Types of Insulation Allowed for FPSF

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 (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average).

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 FPSF application: Type II expanded polystyrene (EPS) - 2.4R per inch; Type IX expanded polystyrene (EPS) - 3.2R per inch; Types IV, V, VI, VII extruded polystyrene (XPS)- 4.5R per inch.

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.

Two factors which affect the insulating performance of polystyrene insulation are insulation compression from overburden and structural loads, and absorbed moisture from the soil. In general, as the amount of insulation compression and absorbed moisture increase, insulation thermal efficiency decreases.

Since the insulations thermal efficiency is directly proportional to its thickness, insulation compression from excessive overburden stress is detrimental. As a general guideline, allowable long-term compressive stresses for polystyrene insulation should be limited to only 30% of the rated compressive strength of EPS (of any density, and when EPS is used) and only 20% of the rated compressive strength of XPS (when XPS is used).

The reasoning for the reduced allowable stresses is that conventional compressive strength tests are performed rapidly, and dont account for long term creep. Creep is defined as continued deformation under constant load. Laboratory testing of EPS and XPS has shown that long term creep can be significant and must be accounted for. Limiting the allowable compressive stresses helps to significantly reduce the amount long term creep compression in the insulation, ensuring thermal efficiency is maintained over the life of the insulation.

The other factor that can reduce insulation thermal efficiency is moisture. The thermal conductivity of the insulation will be increased (not good) as moisture is absorbed. Long-term immersion studies of insulation show that little moisture is absorbed and that R-values were still above 90% that of dry insulation R-value. Good drainage practice can further reduce the amount of moisture that is absorbed by the insulation.

Careful consideration of both of these factors is important when selecting the type, thickness and placement of insulation for FPSF use.

Prof. Horvath submitted a discussion of the paper "Status of ASCE Standard on Design and Construction of Frost Protected Shallow Foundations" by L.S. Danyluk and J.H. Crandell (This paper is an excellent summary of the Frost Protected Shallow Foundation Concept.)

This discussion was published in the February 1999 issue of ASCE'sJournal of Geotechnical and Geoenvironmental Engineering. Browse to http://www.pubs.asce.org/WWWdisplay.cgi?9704940 to view a copy of Prof. Horvaths discussion manuscript.

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.

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