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DESIGN GUIDE FOR FROST-PROTECTED SHALLOW FOUNDATIONS

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 years1. 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.

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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:

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 (i.e., 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.

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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. Unhealed areas of homes such as garages may be constructed in this manner.

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.

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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.

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-million2 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.

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.

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 TEMPERATURE, T CLASSIFICATION TYPICAL TYPE OF STRUCTURE
T > 63°F (17°C) Heated Homes, Offices
41°F (5°C) < T < 63°F (17°C) Semi-heated Agricultural, Seasonal Use .
T < 41°F (5°C) Unheated Garages, Exposed Slabs

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 (AFT) for the site location must be known. An AFT 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 BUILDINGS1

Air Freezing Index
(°F days)2		 Vertical Insulation R-Value3,4 Horizontal
insulation
R-Value3,5		 Horizontal insulation dimensions
per figure No.16 (inches)		 Minimum Footing Depth (inches) D
1,500 or less 4.5 - NR NR NR NR NR 12
2,000 5.6 NR NR NR NR NR 14
2,500 6.7 1.7 4.9 12 24 40 16
3,000 7.8 6.5 8.6 12 24 40 16
3,500 9.0 8.0 11.2 24 30 60 16
4,000 10.1 10.5 13.1 24 36 60 16

1 Insulation 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.

2 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 II expanded polystyrene – 2.4R per inch; Types IV, V, VI, VII extruded polystyrene – 4.5R per inch; Type IX expanded polystyrene – 3.2R per inch. NR indicates that insulation is not required.

4 Vertical insulation shall be expanded polystyrene insulation or extruded polystyrene insulation. 3 Horizontal insulation shall be extruded polystyrene insulation.

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

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Figure 5. FPSF design parameters for heated buildings.

Step 1: Determine the Site’s design Air Freezing Index, Fn Select the 100-year return period design air freezing index, F100, 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 Fn 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 Fn values at return periods less than 100 years.

Step 2: Calculate the R-value for the Floor Slab Cross Section, Rf. Calculate the thermal resistance of the design floor slab, Rf 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 for all materials, including insulation.

If the floor cross section and resulting thermal resistance of the floor slab varies over its area, calculate Rf 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 m2-°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.

Step 3: Determine the Required R-value of Vertical Wall Insulation, Rv Determine the minimum required thermal resistance of the vertical wall insulation, Rv, from Table A4 given h from Figure 5, Fn from Step 1, and Rf from Step 2.

Step 4: Select Thickness of Vertical Wall Insulation Based on the required Ry 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, VII 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.

Step 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 Fn 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, Hf, from Table A5. When the designer desires to limit the foundation depth to 16 inches in severe climates (Fn > 2,250°F-days), select the minimum width the wing shall extend from the foundation wall, Dhw, and the required minimum thermal resistance of insulation, Rhw, from Table A6.

Step 6: Select Thickness of Horizontal Wing Insulation for Walls Based on the required Rhw 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.

Step 7: Select Foundation Depth or Horizontal Wing Insulation at Corners Since more heat loss occurs at building corners 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 (Fn > 2,250°F-days).

Where horizontal wing insulation is not desired in any climate, select the minimum foundation depth at corners, Hfc, from Table A5.

For a uniform foundation depth, Hf = Hhc, use Table A5 with 5.7R wing insulation applied to the comer regions. When tfe designer desires to limit the foundation depth at corners to 16 inches, select the minimum width the wing shall extend from the foundation wall, Dhc, and the required minimum required thermal resistance of insulation, Rhc, from Table A7.

Based on the required Rhc, select an appropriate thickness of XPS wing insulation using an effective resistivity of 4.5 R per inch. The minimum distance the corner protection must extend from the corners, Lc, is also determined from Table A7.

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.

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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.

Step 1: Determine Air Freezing Index, Fn, & Mean Annual Temp., MAT Select the 100-year return period design air freezing index, F100, from Figure Al or Table A3. This information was prepared by die National Oceanic and Atmospheric Administration’s National Climatic Data Center specifically for use in FPSF design.

The Fn 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 Fn values at return periods less than 100 years.

Step 2: 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, Dg, determined from Table A8 and shown in Figure A3. In unheated building foundations, Dg 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. Dg may be reduced by 1 inch for every inch the insulation is buried beyond the 10 inch minimum cover.

Step 3; Select the Minimum Effective R-value of Ground Insulation, Rg Select the minimum required R-value, Rg, required for the ground insulating layer from Table A8 based on Fn and MAT from Step 1. Rg 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. Rg 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.

Step 4: Select Thickness of Ground Insulation Based on the required Rg 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.

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.

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 comer areas.

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 woo,’ 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:

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.

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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.

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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.

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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).

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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, Engineers. 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.

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 dieir 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 comers 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 (i.e., 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.

FPSF DESIGN EXAMPLES

The following examples demonstrate the design process for FPSFs presented in Variations from the designs and details shown which meet the minimum requirerr guide would also be acceptable.

EXAMPLE A:

Site Information

Design Steps

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

Step A1 From Figure A1 for the Chicago area, F100 = 1,500 °F-day (Table A3 gives F100= 1.433 °F-days)

Step A2 Rf for the floor slab cross section = Rf.carpet + Rf.concrete
 Rf - 2-08 + 0.05 R/n x 4 in) = 2.28 R.
(See Table A2 for values.)

Step A3 From Table A4 for h = 12” and Fn – 1,500 °F-day:
Rv = 4.5 hr-ft-°F/Btu.

Step A4 The minimum thickness of wall insulation is Rv/reff
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.

Step A5: From Table A5 for Fn = 1,500 °F-day or less, a 12” foundation depth .. is acceptable and no wing insulation is required along the wall. The design may proceed to Step 7.

Step A7; From Table A5 for Fn = 1,500 °F-day or less, a 12” foundation depth at the corners is acceptable and no corner wing insulation is required.

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EXAMPLE B:

Site Information

Design Steps

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

Step B1 From Figure A1 for tne Bismarck area, F100 = 3,700 °F-day Step : (Table A3 gives F100 = 3,359 °F-day).

Step B2 Rf for the floor slab Cross Secti0n – Rf.carpet + Rf.concrete + Rf.xps
Rf – 1.23 + (0.05 R/in x 4”) + (4.2 R/in x 1”) = 5.63 R.
(See Table A2 for values)

Step B3 From Table A4 for h = 12”, Rf = 5.6, and Fn = 3,700 °F-day:
Rv – 5.7 hr-ft-°F/Btu.

Step B4 The minimum thickness of wall insulation = Rv /reff
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.

Step B5: From Table A5 for Fn = 3,700 °F-day, a minimum foundation depth of Hf = 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, Dhw, of 36” with a required thermal resistance, Rhw, of 7.7 hr-ft-°F/Btu.

Step B6 The minimum thickness of XPS wing insulation = Rhw/reff
7.7/4.5 = 1.7 inches minimum of XPS required therefore, use 2.0 inch thick XPS wing insulation along the walls.

Step B7 From Table A5 for Fn = 3,700 °F-day a minimum foundation depth of Hfc = 50” for a distance of Lc = 60” from the corner would avoid the use of wing insulation at the corners, but require a stepped footing.

Alternatively, from Table A5, using 5.7R wing insulation with Dhc = 24” and Lc = 60” would allow a uniform foundation depth of 30” with no 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, Dhc, of 36” with a required thermal resistance, Rhc, of 11.7 hr-ft-°F/Btu extended for a distance, Lc, of 60” along the walls. The minimum thickness of XPS wing insulation = Rhc/reff or 11.7/4.5 = 2.6 inches minimum of XPS. Therefore, use 3.0” of XPS wing insulation at the corner locations

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Example C

Site Information

Unheated Areas entire building

Design Steps

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

Step C1 From Figures Al and A2 for the Duluth area, F100 = 3,000 °F-day and MAT = 38 °F.

Step C2: From Table A8 for Fn = 3,000 °F-day, Dg = 79 inches, therefore extend the ground insulation 6' 7" outside the building.

Step C3: From Table A8 for MAT = 38 °F and Fn = 3,000 °F-day, Rg = 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 Rg - 18.2 - 0.6 = 17.6 hr-ft-°F/Btu.

Step C4: The minimum thickness of XPS ground insulation = Rg/reff

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 pcf 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 pcf XPS (Type VI) with a compressive resistance of 40 psi (5760 psf).

Using a safety factor of 3, the allowable bearing load is 5760/3 = 1920 psf. This bearing 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.

#

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 IJord (Frost Action in Soil). Nr. 17, Oslo, Norway (November 1976); in Norwegian.

Comite Europeen 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 Freebome. 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 (1991).

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.)

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, Blindem 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 U report.

APPENDIX

DESIGN DATA

 

Table A1. SYMBOLS, UNITS, AND CONVERSION FACTORS

Symbol

Definition

SI units

English Units
B Width (smaller dimension) of building m ft or in
Dh Width of horizontal wing insulation m ft or in
Dhc Width of horizontal wing insulation at corner m ft or in
Dhw Width of horizontal wing insulation along wall m ft or in
Dg Width of ground insulation beyond the perimeter of an unheated building m ft or in
F100 100-year return period design freezing index °C-hr °F-days
Fn Freezing index likely occurring once in n years °C-hr °F-days
h Floor height above finished grade m ft or in
hf, Hfc Foundation depth along walls or at corners m ft or in
Hv Depth of vertical wall insulation into ground m ft or in
Lc Length (along a wall) to extend corner insulation m ft or in
Rf Nominal R-value of floor/slab construction (average of outer 4 feet of floor) m2-°C/W hr-ft2-°F/Btu
Rv Required R-value of vertical wall insulation m2-°C/W hr-ft2-°F/Btu
Rh Required R-value of horizontal wing insulation m2-°C/W hr-ft2-°F/Btu
Rhc Required R-value of horizontal wing insulation at corner m2-°C/W hr-ft2-°F/Btu
Rhw Required R-value of horizontal wing insulation along wall m2-°C/W hr-ft2-°F/Btu
Rg Required R-value of ground insulation for unheated buildings m2-°C/W hr-ft2-°F/Btu
MAT Mean annual temperature (MAT) °C °F
Reff Effective R-value of an insulation material in service conditions m2-°C/W hr-ft2-°F/Btu
reff Effective resistivity of an insulation material in service conditions (R,.ff per inch) m-°C/W hr-ft2-°F/Btu-in


Table A1b

Unit Conversion Factors


Length 1m = 100cm - 3.28ft = 39.4in
Mass 1kg = 2.2 lbm (or 70.8slugs)
Weight 1N = 0.225 lbf
Area 1m2 - 10,000cm2 - 10.8ft2 - 1550in2
Volume 1m3 = 1x10W = 35.3ft3 = 6.1x104in3
Density 1kg/m3 - 0.0624lbf/ft3
Pressure 1kPa - 0.145psi
Temperature °F= 1.8x(°C) + 32
Freezing index 1°C-hr - 0.075°F-days
Thermal properties
- conductance 1W/m2-°C - 0.176Btu/hr-ft2-T
- resistance 1m2-°C/W - 5.86hr-ft2-°F/Btu {R-value}
- conductivity 1W/m-°C = 6.933Btu-in/hr-ft2-T
- resistivity 1m-°C/W - 0.144 hr-ft2-°F/Btu-in

 

Table A2.

THERMAL PROPERTIES OF SOME FOUNDATION AND FLOOR CONSTRUCTION MATERIALS FROM ASHRAE FUNDAMENTALS (1989) AND ASTM STANDARD C 578-92

BUILDING MATERIALS Density lb/ft3 Resistivity hr ft2-°F/Btu
(R-value per inch, dry condition)
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 -- 2.1
Cement Mortar 120 0.15
Concrete 140 0.05
6 mil Plastic - Negligible
EPS Insulation, Type II 1.3 4.0
EPS Insulation, Type DC 1.8 4.2
XPS Insulation, Types IV, V, VI, VII 1.6 - 3.0 5.0
FINISH FLOORING MATERIALS

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

 

Table A4 MINIMUM THERMAL RESISTANCE OF VERTICAL WALL INSULATION

Rv (hr-tf-T/Btu)

Fn (°F-day) 0.0 < Rf <6.0 6.0 < Rff < 15.0 15.0 < Rf <28.0
h ≤12 in h-24in h ≤12 in h- 24 in h ≤12 in h-24in
375 or less 0.0 3.0 4.5 5.7 5.7 8.5
750 3.0 4.6 5.7 5.7 8.5 11.4
1,500 4.5 5.7 5.7 5.7 8.5 11.4
2,250 5.7 5.7 5.7 7.4 8.5 14.2
3,000 5.7 5.7 6.8 8.5 9.7 15.3
3,750 5.7 6.8 8.0 9.7 11.4 17.0
4,500 6.8 8.0 10.2 11.9 13.6 19.3

 

Table A5

MINIMUM FOUNDATION DEPTHS WITHOUT WING INSULATION OR WITH WING AT CORNERS ONLY


 Foundation Depth Along Walls With No Wing Foundation Depth At Corners With No Wing Foundation Depth at Corners With R5.7 Wing At Corners Only
Fn (°F-day) hf(in) Lc(in) hfc(in) Lc(in) hfc(in) Dhc(in)
1,500 or less 12 - 12 -- 12 --
2,250 14 - 14 -- 14 --
2,625 16 40 24 40 16 20
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 71 80 43 32

 

Table A6

MINIMUM THERMAL RESISTANCEOF WING INSULATION, Rhw, FOR USE ALONG WALLS

(16" FOOTING DEPTH)


 R-values For Various Wing Widths Along Walls, Dhc (inches)
Fn (°F-day) 12 18 24 30 36 42 48
2,250 or less 0.0  

   
2,625 2.5  

   
3,000 6.5 6.1 5.3 4.5    
3,375   8.2 7.4 6.5 1111  
3,750     9.1 8.5 7.7 1111
4,125     11.2 10.2 9.6 8.9
4,500    
12.3 11.4 10.7 10.0

 

Table A7

MINIMUM THERMAL RESISTANCEOF WING INSULATION, Rhc, FOR USE AT CORNERS

(16" FOOTING DEPTH)


 R-values For Various Wing Widths At Corners, Dhc (inches)
Fn (°F-day) Lc(in) 16 24 30 36 42 48
2,250 or less 0 0.0    

 
2,625 40 6.5 4.9 4.0      
3,000 40 9.6 8.6 8.0 7.4

3,375 60
11.1 10.5 9.8 9.1
3,750 60
13.1 12.5 12.0 11.2 10.8
4,125 60
  ! 14.5 13.7 13.0 12.5
4,500 80


15.9 15.1 14.8

 

Table A8

MINIMUM THERMAL RESISTANCE, Rg OF GROUND INSULATION AND HORIZONTAL EXTENSION, Dg FOR UNHEATED BUILDINGS

Mean Annual Temperature (°F): 32 36 38 40 ≥41
Fn (°F-day) Dg (inches):




750 or less 30 5.7 5.7 5.7 5.7 5.7
1,500 49 13.1 9.7 8.5 8.0 6.8
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 — —
4,500 108 37.5 31.8 — ~ ~

#

#

Maximum
Size Limit		 Fdim °F-days)
≤ 2,250 2,250 to 3,000 3,000 to 3,750 ≥3,750
L. 13'-0" 11'-6" 10'-0" 6'-6"
L2 9'-9" 8'-8" 7'-6" 4'-11"
L3 6'-6" 5'-10" 5'-0" 3'-3"


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