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
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Penalty for Private Use $300

EPA/600/S8-90/063

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