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PATH Case Study

Frost-Protected Shallow Foundations Save Time, Money


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"The use of FPSFs are now approved by the International Code Council for residential and commercial buildings, but local code approval may still be an issue in some areas. Still, my experience has been that when armed with the right information and accompanied by a knowledgeable engineer, I can get building officials to approve the use of FPSFs."


The insulation used to protect FPSFs is made from rigid polystyrene foam--either expanded polystyrene (EPS) or extruded polystyrene (XPS), also known as blueboard.

"I use XPS or blueboard for its higher R-value (4.5 per inch) and also because it is readily available," Fosdick says. "The insulation usually comes in 4'x8' sheets of varying thickness ranging from 1 to 3 inches."

FPSFs offer excavation, construction, material and labor cost savings. One case study in Denver showed savings of 15 to 17 percent compared to a conventional foundation. An Iowa builder reported a $10 to $15 savings per linear foot when compared to basement foundations. Cost savings will vary by regional code-specified frost depth; colder climates offer greater opportunities for savings than more temperate climates.

"In southeast Colorado, we typically use 2" XPS under the slab and on the vertical surfaces of the foundation, which provides an insulation value of approximately R-9. Since more heat loss occurs at building corners, our standard practice is to install insulation horizontally at the corners. In the colder mountain climates, we use horizontal wings that extend 12" around the entire perimeter of the home. Only XPS can be used for horizontal insulation applications, but either XPS or EPS is suitable for insulating vertical surfaces, depending on the R-value required."*


FPSFs are generally cost effective if the frost line is 30 inches or deeper. The NAHB Research Center's Revised Builder’s Guide to Frost-Protected Shallow Foundations provides guidance. Use the Air Freezing Index (AFI) for your location to select the proper footing depth, the effective R-value for vertical and horizontal insulation, and the recommended size for horizontal wing insulation where needed.

Vertical wall insulation should be installed first. If wing insulation is required around the entire perimeter, it should have at least 10" of ground cover and be installed directly on the subgrade, flush with the vertical wall insulation.

Technology Highlights

This project included the following PATH-profiled technologies:

Section detail showing properly placed insulation protection.

Cast-in-place concrete walls fitted with 2-inch extruded polystyrene insulation.

"As with any type of foundation, proper drainage is crucial. Insulation works better when dry, so ensuring that water drains away from the building is very important," Fosdick says. "Creating a gravel or sand base improves drainage and helps create a smooth surface for horizontal wing insulation as well. We cover exposed insulation above ground with stucco to protect against UV rays and below-ground portions with sheetmetal to protect against termite damage. Stucco systems, durable coatings, and pre-coated insulation products are just a few methods to consider. Sheetmetal flashing can be effective from a durability and cost standpoint."

"When you have a walkout basement where the grade comes down the sides of the house, you also have to be mindful of the critical above- and below-grade marks and where dampproofing is required. When we first start a job, we grade the site, excavate the foundations, prepare the subgrade, and place either sand as a base or washed gravel with a French drain, if required by the engineer. For the base under the slab, we use a 3/8 minus, free-flowing material to ensure proper drainage. Since our exterior walls are made of concrete, we have our wall forms laid out around the perimeter of the house."

"After laying out forms, we place our insulation on a sand bed. Blueboard is placed at the bottom 20" of the wall and polyiso above the blueboard from 3" above the finish floor level to the top of the wall. Panel tie connectors are used to attach the polyiso insulation to the concrete walls. The special connectors that anchor the polyiso in place are arranged in a pattern consistent with code requirements for mesh attachment: every 2 feet horizontally, and every 6 inches vertically, allowing for conventional stucco application over the polyiso board."

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Content updated on 10/6/2006

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