-slab space is
in direct communication with the indoor
space, then sealing the known openings
may be sufficient to reduce the indoor
radon concentrations. Having unblocked
openings between the two spaces not
only allows soil gas entry, but also
provides leaks whereby the pressure field
of an SSD system may be truncated.
Professional judgment is still the most
important element in the design and
installation of radon mitigation systems.
Research is also continuing relevant to
design criteria for sub-slab mitigation
systems in the same (and other) areas of
Florida, across the U.S. and in other parts
of the world. The University of Florida, in
particular, is contributing much
complimentary research to houses in a
different part of the state. Other local
mitigators who have worked through
problems and situations unique to their
areas and/or building practices are also
good potential sources of information on
possible changes or permutations in
these guidelines. Two years is too short a
time frame, considering the life of a
house, to, be able to state definitely that
these guidelines will be the final word in
SSD systems in low-permeability soils.
Because radon mitigation is a field
growing in breadth and application,
readers are encouraged" to seek
additional information. EPA Regional
Offices and appropriate state and- local
agencies should be good sources of the
latest information or of suggestions for
how to obtain such information.
This report includes a description of
background information necessary or
useful to know before installing a system,
keys to the selection of good suction hole
locations, fans and pipe sizes, installation
suggestions for suction holes, piping,
fans, and exhausts, and recommenda-
tions of system indicators and labeling. A
section on commercial equipment is
included to help identify potential sources
of supply for products that may be
unfamiliar or unavailable to the reader.
Background Information
Before a mitigator or homeowner starts
to design a radon mitigation system, it
should be established that indoor radon
is a problem. With all of the publicity that
radon has received from often-times less-
than-informed sources, home owners
may be acting or reacting without
knowing the seriousness or even the
certainty of their problem. It is reason.-'
and ethical for a mitigator,
communicate to the homeowner i.
recommended EPA protocols for
screening and follow-up measurements.
Several EPA publications present
guidance for making reproducible
measurements of radon concentrations in
residences, including recommendations
for using the results to make well-
informed decisions about the need for
additional measurements or remedial
action.
Once it is determined that the house in
fact does have elevated radon
concentrations, before any other action is
taken, certain basic house information
needs to be obtained. Many of the items
useful to investigate are given and
discussed and appropriate forms are
suggested for use, including house
summary information, radon entry point
determination, house differential pressure
readings, and sub-slab communication
and permeability measurements.
Sub-Slab Depressurization
Design Process
Once the decision has been made to
install an SSD system for radon
mitigation, the first and most critic-'
question to answer is how many sue
holes will be needed to remedy
problem and where to put them. If the
house has more than one slab separated
by footings or a foundation wall, then for
determining the number of suction holes,
each slab is treated separately. The
following process should be conducted
for each separate slab area. The single
most useful diagnostic tool to use as
input in this determination is the sub-slab
pressure field extension measurement.
The mitigator should have obtained a
reasonable feel for types of
communication present under the slab.
Based on the results of this test and a
figure presented in this report, the
mitigator is instructed how to
approximate the coverage area of a
suction hole. When this information is
combined with the slab area and
geometrical considerations, the number
and placement of the suction holes can
be determined. After considering how
moisture variation and other factors may
adversely affect the ability to move gas
through the soil, this document
recommends that a mitigator be
conservative in estimating system
performance when designing the system.
Actually placing the suction holes i«
very dependent on the structural feati
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of the house. Some guidance is given in
\ report for unfinished and finished
sments and for slab-on-grade houses
..in adequate access to all slab areas
from interior placements of suction holes
and those without. Installation guidance
and tips for drilling the holes, excavating
the pits, and finishing the installation are
provided for several commonly installed
systems.
While the pressure field extension
measurements of the sub-slab
communications diagnostic give
information useful for suction hole
placements,the pressure and flow
measurements indicate sub-slab flow
characteristics. Fan performance curves
indicate ranges of pressures and flows
where the fans are effective. The report
illustrates and describes how to interpret
the sub-slab flow curves and fan curves
to estimate the potential effectiveness of
various fans on sample types of fill. The
influence of other factors such as fan
durability, costs, noise, and installation
features of various fans is also discussed
from the perspective of fan selection.
Generally most mitigators use PVC
pipe when installing SSD systems. It is
lightweight, easy to cut and handle,
convenient for fittings and accessories,
strong in its glueing characteristics,
noncorrosive, and smooth so as to offer
low resistance to air movement. For
permeable sub-slab environments
conducive to high volumes of air flow, 4-
n, (100 mm) PVC piping is generally
used. For the low flows resulting from the
low-permeability soils addressed in this
document, 2-in. (50 mm) or larger PVC
piping is usually adequate. The smaller
piping has the added advantages of
being lighter and easier to handle, less
obtrusive to the homeowner and easier to
conceal if desired, and usually less
expensive for the piping, fittings, and
accessories. Therefore, an important
determination is what size of pipe is the
best to use for the given mitigation
project.
A figure is provided to help the
mitigator estimate the size of pipe to
select for a house dependent on the
projected flow in the system and the
approximate length of pipe to be used. A
sample house is used with appropriate
numbers provided so that the reader can
experience the use of the figure.
Descriptions of how to calculate friction
loss for both the pipe and connectors are
given with the help of a table and a
realistic example. Other factors to
consider in the process of pipe selection
are indicated.
Some generic guidelines for the proper
design and installation of the piping and
for the fan placement are given for
several types of mitigation systems.
Additional installation tips from
experienced contractors are passed
along to the reader as well. For houses
requiring roof penetrations, a section
containing guidance and suggestions is
also provided.
Finally, the successful installation of a
mitigation system should not be
considered complete unless some sort of
system monitoring and labeling is
provided for the benefit of the current and
future home owners and others who may
be working with or around the system.
Some of the available options for
accomplishing these aspects of the
process are included.
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C. S. Fowler, A. D. Williamson, B. E. Pyle, F. E. Belzer, and R. N. Coker are with
Southern Research Institute, Birmingham, AL 35255-5305.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Engineering Design Criteria for Sub-Slab
Depressurization Systems in Low Permeability Soils," (Order No. PB 90-
257 767'i'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
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S8-90/063
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