Tables

Table Al.
Table A2.
Table A3.
Table A4.
Table AS.
Table A6.
Table A7.
Table A8.

Figures

Figure Al.
Figure A2.
Figure A3.
Figure A4.

Table of Contents

Symbols Units and Conversion Factors ....... 6.6. cece eee eee eeee A-l1
Thermal Properties of Some Construction Materials ...............4-- A2
Air Freezing Indices For Select Locations ...........e eee seeeeeeeee A-3
Minimum Effective R-Value Of Vertical Wall Insulation .............. Ag

Minimum Foundation Depth Without Wing Insulation
Minimum Thermal Resistance of Wing Insulation (Walls) . . .
Minimum Thermal Resistance of Wing Insulation (Corners) .
Minimum Thermal Resistance of Ground Insulation ...............06%

USS. Air Freezing Index Contour Map ............. eee aee A-4, A-5, A-6
Mean Annual Temperature Contour Map for the U.S. ............. A-7, A-8
Ground Insulation Applications and Requirements . . wee ALL
Definition of a Small Unheated Part of a Floor Slab..............005 A-12



REFERENCES

American Society of Testing and Materials (ASTM). ASTM C 578 - 92 Standard Specification
for Rigid, Cellular Polystyrene Thermal Insulation. Philadelphia, PA (1992).

___- Moisture Control in Buildings-Chapter 4: Effects of Moisture on the Thermal Performance
of Insulating Materials. ASTM Manual Series: MNL 18. Heinz R. Trechsel, Editor.
Philadelphia,PA (1994).

Building Officials and Code Administrators International, Inc., (BOCA) National Building Code.
Country Hills, IL (1990).

Committee on Frost Actions in Soils. Frost I Jord (Frost Action in Soil). Nr. 17, Oslo, Norway
(November 1976); in Norwegian.

Comité Européen de Normalisation (CEN). Building Foundations--Protection against Frost
Heave. Preliminary draft for proposed European Standard N185, CEN TC 89/WG5
(August 1992).

Council of American Building Officials (CABO). Model Energy Code. Falls Church, VA
(1992).

: One- and Two-Family Dwelling Code. Falls Church, VA (1992).

Crandell, Jay H., Peter M. Steurer, and William Freeborne. Demonstration, Analysis, and
Development of Frost Protected Shallow Foundations and Freezing Index Climatography
for Residential Construction Applications in the United States. Proceedings of the 7th

International Cold Regions Engineering Specialty Conference. Edited by D.W. Smith and
D.C. Sego. Canadian Society for Civil Engineering, Montreal Quebec, (1994).

Farouki, Omar. European Foundation Designs for Seasonally Frozen Ground. U.S. Army Corps
of Engineers, Cold Regions Research & Engineering Laboratory, Monograph 92-1,
Hanover, NH (March 1992).

International Conference of Building Officials (ICBO). Uniform Building Code. Whittier, CA
4991).

Jones, C.W., D.G. Miedema, and J.S. Watkins. Frost Action in Soil Foundations and Control
of Surface Structure Heaving. U.S. Department of the Interior, Bureau of Reclamation,
Engineering Research Center, Denver, CO (1982).

Labs, Kenneth, et al. Building Foundation Design Handbook. Prepared for the Oak Ridge
National Laboratory by the University of Minnesota/Underground Space Center (May
1988); distributed by NTIS, Springfield, VA. (Note: this publication is also available in
shortened form as the Builder’s Foundation Handbook.)


Appendix: Design Data

Figure A1. United States Air-Freezing Index Contour Map

The air-freezing index method for the United States is described as cumulative degree days above and below 32°F. It is used
as a seasonal measure of the combined magnitude and duration of air temperatures below freezing. The index was computed
over a 12-month period (July-June) for each of the 3,044 stations used in the above analysis. (Stuerer, 1989) Data from the
1951-80 period were fitted to a Weilbull probability distribution to produce an estimate of the 100-year return period.
Probability estimates were verified by analysis of representative, long-term weather records. (Stuerer and Crandell, 1993)

‘As with most contour maps, the analyzed isopleths are somewhat generalized and may depart from actual conditions.
Topographic variability, proximity to bodies of water, and urban heat effects must be considered when interpolating values
for specific locations. Dashed lines are used in areas which contain few or no stations. Micro climate effects may produce
a variability of + 500 °F-days in the 100-year retum air freezing index estimate.

‘This analysis was prepared by the National Oceanic and Atmospheric Administration’s National Climatic Data Center under
contract to the NAHB Research Center.(Steurer, 1989)

A-4



FPSF DESIGN PROCEDURE

This design guide contains two approaches: a simplified design and a detailed design. The
simplified method streamlines the design process of FPSFs for heated buildings. The procedure
is in a form suitable for adoption by the major model building codes and is intended to promote
widespread acceptance and use of the technology. In consolidating the design steps for the
simplified method, R-values for the vertical insulation were established so that the performance
level of various conditions, including slab surface temperatures, were conservatively
accommodated. Therefore, more economical construction costs may be obtained when the
detailed design procedure is followed. The detailed design procedure must be used when
buildings include unheated areas such as attached garages.

SIMPLIFIED FPSF DESIGN METHOD

To use the simplified approach, the air freezing index (AFI) for the site location must be known.
An AFI contour map is provided in the Appendix to assist in establishing this value. Insulation
R-value and dimensions, and the depth of the footings are then determined from Table 2 and
Figure 4, Horizontal insulation shall be bedded firmly on smooth ground or granular base.
When foundation depths greater than 12 inches are required by Table 2, the increase in depth
may be satisfied by substituting compacted gravel, crushed rock, sand, or approved non-frost
susceptible materials.

Table 2, MINIMUM INSULATION REQUIREMENTS
FOR FROST PROTECTED FOOTINGS

IN HEATED BUILDINGS'
Air Freezing || Vertical | Horizontal tnsijation se mensions “sing
Index Insulation per Figure No. 16 (inches)
(°F days)" R-Value** (inches)
along walls | at corners A B Cc D
1,500 or less 45 NR NR NR NR NR 12
2,000 5.6 NR NR NR NR NR 14
2,500 6.7 17 49 12 24 40 16
3,000 || 78 6.5 86 12 24 40 16
3,500 9.0 8.0 11.2 24 30 60 16
4000 | 10.1 105 | 13.1 24 36 60

2 fnsulation requirements are for protection against frost damage in heated buildings. Greater values may be required to meet energy conservation
standards. Interpolation between values is permissible.

? See Appendix for Air Freezing Index values.

3 Insulation materials shall provide the stated minimum R-values under long-term exposure to moist, below-ground conditions in freezing climates.
‘The following R-values shall be used to determine insulation thicknesses required for this application: Type I expanded polystyrene - 2.4R per
inch; Types IV, V, VI, VI extruded polystyrene - 4.5R per inch; Type IX expanded polystyrene - 3.2R per inch. NR indicates that insulation
is not required.

* Vertical insulation shall be expanded polystyrene insulation or extruded polystyrene insulation.

5 Horizontal insulation shall be extruded polystyrene insulation.





Appendix: Design Data

3 Unheated Part
(7) Heated Part

>13'

EE

L L
am a 4 aA
> 13! >13" <ly

Fun CF-days)
Size Limit
$2,250 | 2,250 to 3,000 | 3,000 to 3,750 23,750
L, 13°-0" 117-6" 10°-0" 6-6"
L 9.9" 8" 76 ul"
6-6" 3-10" 3-0"

Figure A4. Definition of a small unheated part of a floor slab.

A-12



Appendix: Design Data

Table A6
MINIMUM THERMAL RESISTANCE
OF WING INSULATION, Rhw,
FOR USE ALONG WALLS
(16" FOOTING DEPTH)

2,250 or less

Table A7
MINIMUM THERMAL RESISTANCE
OF WING INSULATION, Rhc,
FOR USE AT CORNERS
(16" FOOTING DEPTH)

R-values For Various Wing Widths At Corners, D,,. (inches)

16 24 30 36 2 8
2,250 or less 0 0.0 S
2,625 40 65 49 40
3,000 40 9.6 8.6 8.0
3,375 60 2 ui 105 98 OL
3,750 60 Bis 0S—«iB
4,125 60 45 13.7130 125

4,500 80 2 — 189 15.1 148

A-10



RECOMMENDED CONSTRUCTION METHODS AND DETAILS

PROTECTIVE
COATING

PER LOCAL
CODE

] ~|
;

Figure 8, Typical FPSF monolithic slab in a mild climate.

PROTECTIVE
FACING ON

12"
Min.

HEATED SPACE

WING INSULATION

Figure 9. Typical FPSF monolithic slab with horizontal wing Insulation for

16

colder climates.





ACKNOWLEDGEMENTS

This report was prepared by the NAHB Research Center for the U.S. Department of Housing and
Urban Development. Special appreciation is extended to NAHB members Bill Eich, Gerald Eid,
Tim Duff, Jess Hall and their staffs, for constructing the five demonstration homes; Richard
Morris, National Association of Home Builders, for initiating the technology transfer into the
United States; Peter Steurer, National Oceanic and Atmospheric Administration, for providing
and verifying proper methods of estimating long-term air-freezing indexes; Dow Chemical
Company, Mid-America Molders, U.C. Industries, Inc., and Amoco Foam Products, for donation
of insulation; and William Freeborne, U.S. Department of Housing and Urban Development, for
guidance and review throughout the demonstration, testing, and development of this design guide
for frost protection of shallow foundations.

In addition, the NAHB Research Center gives special recognition to The Society of the Plastics
Industry, Inc., for several years of support that has helped to re-introduce this technology to the
United States. Their sponsorship has promoted activities such as technology transfer from
Europe, finite element computer modelling of heat transfer through foundations, development of
design procedures for frost protection of foundations, formulation of important U.S. climate data
for use in the design procedure, and initiating an interest in this technology at the level of the
major U.S. building codes. These previous activities have contributed tremendously to this
design document and development project.

The NAHB Research Center staff responsible for this document are:

Jay H. Crandell, P.E., principal investigator
Eric M. Lund, technical assistance

Mike G. Bruen, P.E., technical assistance
Mark S. Nowak, reviewer



FPSF DESIGN EXAMPLES

Example C Site Information

Approximate Location Duluth, MN

Building Type 16 X 24 foot unheated garage

Foundation Type 8 inch wide concrete stemwall, with slab-on-grade
Floor Finish none

Slab/Floor Insulation none

Unheated Areas entire building

Design Steps

The design of this FPSF follows the basic procedure for unheated buildings.

From Figures Al and A2 for the Duluth area, Fi) = 3,000 °F-day and
MAT = 38 °F,

From Table A8 for F, = 3,000 °F-day, D, = 79 inches, therefore
extend the ground insulation 6’ 7" outside the building.

From Table A8 for MAT = 38 °F and F, = 3,000 °F-day,

R, = 18.2 hr-ft-"F/Btu. In order to reduce the amount of insulation
required in Step 4, increase the minimum 6" non-frost susceptible soil
layer to 8" and design for R, = 18.2 - 0.6 = 17.6 hr-ft-"F/Btu.

The minimum thickness of XPS ground insulation = R/T.
17.6/4.5 = 3.9 inches (min) of XPS required, therefore use a 4" layer
of XPS ground insulation.

Because the foam in this case is bearing structural loads from footings,
a higher density polystyrene is used for the required compressive
strengths. The compressive resistance of 1.6 pef XPS (Type IV) per
ASTM C 578-92 is 25 psi (3600 psf). For greater bearing capacity a

_ more dense type may be specified, such as 1.8 pef XPS (Type VI)

__ with a compressive resistance of 40 psi (5760 psf). Using a safety
actor of 3, the allowable bearing load is 5760/3 = 1920 psf. This
earing capacity is sufficient for this single-story, light-frame

construction example including normal dead, live, and snow loads. It
is important to ensure that the insulation is placed firmly on a smooth
_ layer of bedding material such as sand or gravel.

27


RECOMMENDED CONSTRUCTION METHODS AND DETAILS

GENERAL

The construction of FPSF is similar to that of conventional foundation construction except for
the insulation details. Many of the steps, such as rough grading, foundation layout, the
preparation of subgrade, wall forming, steel reinforcing and casting of the concrete, are.all similar
to. conventional practices.

FPSF can be constructed using one of many approaches, including a monolithic slab-on-grade,
a independent slab and stem wall, or a permanent woc.' foundation. Although the details shown
here illustrate techniques for the construction of new homes, the FPSF technique can also be
applied to additions to homes with existing conventional foundations, and even walk out
basements. Each of these options is explained in this section.

The FPSF construction details shown here reflect satisfactory practices for several typical
situations. The drawings shown are generic in that they must be modified to meet the
requirements specific to the site, such as insulation dimensions. Of key importance is the proper
placement and sizing of the insulation.

Regardless of the option chosen, a few issues apply to the construction of any FPSF:

* Cold bridges. Cold bridges are created when building materials with high thermal
conductivity, such as concrete, are directly exposed to outside temperatures (see Figure 7).
Foundation insulation should be placed such that continuity is maintained with the insulation of
the house envelope. Cold bridges may increase the potential for frost heave, or at the least,
create localized lower temperatures or condensation on the slab surface. Care must be taken
during construction to ensure proper installation of the insulation.

IE)

Unheated Building

Figure 7. lilustrations of cold bridges

13


RECOMMENDED CONSTRUCTION METHODS AND DETAILS

UNHEATED BUILDINGS

Additional measures are required for an unheated building. While a drainage layer is only
recommended under wing insulation for heated buildings, a 6-inch drainage layer is required
under unheated FPSF designs. Additionally, the horizontal ground insulation extends not only
as a wing beyond the perimeter of the building, but continues under the entire unheated portion
of the building. This insulation layer can be installed either directly under the slab as shown in
Figure 13, or entirely at one level as shown in Figure 14. In either case, the compressive load
of the building on the insulation must be determined to compare to the compressive resistance
of the foam (see design examples). The horizontal insulation must have a minimum of 10 inches

of soil cover.

UNHEATED SPACE

}
‘MINIMUM |
10" |
b
[Ii
\__ MIN. THICKNESS GRAVEUSAND LAYER
Figure 13. FPSF design for unheated space with
independent slab and stem wall.
zl onexreseie
Po
nun |
w | NON FROSTISUSCEPTIBLE FILL

“GROUND INSULATION

(6° MIN. GRAVELISAND LAYER

Figure 14. FPSF design for unheated bulldings with
insulation In single plane.

19


RECOMMENDED CONSTRUCTION METHODS AND DETAILS

* Doorways and Thresholds. At doorways where the threshold overhangs the vertical
wall insulation, the insulation should be cut out as required to provide solid blocking for
adequate bearing and fastening of the threshold. The size of the cut-outs should be minimized.

* Landscaping and wing insulation. In situations where wide horizontal wing insulation
is required (e.g., greater than 3 to 4 foot widths), this may deter the location of large plantings
close to the home. In some of these cases, using thicker wing insulation or increasing the
foundation depth will decrease the required width of the wing insulation.

* Foundation height. Given that most polystyrene insulation boards are typically
available in 24 inch and 48 inch widths, 24 inches becomes a practical height for many
foundations. This provides 16 inches of the foundation below grade and 8 inches above grade.

* Excavation. Generally, lightweight equipment is adequate for FPSFs because little
excavation is required. As with any foundation, organic soil layers (top soil) should be removed
to allow the foundation to bear on firm soil or compacted fills.

* Construction scheduling. The foundation should be completed and the building
enclosed and heated prior to the freezing weather, similar to conventional construction practice.

MONOLITHIC SLAB

Construction of an FPSF monolithic slab is very similar to that of a conventional slab. (See
Figures.8 and 9). The only step requiring additional consideration is whether to place the
insulation before or after the pouring of the slab. Tacking the vertical insulation in place with
nails after the pour is a simple technique, made even easier if it is done when the concrete is
new, or "green".

As an alternative installation technique, the vertical insulation boards may be tacked in place to
the inside of the formwork before the pouring of concrete. This technique allows the insulation
to adhere, or if nails are used, to fasten to the concrete and eliminates the necessity for securing
the insulation to the wall later. If the insulation is installed in this manner, it should be noted
that the foundation line is actually at the inside face of the insulation board, which may affect
overall dimensions used for framing.

If both vertical and horizontal insulation are used, the vertical wall insulation should be placed
first. The horizontal insulation, where used, is then placed directly on the subgrade, or on the

gravel drainage layer, if provided. Measures should be taken to ensure a smooth bed for the
horizontal insulation, and sub-slab insulation, if used.

15


FPSF DESIGN EXAMPLES

Design Details
\
UNHEATED SPACE
a = ~ wee
— ==
S|/Elll cositentenieny gM
an ee ee
4
Syll(Elll
; 6-7" « /
jp
LI 4° xps
= 7 GROUND INSULATION
/

8* THICK GRAVEL/SAND LAYER

Figure 19. Design detail for example C.



FPSF DESIGN PROCEDURE

DETAILED METHOD FOR UNHEATED BUILDINGS

In following the detailed design procedure for unheated buildings, the designer has the flexibility
to increase foundation depths to reduce ground insulation requirements.

Figure 6 illustrates the variables for FPSF design of unheated buildings. The Appendix contains
figures and tables for determining and selecting the design variables. The following steps
outline the detailed design approach for unheated buildings.

LUNMEATED SPACE

GROUND INSULATION, Rg

Figure 6. FPSF design parameters for unheated
buildings.

Determine Air Freezing Index, F,, & Mean Annual Temp., MAT

Select the 100-year return period design air freezing index, Fi, from Figure Al or Table A3. This information was
prepared by the National Oceanic and Atmospheric Administration’s National Climatic Data Center specifically for
use in FPSF design. The F, values are conservative because they are not adjusted for the insulating benefit of a
normal snow cover on the ground. A lower return period value may be used for less important structures or those
that are resilient to infrequent ground freezing, such as detached garages. See Table A3 for F, values at return
periods less than 100 years.

Select Placement of Ground Insulation

A continuous ground insulation layer with a 6-inch gravel or other non-frost susceptible base must be placed below
the entire foundation of unheated buildings. The ground insulation must extend outside the foundation a minimum
width, D,, determined from Table A8 and shown in Figure A3. In unheated building foundations, D, is the same
at both corner and wall locations. Outside the foundation perimeter, the insulation must have a minimum of 10
inches of soil cover. D, may be reduced by 1 inch for every inch the insulation is buried beyond the 10 inch
minimum cover.

| Select the Minimum Effective R-value of Ground Insulation, R,

"Select the minimum required R-value, R,, required for the ground insulating layer from Table A8 based on F, and

MAT from Step 1. R, may be reduced by 0.3 R for every 1-inch the underlying non-frost susceptible layer is
increased beyond the 6 inch minimum thickness. R, may also be reduced by 0.25 R for every 1-inch increase in
soil cover, above the 10 inch minimum, over the ground insulation.

Select Thickness of Ground Insulation

Based on the required R, value determined in Step 3, select an adequate thickness of XPS ground insulation
assuming an effective resistivity of 4.5 R/inch. Recommended nominal thicknesses of XPS are 1", 1-1/2", 2", and
3", In severe climates, insulation may need to be layered to meet the required thickness.

10



INTRODUCTION

The FPSF allows builders to construct a structurally sound foundation at a lower cost than
associated with these practices. Although this document is limited to slab foundations, the
technology may also be used with crawlspace construction when ventilation is properly
controlled. In addition to substantial initial cost savings, FPSFs provide an opportunity for
increased energy savings because of their insulation requirements. These minimum insulation
requirements for frost protection generally exceed existing energy code requirements (ie., the
CABO Model Energy Code) for foundation insulation.

The frost protected shallow foundation technology recognizes the thermal interaction of building
foundations with the ground. Heat input to the ground from buildings effectively raises the frost
depth at the perimeter of the foundation. This effect and other conditions that regulate frost
penetration into the ground are illustrated in Figure 2.

It is important to note that the frost line rises near a foundation if the building is heated. This
effect is magnified when insulation is strategically placed around the foundation. The FPSF also
works on an unheated building by conserving geothermal heat below the building. Unheated
areas of homes such as garages may be constructed in this manner.

Forest
House
Clay
Snow | Road == A
TTT ~ wets
~ ~~ Stuy ~—
Humus
Gravel or Frost Depth Line

Sand

FIGURE 2. Frost Penetration into the Ground Under Various Conditions.

Figure 3 illustrates the heat exchange process in an FPSF, which results in a higher frost depth
around the building. The insulation around the foundation perimeter conserves and redirects heat
loss through the slab toward the soil below the foundation. Geothermal heat from the underlying
ground also helps to raise the frost depth around the building.

FPSFs are most suitable for slab-on-grade homes on sites with moderate to low sloping grades.
The method may, however, be used effectively with walk-out basements by insulating the

~~ foundation on the downhill side of the house, thus eliminating the need for a stepped footing.
FPSFs are also useful for remodeling projects in part because they minimize site disturbance.
In addition to residential, commercial, and agricultural buildings, the technology has been applied
to highways, dams, underground utilities, railroads, and earth embankments.



FPSF DESIGN EXAMPLES

EXAMPLE B: Site Information

Approximate Location Bismarck, ND

Building Type 42 X 26 foot heated home

Foundation Type 8 inch CMU stemwall footing, with slab-on-grade
Floor Finish carpet with rubber pad

Slab/Floor Insulation —_1-inch Type IX EPS (under slab), 4-foot width
Unheated Areas none

Design Steps

The design of this FPSF follows the basic procedure for heated buildings.

From Figure Al for the Bismarck area, Fo) = 3,700 °F-day
(Table A3 gives Fy) = 3,359 °F-day).

|| R, for the floor slab cross section = Recaper + Reconcrete + Rexos
R, = 1.23 + (0.05 R/in x 4") + (4.2 R/in x 1") = 5.63 R.
(See Table A2 for values)

From Table A4 for h = 12", R; = 5.6, and F, = 3,700 °F-day:
R, = 5.7 hr-ft-"F/Btu.

The minimum thickness of wall insulation = RyJtete-

In the case of XPS, 5.7/4.5 = 1.3 inches minimum of XPS required,
therefore use 1.5 inches XPS insulation.

In the case of Type IX EPS, 5.7/3.2 = 1.8 inches (min) required,
therefore use 2.0" of Type IX EPS vertical wall insulation.

_From Table AS for F, = 3,700 °F-day, a minimum foundation depth of
__ H, = 30" would avoid the use of wing insulation along the walls. For
this example, however, the desired foundation depth is 16". From
Table A6, select wing width, D,,,, of 36" with a required thermal

resistance, R,,,, of 7.7 hr-ft-"F/Btu.

_ The minimum thickness of XPS wing insulation = Ryy/Teq
7.7/4,5 = 1.7 inches minimum of XPS required
therefore, use 2.0 inch thick XPS wing insulation along the walls.


COMMON QUESTIONS AND ANSWERS ABOUT FROST-PROTECTED SHALLOW FOUNDATIONS

Question No. 4: What happens if the heating system fails for a time during the winter?

For all types of construction, heat loss through the floor of a building contributes to geothermal heat storage under
the building, which during the winter is released at the foundation perimeter. Using insulated footings will
effectively regulate the stored heat loss and retard penetration of the frost line during a period of heating system
failure or set-back. Conventional foundations, with typically less insulation, do not offer this level of protection and
the frost may penetrate more quickly through the foundation wall and into interior areas below the floor slab. With
ad-freezing (the frozen bond between the water in the soil and the foundation wall), frost does not need to penetrate
below footings to be dangerous to light construction. In this sense, frost protected footings are more effective in
preventing frost damage.

The proposed insulation requirements are based on highly accurate climate information verified by up to 86 years
of winter freezing records for over 3,000 weather stations across the United States. The insulation is sized to prevent
foundation soil freezing for a 100-year return period winter freezing event with a particularly rigorous condition of
no snow or ground cover. Even then, it is highly unlikely that during such an event their will be no snow cover,
sufficiently high ground moisture, and an extended loss of building heat.

Question No. 5: Why are greater amounts of insulation needed at the corners of the
foundation?

Heat loss occurs outward from the foundation walls and is, therefore, intensified at the proximity of an outside corner
because of the combined heat loss from two adjacent wall surfaces. Consequently, to protect foundation comers from
frost damage, greater amounts of insulation are required in the corner regions. Thus, an insulated footing design will
provide additional protection at corners where the risk of frost damage is higher.

Question No. 6: What experience has the U.S. seen with this technology?

Frost protected insulated footings were used as early as the 1930s by Frank Lloyd Wright in the Chicago area, But
since that time, the Europeans have taken the lead in applying this concept over the last 40 years. There are now
over 1 million homes in Norway, Sweden, and Finland with insulated shallow footings which are recognized in the
building codes as a standard practice. In the United States, insulation has been used to prevent frost heave in many
special engineering projects (.c., highways, dams, pipelines, and engineered buildings). Its use on home foundations
has been accepted by local codes in Alaska, and it has seen scattered use in uncoded areas of other states. It is likely
that there are several thousand homes with variations of frost protected insulated footings in the United States
(including Alaska).

To verify the technology in the United States, five test homes were constructed in Vermont, Iowa, North Dakota,
and Alaska. The homes were instrumented with automated data acquisition systems to monitor ground, foundation,
slab, indoor, and outdoor temperatures at various locations around the foundations. The performance observed was
in agreement with the European experience in that the insulated footings prevented the foundation soil from freezing
and heaving even under rigorous climatic and soil conditions (ref. U.S. Department of Housing and Urban
Development, "Frost Protected Shallow Foundations for Residential Construction", Washington, DC, 1993).

~~ Question No. 7: How energy efficient and comfortable -are-slab-foundations-with-frost ———
protected footings?

The insulation requirements for frost protected footings are minimum requirements to prevent frost damage. The
requirements will provide a satisfactory level of energy efficiency, comfort, and protection against moisture
condensation. Since these requirements are minimums, additional insulation may be applied to meet special comfort
objectives or more stringent energy codes.


Contents

List of Tables and Figures 21... 0.6. e ccc cee cece cece eee eee ete e ene eeeee vi
INTRODUCTION ........ cece cece etter tee t tenet tenet en et eneeee 1
FPSF DESIGN PROCEDURE ... 1... - eee e ete e eee e eee tere een eeeeeeenee 5
RECOMMENDED CONSTRUCTION METHODS AND DETAILS ..............- 13
COMMON QUESTIONS AND ANSWERS ABOUT FPSFS ..........-00-eeeeee 21
FPSF DESIGN EXAMPLES ... 6... 2c cc cece eee eet tenet e nee n ene 23
REFERENCES 9.1... 0. eee cece eee eee eee ete ttt ennenee 29
APPENDIX: DESIGN DATA .... 0... c cece eee cece eee nee eenneee A-1





FPSF DESIGN PROCEDURE

SPECIAL CONDITIONS
Small Unheated Areas in Otherwise Heated Buildings

Where small, (as defined in Figure A4 of the Appendix), unheated perimeter parts of an
otherwise heated building are encountered, follow the design procedure for heated buildings and
address the small unheated part as follows:

° Continue the vertical wall insulation of the heated part of the building along the
exterior face of the small unheated part.

° Consider the small unheated area a corner location and provide protection, with
wing insulation or increased foundation depth if desired, according to Step 7 of
the design procedure for heated buildings.

. Provide ground insulation as required for unheated buildings under the small
unheated area.

Large Unheated Areas in Heated Buildings

When an unheated building area does not meet the conditions for a small building part, it is
considered a large unheated area. This situation is commonly encountered in homes with
unconditioned attached garages. When large unheated areas are encountered, regard the heated
and unheated sections as separate buildings and design the foundations accordingly.

Semi-Heated Buildings

If the anticipated operating conditions of the building are such that the lowest average internal
monthly temperature of a building falls between 41 °F (5 °C) and 63 °F (17 °C), the building is
considered semi-heated. In this case, design the foundation as a heated building and increase the
minimum required foundation depth by 8 inches in both wall and corner areas.

Ww



INTRODUCTION

A frost protected shallow foundation (FPSF) is a practical alternative to deeper, more-costly
foundations in cold regions with seasonal ground freezing and the potential for frost heave.
Figure 1 shows an FPSF and a conventional foundation. An FPSF incorporates strategically
placed insulation to raise the frost depth around a building, thereby allowing foundation depths
as shallow as 16 inches, even in the most severe climates. The most extensive use has been in
the Nordic countries, where over one million FPSF homes have been constructed successfully
over the last 40 years’. The FPSF is considered standard practice for residential buildings in
Scandinavia. The objective of this design guide is to assist U.S. builders, designers, code
officials, and others in employing the technology.

Conventional

Figure 1. Schematic of FPSF and Conventional Foundation Systems

In northern U.S. climates, builders mitigate the effects of frost heave by constructing homes with
basements, slabs, or crawlspaces with perimeter footings below the frost line. Other construction
methods include:

Piles or caissons extending below the frost-line

Mat or reinforced structural slab foundations to resist heave
Non-frost susceptible fills and drainage

Adjustable foundation supports

’ Morris, Richard A., Frost-Protected Shallow Foundations: Current State-of-the-Art and Potential Application
in the U.S. Prepared for the Society of Plastics Industry, Inc., NAHB Research Center, Upper Marlboro, MD, August
1988,


RECOMMENDED CONSTRUCTION METHODS AND DETAILS

INDEPENDENT SLAB AND STEM WALL

As noted previously, FPSF can also be adapted to foundations constructed with an independent
stem wall and ground supported slab. (See Figures 10, 11, and 12). The stem wall and slab
technique has the same insulation and drainage requirements as a monolithic slab. If a separate
footing is used, as may be required by local soil conditions or code requirements, the footing
must be located below the insulation. In any case, any horizontal insulation should be a
minimum 10 inches below grade. The wall may be constructed of poured concrete, concrete
masonry, wood or other acceptable materials. Some examples are shown below.

|
FLASHING ~~‘ HEATED SPACE

PROTECTIVE ___, | ee

S/Elll

VERT. WALL INSULATION

<+——— FOOTING
WING INSULATION

Figure 10. Independent block stem wall with insulation on exterior face
of wall.

17


DESIGN GUIDE FOR FROST-PROTECTED SHALLOW
FOUNDATIONS

Prepared for:

U.S. Department of Housing and Urban Development
Office of Policy Development and Research

Prepared by:

NAHB Research Center
Upper Marlboro, MD

Instrument No. DU100K000005897

June 1994



Appendix: Design Data

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AG


INTRODUCTION

In the United States, FPSF technology has been used in engineered structures and is common
practice for residential construction on crawlspace foundations in areas of Alaska. However, the
major model building codes do not specifically recognize the FPSFs equivalence to footings
placed below a prescribed frost depth. Recent amendments to the CABO One- and Two-Family
Dwelling Code and the BOCA National Building Code recognize performance-based criteria for
frost protection but do not specifically mention FPSFs. Consequently, widespread use of the
technology in residential buildings has not yet occurred. Efforts are underway to obtain U.S.
building code approval.

This document, based on the European standards, presents an FPSF design procedure for slab-on-
grade buildings, demonstrates the procedure with design examples, answers common questions
about FPSFs, and recommends specific construction methods and details. Additionally, a
simplified design method is presented in a form suitable for adoption by the major model
building codes.



INTRODUCTION

i, | Building Heat Loss
Ai
Cold Air | | Througn the Floor

Cooling of
Frozen Sol Beothermal
Heat Flow Heat (Warm Soil)
+°" Latent Heat of * Increasing lor insulation wil decrease |
Freezing (H O,n Soil) heat flow to the foundation, and more !
\ Perimeter (FPSF) insulation is required, |
— Frost Line oo

FIGURE 3. FPSF Heat Flow Diagram for a Heated Building with
Optional Floor Insulation.

FPSFs have seen notable technological development and increased application in Europe over the
past 40 years. Their most practical advancements in the understanding and application have
come from the Nordic countries.

In the United States, slab-on-grade houses were constructed around the turn of the century in cold
climates near Chicago. During the Depression, Frank Lloyd Wright designed and built a type
of FPSF to meet affordability needs and used FPSF techniques in his "Usonian" style homes with
shallow slab-on-grade foundations.

In the 1950s, Swedish and Norwegian researchers constructed the first experimental houses using
insulated shallow foundations. These demonstration sites provided practical experience and
empirical data on the FPSF technology. By 1972, nearly 50,000 slab-on-grade foundations had
been built in Sweden, and the FPSF technology had gained wide acceptance. The Norwegians
also recognized the need to address design recommendations for unheated portions of buildings
such as air-lock entries and exterior stairways.

In the 1970s, the Scandinavian nations consolidated their research efforts in an attempt to address
the FPSF technology. In 1972, the Royal Norwegian Council for Scientific and Industrial
Research appropriated the equivalent of $10-million’ for the compilation and advancement of

~ the Scandinavian work to date. The effort led to the 1976 publication, Frost I Jord ("Frost

Action in the Ground"). Scandinavian engineers consider Frost I Jord a reliable guide for design
against frost action in soils. Based on the results of the Frost I Jord Project, the Norwegian
Building Research Institute started publication in 1978 of "Building Details" related to FPSF
design and construction.

21988 exchange rate.



Appendix: Design Data

Table A2.

THERMAL PROPERTIES OF SOME FOUNDATION AND
FLOOR CONSTRUCTION MATERIALS
FROM ASHRAE FUNDAMENTALS (1989)

AND ASTM STANDARD C 578-92

Resistivity hr-ft?-°F/Btu-in

Description Density Ib/ft® [R-value per inch,
dry condition]

BUILDING MATERIALS

Plywood 34 1.25
Waferboard 37 1.59.
Particleboard, Low-Density 37 1.41
Particleboard, High-Density 62.5 0.85
Particleboard, Underlayment 40 1.31
Wood Subfloor - 1.25
Softwoods 35 0.9
Hardwoods 40 0.8
Brick 100 0.25

8" CMU with Perlite Fill - 21
Cement Mortar 120 0.15
Concrete 140 0.05

6 mil Plastic - Negligible
EPS Insulation, Type I 13 4.0
EPS Insulation, Type IX 18 4.2
XPS Insulation, Types IV, V, VI, VI 1.6 - 3.0 5.0

[ros FLOORING MATERIALS

Carpet and Fibrous Pad 2.08 (R)
Carpet and Rubber Pad 1.23 (R)



FPSF DESIGN EXAMPLES

Design Details
PROTECTIVE YT
COATING | 7 canPEr
Z 4
C ~
n «
< = 1" XPS OR 2" Type Il EPS
- 4 VERTICAL INSULATION
e- 8

Figure 17. Design detail for example A.





FPSF DESIGN EXAMPLES

The following examples demonstrate the design process for FPSFs presented in this guide.
Variations from the designs and details shown which meet the minimum requirements of this

guide would also be acceptable.
EXAMPLE A: Site Information

Approximate Location Chicago, IL

Building Type 52 X 26 foot heated home
Foundation Type monolithic slab-on-grade
Floor Finish carpet with fibrous pad
Slab/Floor Insulation _ none

Unheated Areas none

Design Steps

The design-of this FPSF follows the basic procedure for heated buildings.

From Figure Al for the Chicago area, Fi) = 1,500 °F-day (Table A3
gives Fi) = 1,433 °F-days)

R, for the floor slab cross section = Recaret + Reconcrete
R, = 2.08 + (0.05 R/in x 4 in) = 228K.
_ (See Table A2 for values.)

From Table A4 for h = 12" and F, = 1,500 °F-day:
R, = 4.5 br-ft-°F/Btu.

_ The minimum thickness of wall insulation is R/tay.

In the case of XPS, 4.5/4.5 = 1.0 inch minimum XPS required.

In the case of Type II EPS, 4.5/2.4 = 1.9 inches (min) required.
Therefore, use 2.0 inches of Type II EPS.

‘rom Table AS for F, = 1,500 °F-day or less, a 12" foundation depth
is acceptable and no wing insulation is required along the wall. The

lesign may proceed to Step 7.

From Table AS for F, = 1,500 °F-day or less, a 12" foundation depth

at the comers is acceptable and no comer wing insulation is required.

23


Tables

Table 1.
Table 2.

Figures

Figure 1.
Figure 2.
Figure 3.

Figure 4.

Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.

Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.

Figure 16.
Figure 17.
Figure 18,

List of Tables and Figures

Classification of Building Based on Indoor Air Temperature ............ 5
Minimum Insulation Requirements for Frost Protected

Footings With Heated Buildings ........... 0... cee eee eee eee eee 6
Schematic of FPSF and Conventional Foundation Systems ............. 1
Frost Penetration into the Ground Under Various Conditions ........... 2
FPSF Heat Flow Diagram for a Heated Building with Optional Floor

Insulation 0... eee eee eee nents 3
FPSF Design Parameters for Heated Buildings Using the Simplified

Design Procedure 2.1... 2. eee e eect eee eee ere ete teen eens 7
FPSF Design Parameters for Heated Buildings ......... 00s ee eens 8
FPSF Design Parameters for Unheated Buildings ............--.000- 10
Illustrations of Cold Bridges ..... 0... cece cece cece eee teens 13
Typical FPSF Monolithic Slab in Mild Climate .............--0000- 16
Typical FPSF Monolithic Slab with Horizontal Wing Insulation for Colder
Climates 2... eee eter teen eee e ene 16
Independent Block Stem Wall with Insulation on Exterior Face of Wall... 17
Independent Stem Wall with Insulation on Exterior Face of Wall ....... 18
Typical Permanent Wood FPSF ....... 0... 0 eee rece eee eee eee 18
FPSF Design for Unheated Space with Independent Slab and Stem Wall .. 19
FPSF Design for Unheated Buildings With Insulation in Single Plane .... 19
Insulation Detail for Small Unheated Portion in an Otherwise Heated

— Building oe ee ee ee et ee eee eee ree tere eetereee?e 20

Insulation Detail for a Large Unheated area (e.g., Attached Garage) ..... 20
Design detail for example A... 1... eee ete eect eee eens 24
Design detail for example Bo... 16. eee eceeeeeeee 26
Design detail for example C 1.2... eee cece eet eee 28

Figure 19.

vi



RECOMMENDED CONSTRUCTION METHODS AND DETAILS

* Drainage. Good drainage is important with any foundation and FPSF is no exception.
Insulation performs better in drier soil conditions. Ensure that ground insulation is adequately
protected from excessive moisture through sound drainage practices, such as sloping the grade
away from the building. Insulation should always be placed above the level of the ground water
table. A layer of gravel, sand, or similar material is recommended for improved drainage as well
as to provide a smooth surface for placement of any horizontal wing insulation. A minimum 6-
inch drain layer is required for unheated FPSF designs. Beyond the 12-inch minimum foundation
depth required by building codes, the additional foundation depth required by an FPSF design
may be made up of compacted, non-frost susceptible fill material such as gravel, sand, or crushed
rock.

* Slab surface temperatures (moisture, comfort, and energy efficiency). The
minimum insulation levels prescribed in this design procedure protect the foundation soil from
frost. They also provide satisfactory slab surface temperatures to prevent moisture condensation
and satisfy a minimum degree of thermal comfort. Since the design procedure provides minimum
insulation requirements, the foundation insulation may be increased to meet special needs
concerning these issues and energy efficiency. Successfully limiting cold bridging is critical --
use of the stem wall and slab technique (described later), in effect, adds a second thermal break
between the slab and stem wall. Increasing the vertical wall insulation thickness above the
minimum requirements for frost protection will also improve energy efficiency and thermal
comfort, Selection of a finish floor material such as carpeting decreases the surface contact
between occupant and the slab, giving a warmer feel.

* Heated slabs and energy efficiency. This design procedure can be applied to all slab-
on-grade techniques, including those with in-slab heat which provide excellent thermal comfort.
If an in-slab heating system is used, additional insulation below the slab and around the perimeter
is recommended for improved energy efficiency.

* Protecting the insulation. Because the vertical wall insulation around a foundation
extends above grade and is subject to ultraviolet radiation and physical abuse, that portion must
be protected with a coating or covering that is both tough and durable. Some methods to
consider are a stucco finish system or similar brush-on coatings, pre-coated insulation products,
flashings, and pressure treated plywood. The builder should always verify that such materials
are compatible with the insulation’ board. The protective finish should be applied before
backfilling, since it must extend at least four inches below grade. Also, polystyrene insulation
is easily broken down by hydrocarbon solvents such as gasoline, benzene, diesel fuel, and tar.
Care should be taken to prevent insulation damage during handling, storage, and backfilling.
Also, where termites are a concern, standard preventative practice such as soil treatment, termite

shields, etc. is suggested.

* Insulation specifications. Because some insulation materials resist water absorption
less effectively than others, which in turn degrades their thermal resistance (R-values), insulation
material should be specified carefully. The following effective R-values shall be used to
determine insulation thicknesses required for this application: Type II expanded polystyrene - 2.4
R per inch; Types IV, V, VI, VII extruded polystyrene - 4.5 R per inch; Type IX expanded
polystyrene - 3.2 R per inch. Special applications, such as bearing structural loads from footings,
may require higher density polystyrenes for the required compressive strengths. The builder is
referred to manufacturers for product-specific information.

14



APPENDIX

DESIGN DATA







RECOMMENDED CONSTRUCTION METHODS AND DETAILS

UNHEATED AREAS IN OTHERWISE HEATED BUILDINGS

Many heated buildings may have small portions of its footprint which are unheated, such as
entries and porches, and therefore require special consideration (see Figure 15). The design for
these cases is outlined in steps earlier in this design guideline. In the case of a home with an
attached garage, a cold bridge is created between the two slabs, requiring insulation to break the

bridge (see Figure 16).

S/T

Re

NON FROST-SUSCEPTIBLE FILL

WING INSULATION (DEPENDS ON DESIGN OPTION AND CLIMATE)

Figure 15. Insulation detail for small unheated portion (e.g.
recessed entry) in an otherwise heated building.

STE

‘MUST MEET FLt
8° MIN. THICKNESS \WIHOUSE INSULATION
GRAVEUSAND LAYER ——!
aRAGE AREX

I
| | GARAGE INSULATION
LUSH

Figure 16. Insulation detall for a large unheated area (e.g.
attached garage).

20





Appendix: Design Data

Figure A1. Continued

————— =
The air-freezing index method for the United States is described as cumulative degree days above and below 32°F. It is used

as a seasonal measure of the combined magnitude and duration of air temperatures below freezing. The index was computed
over a 12-month period (July-June) for each of the 3,044 stations used in the above analysis. (Stuerer) Data from the 1951-80
period were fitted to a Weilbull probability distribution to produce an estimate of the 100-year return period. Probability
estimates were verified by analysis of representative, long-term weather records. (Stuerer and Crandell, 1993)

As with most contour maps, the analyzed isopleths are somewhat generalized and may depart from actual conditions,
Topographic variability, proximity to bodies of water, and urban heat effects must be considered when interpolating values
for specific locations. Dashed lines are used in areas which contain few or no stations. Micro climate effects may produce
a variability of + 500 °F-days in the 100-year retum air freezing index estimate.

This analysis was prepared by the National Oceanic and Atmospheric Administration’s National Climatic Data Center under
contract to the NAHB Research Center. (Stuerer, 1989)



Appendix: Design Data

Table A3,

Estimates of the Air-Freezing Index (°F-days) at Various Locations and Return Periods

MEAN ANNUAL RETURN PERIOD ESTIMATES (F,,
LOCATION TEMPERATURE
(CF) 100-YR | 50-YR | 25-YR | 5-YR| 2-YR
ALASKA, ELMENDORF AFB 35.0 3427 3317 3190 | 2777 | 2347
COLORADO, DENVER 50,3 712 653 589 408 261
CONNECTICUT, HARTFORD 49.7 938 880 815 619 Aga
IDAHO, IDAHO FALLS 43.8 2353 2202 | 2035 | 1537] 1092
ILLINOIS, CHICAGO 50.6 1433 1334 1225 904 625
INDIANA, SOUTH BEND 49.4 1379 1286 1183 880 270
IOWA, FORT DODGE 474 2132 2031 1917 | 1561 | 1216
KANSAS, TOPEKA 54.1 1000 919 831 581 376
KENTUCKY, LEXINGTON 54.9 720 642 560 343 190
MAINE, PORTLAND 45.0 1407 1353 1290 | 1091] 889
MICHIGAN, LANSING 47.2 1529 1454 1370 | 1107] 854
MINNESOTA, DULUTH 38.2 3126 3060 | 2984 | 2729] 2448
MISSOURI, JEFFERSON CITY 55.1 903 804 700 427 234
MONTANA, LEWISTOWN 419 2473 2319 2147 | 1632] 1170
NEBRASKA, NORTH PLATTE 48.1 1686 1582 1466 | 1118 S05 |
NEVADA, ELKO 46.2 1526 1398 1258 867 551
NEW HAMPSHIRE, CONCORD 45.3 1599 1537 1466 | 1239] 1010
NEW YORK, SYRACUSE 417 1213 1163 1106 925 744
NORTH DAKOTA, BISMARCK. 41.3 3359 3239 3102 | 2659 | 2205
OHIO, MANSFIELD 48.2 1367 1278 1178 883 622
OREGON, BAKER 45.6 1446 1312 1167 m7 466
PENNSYLVANIA, STATE COLLEGE 49.3 1169 1082 987 112 479
SOUTH DAKOTA, REDFIELD 43.9 3005 2885 2748 | 2310} 1872
| UTAH, OGDEN" 508 1081 968 849 532 301
VERMONT, BURLINGTON 44.1 2054 1958 1905 | 1646} 1379
VIRGINIA, BIG MEADOWS 47.2 1149 1062 966 690 458
WASHINGTON, SPOKANE 47.2 1232 1120 998 664 405
WEST VIRGINIA, ELKINS 49.4 1045 950 848 567 347
WISCONSIN, WAUSAU 42.4 2492 2427 | 2353 | 2106} 1840
WYOMING, SHERIDAN 44.6 2283 2157 2015 | 1584] 1181

A-3


FPSF DESIGN PROCEDURE

FLASHING

PERMANENT WOOD FOUNDATION

| PER BUILDING CODE
i

CONCRETE, MASONRY, OR
INSULATION PROTECTION

12" MAX.

VERTICAL WALL INSULATION
HORIZONTAL WING INSULATION
GRANULAR BASE (AS REQ'D)

INSULATION DETAIL

HORIZONTAL INSULATION

FOUNDATION PERIMETER

INSULATION PLAN

Figure 4. FPSF design parameters for heated buildings using the simplified
design procedure.


FPSF DESIGN PROCEDURE

for all materials, including insulation. If the floor cross section and resulting thermal resistance of the floor slab
varies over its area, calculate R, as the average over the perimeter 39 inches (1 m) of the floor. In superinsulated
slabs where the calculated R, value exceeds 28 R (5.0 m?-°C/W), the designer must follow the design procedure for
unheated buildings, since the heat from the building is substantially blocked from moving into the ground and
protecting the foundation.

Determine the Required R-value of Vertical Wall Insulation, R,

Determine the minimum required thermal resistance of the vertical wall insulation, R,, from Table A4 given h from
Figure 5, F, from Step 1, and R, from Step 2.

Select Thickness of Vertical Wall Insulation

Based on the required R, value from Step 3, select an adequate thickness of vertical XPS or EPS insulation using
the following effective resistivities: Type II EPS - 2.4 R per inch, Type IX EPS - 3.2 R per inch and Types IV, V,
VI, VIE XPS - 4.5 R per inch. Common nominal thicknesses are 1", 1-1/2", 2", and 3". The insulation shall extend
from the bottom of the footing to the exterior wall envelope as shown in Figure 5.

Select Foundation Depth or Horizontal Wing Insulation for Walls

Horizontal wing insulation is placed below ground extending outward from the vertical wall insulation as shown in
Figure 5. For climates where F, is less than 2,250°F-day, wing insulation along the walls is not required and the
designer may proceed to Step 7. In more severe climates, where horizontal wing insulation is not desired, select the
minimum foundation depth, H,, from Table AS. When the designer desires to limit the foundation depth to 16 inches
in severe climates (F, > 2,250°F-days), select the minimum width the wing shall extend from the foundation wall,
D,,» and the required minimum thermal resistance of insulation, R,,, from Table A6.

Select Thickness of Horizontal Wing Insulation for Walls

Based on the required R,,, value determined in Step 5, select an adequate thickness of XPS wing insulation using
an effective resistivity of 4.5 R per inch. Wing insulation must have at least 10 inches of ground cover and meet
flush with the vertical wall insulation.

Since more heat loss occurs at building comers than through mid-wall sections of heated buildings, additional frost
protection in the form of horizontal wing insulation or a deeper foundation is required for more severe climates (F,
> 2,250°F-days). Where horizontal wing insulation is not desired in any climate, select the minimum foundation
depth at comers, H,,, from Table AS. For a uniform foundation depth, H, = H,,, use Table AS with 5.7R wing
insulation applied to the corner regions. When the designer desires to limit the foundation depth at comers to 16
inches, select the minimum width the wing shall extend from the foundation wall, D,., and the required minimum
— —- + required-thermal resistance -of-insulation, R,. from—Table-A7._Based-on_the required_R,,, select-an-appropriate-——..—-———
thickness of XPS wing insulation using an effective resistivity of 4.5 R per inch. The minimum distance the comer
protection must extend from the corners, L,, is also determined from Table A7.

_ Select Foundation Depth or Horizontal Wing Insulation at Corners


Foreword

It is a rare occurrence for an innovative building technology to offer both lower initial costs and
increased energy savings. The frost-protected shallow foundation technique (FPSF) outlined in
this document is such a technology. Although the technique is new to United States builders and
code officials, it has been used effectively in Scandinavia in more than one million homes.

Until now, U.S. home builders have placed their foundations below the frost line established by
the local building department. As a result, in many regions of the country, deep crawl spaces
and basements have become the norm adding significantly to the cost of construction. The frost-
protected shallow foundation technology offers builders an opportunity to excavate less earth,
use less material, and build homes more cost effectively.

This design guide presents an FPSF design procedure for slab-on-grade buildings and
demonstrates the procedure with design examples. It is the result of nearly 10 years of research
on the subject in the United States, as well:as more than 40 years of research in Scandinavia.
In addition, a simplified design method is presented in a form suitable for adoption by the major
model building codes.

The Design Guide For Frost-Protected Shallow Foundations provides residential builders
throughout cold-region communities with technical information that can reduce the cost of
housing and thereby expand housing affordability and opportunity.

lake Gb —

Assistant Secrétary for Policy
Development and Research



REFERENCES

Morris, Richard A. Frost-Protected Shallow Foundations: Current State-of-the-Art and Potential
Application in the U.S. Prepared for Society of the Plastics Industry, Inc. NAHB
Research Center, Upper Marlboro, MD (August 1988).

Norwegian Building Research Institute. Frost-Protected Shallow Foundations for Houses and
Other Heated Structures, Design Details. Forskningsveien 3b, Postboks 322, Blindern
0314, Oslo 3, Norway. Translated by the NAHB Research Center January 1988).

Southern Building Codes Congress International, Inc. (SBCCI). Standard Building Code.
Birmingham, AL (1991).

Steurer, Peter M. and Jay H. Crandell. Comparison of the Methods Used to Create an Estimate
of the Air-Freezing Index. National Oceanic and Atmospheric Administration, National
Climatic Data Center, Asheville, NC (March 1993).

Steurer, Peter M. Methods Used to Create an Estimate of the 100-Year Return Period of the Air-
Freezing Index. U.S. Department of Commerce, National Oceanic and Atmospheric
Administration, National Climatic Data Center, Asheville, NC (1989); Appendix of SPI
Phase I report.

30



RECOMMENDED CONSTRUCTION METHODS AND DETAILS

HEATED SPACE

VERT. WALL INSULATION

NO WING INSULATION

Figure 11. independent stem wall with insulation on exterior face of
wall.

HEATED SPACE

PROTECTIVE n ae :
FACING ——>Bt - - —

<;——_——_ PERMANENT WOOD FOUNDATION

VERT. WALL INSULATION
(AS REQ'D.)

PRESSURE TREATED FOOTING
GRAVEL BASE

(AS REQ'D)

WING INSULATION
(OPTIONAL)

Figure 12. Typical permanent wood FPSF.

18



Notice

The U.S. Government does not endorse products or manufacturers.
Trade or manufacturers’ names appear herein solely because they are
considered essential to the object of this report.

The contents of this report are the views of the contractor and do not
necessarily reflect the views or policies of the U.S. Department of Housing
and Urban Development or the U.S. Government.





FPSF DESIGN EXAMPLES

rom Table A5 for F, = 3,700 °F-day a minimum foundation depth of
, = 50" for a distance of L, = 60" from the corner would avoid the
use of wing insulation at the comers, but require a stepped footing.
Alternatively, from Table A5, using 5.7R wing insulation with D,, =
4" and L, = 60" would allow a uniform foundation depth of 30" with
0 wing required along the midwall areas. For this example, the
desired foundation depth is 16". From Table A7 by interpolation

- between values, select a wing width, D,,, of 36" with a required
thermal resistance, R,,, of 11.7 hr-ft-"F/Btu extended for a distance, L,,
f 60" along the walls. The minimum thickness of XPS wing
insulation = Ry~/Teg or 11.7/4.5 = 2.6 inches minimum of XPS.
Therefore, use 3.0" of XPS wing insulation at the comer locations.

Design Details

36" 36"

|- 1.5" XPS OR 2" Type IX EPS
Vertical wall insulation
@ all locations

__— Horizontal wing insulation
2" XPS @ walls

3" XPS @ corners Plan View

Figure 18. Design detail for example B.



Appendix: Design Data

Table A1.
SYMBOLS, UNITS, AND CONVERSION FACTORS

Raz per inch)

English
Symbol Definition SI units Units
B Width (smaller dimension) of building m ft or in
Dd, Width of horizontal wing insulation m ft or in
Di. Width of horizontal wing insulation at comer m ft or in
Dw Width of horizontal wing insulation along wall m ft or in
D, Width of ground insulation beyond the perimeter of an unheated m ft or in

building
Fro 100-year return period design freezing index °C-hr °F-days
F, Freezing index likely occurring once in n years °C-hr °F-days
h Floor height above finished grade m ft or in
hy, hy Foundation depth along walls or at corners m ft or in
h, Depth of vertical wall insulation into ground m ft or in
L Length (along a wall) to extend comer insulation m ft or in
R, Nominal R-value of floor/slab construction (average of outer 4 mC/W | br-ft?-°F/Btu
feet of floor)

R, Required R-value of vertical wall insulation mC/W | hr-ft-°F/Btu
R, Required R-value of horizontal wing insulation m-C/W | hr-ft-°F/Btu
Ree Required R-value of horizontal wing insulation at corner m"C/W hr-f?-"F/Btu
Ru Required R-value of horizontal wing insulation along wall mC/W hr-f?-"F/Btu
R, Required R-value of ground insulation for unheated buildings m°C/W hr-f?-"F/Bu
MAT Mean annual temperature (MAT) me °F
Rey Effective R-value of an insulation material in service conditions mC/w | br-ft?-°F/Btu
Tes Effective resistivity of an insulation material in service conditions m-C/W | hr-ft?-°F/Ba-in

Unit Conversion Factors

Length 1m = 100cm = 3.28ft = 39.4in
Mass ikg = 2.2 Ibm (or 70.8slugs)
Weight 1N = 0.225 Ibf
Area 1m? = 10,000cm? = 10.8f? = 1550in®
Volume im? = 1x10%cm’ = 35.3ft = 6.1x10‘in®
Density Ikg/m? = 0.0624Ibf/ft°
Pressure 1kPa = 0,145psi
Temperature °F = 1.8x(°C) + 32
Freezing index 1°C-hr = 0.075°F-days
Thermal properties
- conductance 1Wim?-°C = 0.176Btwhr-ft-F
- resistance Im*°C/W = 5.68hr-f?-"F/Btu_ {R-value}
- conductivity 1W/m-°C = 6.933Btu-in/hr-f?-F
_ “| = resistivity Tm-*C/W = 0.144 hr-ft?°F/Btu-in



Common Questions and Answers
About Frost-Protected Shallow Foundations

A number of questions have been posed by building officials, builders, engineers, and others
about frost-protected shallow foundations (FPSF). The most common questions and their answers
are provided here to benefit those interested in this technology.

Question No. 1: How does insulation stop frost heave from occurring?

Frost heave can only occur when all of the following three conditions are present: 1) the soil is frost susceptible
(large silt fraction), 2) sufficient moisture is available (soil is above approximately 80 percent saturation), and 3) sub-
freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of frost
damage. Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene
insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly
effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat
into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation
projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage.
Finally, because of the insulation, the frost line will rise as it approaches the foundation. Since frost heave forces
act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.

Question No. 2: Does the soil type or ground cover (e.g., snow) affect the amount of
insulation required?

By design, the proposed insulation requirements are based on the worst-case ground condition of no snow or organic
cover on the soil. Likewise, the recommended insulation will effectively prevent freezing of all frost-susceptible
soils. Because of the heat absorbed (latent heat) during the freezing of water (phase change), increased amounts of
soil water will tend to moderate the frost penetration or temperature change of the soil-water mass. Since soil water
increases the heat capacity of the soil, it further increases the resistance to freezing by increasing the soil’s "thermal
mass" and adding a significant latent heat effect. Therefore, the proposed insulation requirements are based on a
worst-case, silty soil condition with sufficient moisture to allow frost heave but not so much as to cause the soil itself
to drastically resist the penetration of the frost line. Actually, a coarse grained soil (non-frost susceptible) which is
low in moisture will freeze faster and deeper, but with no potential for frost damage. Thus, the proposed insulation
recommendations effectively mitigate frost heave for all soil types under varying moisture and surface conditions.

Question No. 3: How long will the insulation protect the foundation?

This question is very important when protecting homes or other structures which have a long life expectancy. The
ability of insulation to perform in below-ground conditions is dependent on the product type, grade, and moisture
resistance. In Europe, polystyrene insulation has been used to protect foundations for nearly 40 years with no
experience of frost heave. Thus, with proper adjustment of R-values for below-ground service conditions, both

— —~ — extruded polystyrene (XPS) and expanded polystyrene (EPS) can be used with assurance of performance. In the
United States, XPS has been studied for Alaskan highway and pipeline projects, and it has been found that after 20

years of service and at least 5 yrs of submergence in water that the XPS maintained its R-value (ref. McFadden and
Bennett, Construction in Cold Regions: A Guide for Planners, Engine: Contractors, and Managers, J. Wiley &
Sons, Inc., 1991. pp328-329). For reasons of quality assurance, both XPS and EPS can be readily identified by
labelling corresponding to current ASTM standards.

21


FPSF DESIGN PROCEDURE
APPLICATIONS/LIMITATIONS

This procedure addresses the design of frost-protected shallow foundations which use insulation
to prevent frost heave in cold climates. It is specific to slab-on-grade foundations on residential
homes but may also be used on commercial and agricultural structures. This design procedure
does not apply to buildings on permafrost, to areas with mean annual temperatures less than 32°F
(0°C), or to crawl-space construction.

The proper specification of insulation products is paramount to the success of an FPSF
application. Few insulation products are able to maintain a dry R-value in a moist, below-ground
environment over any great length of time. Insulation products specified for an FPSF must be
rated with an effective R-value that can be maintained in such an environment for the expected
life of the structure. Because some insulation materials resist water absorption less effectively
than others, which in turn degrades their thermal resistance (R-values), insulation material should
be specified carefully. The reader is directed to references listed in the bibliography for
additional information on this issue.

Polystyrene insulation for below-ground, frost-protection applications in the U.S. must comply
with the only available U.S. standard, ASTM C 578-92 for Rigid, Cellular Polystyrene Thermal
Insulation. Although this standard does not provide for adjustment of thermal resistance in
potentially moist, below-ground conditions, appropriate adjustment factors for EPS and XPS have
been determined based on international knowledge and experience. The reader is referred to
manufacturers for product-specific information.

This design procedure specifies insulation and foundation depths which ensure protection against
frost heave damage for all types of soils. The procedure is conservative in that it assumes a
100-year return winter and no insulating ground vegetative or snow cover. The designer assumes
responsibility for compliance with all local building and energy codes. This document addresses
heated, unheated, and semi-heated structures as based on the expected average indoor monthly
temperature range of the building from Table 1.

Table 1. Classification of building based on indoor air temperature, T.

AVERAGE MONTHLY INDOOR CLASSIFICATION TYPICAL TYPE OF
TEMPERATURE, T STRUCTURE

T > 63°F (17°C) Heated Homes, Offices

41°F (S°C) < T < 63°F (17°C)
T < 41°F SC)

Agricultural, Seasonal Use
Garages, Exposed Slabs



Appendix: Design Data

Table A4
MINIMUM THERMAL REISTANCE OF VERTICAL WALL INSULATION
R, (hr-ft?-°F/Btu}
F, 0.0<R, < 6.0 6.0 <R, < 15.0 15.0 < R, < 28.0
(F-day) jl A - - A A
hsi2in h=24in h<i2in h=24in hsi12in h= 24 in
375 or less 0.0 3.0 45 5.7 57 8.5
750 3.0 46 5.7 5.7 85 11.4
1,500 45 5.7 5.7 5.7 8.5 11.4
2,250 5.7 5.7 57 TA 8.5 14.2
3,000 5.7 5.7 68 8.5 97 15.3
3,750 5.7 6.8 8.0 9.7 114 17.0
4,500 68 8.0 10.2 119 13.6 19.3

Note: Interpolation is allowed to obtain design values for a site’s F, and h.

Table AS
MINIMUM FOUNDATION DEPTHS

WITHOUT WING INSULATION
OR WITH WING AT CORNERS ONLY

Foundation Depth Along eanentne Foundation Depth at Corners With
Walls With No Wing No Wing RS5.7 Wing At Corners Only
F, (F-day)
hin) L, Gn) | hy, (in) L, (in) h;, Gin) | D,, (in)
1,500 or less 12 - 12 - 12 -
2,250 14 ~- 14 ~ 14 ~
2,625 16 —40—_ — 24 46- 16 10° 1
3,000 20 40 32 40 20 20
3,375 24 60 40 60 24 20
3,750 30 60 51 60 30 24
4,125 36 60 63 60 36 32
4,500 43 80 n 80 4B 32 |

AS


Appendix: Design Data

Table A8
MINIMUM THERMAL RESISTANCE, R, OF GROUND INSULATION
AND HORIZONTAL EXTENSION, D,, FOR UNHEATED BUILDINGS

Mean Annual Temperature (CF): | 32 36 38 40 | 241

F, (F-day): D, (inches):

750 or less 30 5.7 | 5.7 | 5.7 | 5.7 | 5.7
1,500 49 13.1] 9.7 | 85 | 80 | 68
2,250 63 19.4 | 15.9 | 13.6 | 11.4 } 10.2
3,000 79 25.0 | 21.0 | 18.2 | 15.3 | 14.2
3,750 91 31.2 | 26.1 | 22.7] - -
4,500

t

Ground Insulation a]

Ground “F
Insulation

pot

Plan View Plan View

Elevation Elevation Elevation

Slab-on-Grade Foundation Wall Column

Figure A3. Ground insulation applications and requirements.

A-11


PSF DESIGN PROCEDURE

DETAILED METHOD FOR HEATED BUILDINGS

In practice, there are many different combinations of vertical and horizontal insulation details,
R-values, and footing depths that can be used in an FPSF. The detailed design approach is a
flexible approach that allows the designer to utilize experience and select the preferred method
of construction for a given site. For example, the designer may opt to provide vertical wall
insulation only, wing insulation only at the corners, or provide wing insulation around the entire
building. The designer also has the flexibility to step the footing to increase foundation depths,
add wing insulation to reduce required foundation depths, or select the width of wing insulation
in meeting the minimum requirements in the design process.

Figure 5 illustrates the variables for FPSF design. The Appendix contains figures and tables for

determining and selecting the detailed design variables. The following steps outline the detailed
design approach for heated buildings.

Ray

| Vertical wall insulation, Ry Ry

* Rae
L_ Horizontal wing insulation, RywOr Rho

a8 required
Plan View ,

Figure 5. FPSF design parameters for heated buildings.

Determine the Site’s design Air Freezing Index, F,

Select the 100-year return period design air freezing index, Fo, from Figure Ai or Table A3. ‘This information was
prepared by the National Oceanic and Atmospheric Administration’s National Climatic Data Center specifically for
use in FPSF design. The F, values are conservative because they are not adjusted for the insulating benefit of a
normal snow cover on the ground. A lower return period value may be used for less important structures or those
that are resilient to infrequent ground freezing. See Table 3 for F, values at return periods less than 100 years.

Calculate the R-value for the Floor Slab Cross Section, R,

Calculate the thermal resistance of the design floor slab, R,, considering all insulating materials in the cross-section
including any floor coverings. When determining Rf, dry condition R-values, as presented in Table A2, shall be used.

8