The Dirt on Below-Grade Insulation

Originally posted online at Construction Superintendent

Understanding rigid foam insulation for foundation and under-slab applications

Contractors are increasingly called upon to install rigid foam insulation under concrete slabs and on building foundations.

Contractors are increasingly called upon to install rigid foam insulation under concrete slabs and on building foundations.

Up to one-quarter of a building’s energy loss is due to lack of insulation in below-grade areas, including the foundation and under slabs. Now that high-performance building envelopes are common above ground, the relative amount of total heat lost below grade will grow if these spaces are not addressed.

As a result, superintendents increasingly will encounter below-grade and under-slab insulation on all building types. To help increase understanding of how two common rigid-foam insulations perform in these settings, this article evaluates moisture absorption and thermal performance. It also discusses installation procedures for below-grade and under-slab insulation.

Rigid foam insulation
Two common rigid foam insulations specified for below-grade applications are expanded polystyrene (EPS) and extruded polystyrene (XPS).

EPS

An easy way to recognize EPS on the jobsite is that it is commonly white. This insulation is made of expanded polystyrene beads fused into sheet stock and blocks of various densities, compressive strengths and sizes. Historically used as a stable roof insulation, EPS has gained wide acceptance in wall, below-grade and under-slab applications due to its low-moisture absorption, strength and stable, long-term thermal performance. EPS insulation blocks can be custom-cut into a variety of shapes and sizes to meet wide ranging job specifications.

Building professionals have used EPS successfully in below-grade applications for decades. As of 2013, the International Code Council explicitly permits EPS throughout frost protected shallow foundations, under slabs and any other below-grade application.

XPS
To make XPS, manufacturers combine and melt polystyrene with blowing agents and additives, then force the liquid mixture through an extrusion die in a continuous feed, where it is shaped, cooled and trimmed to size. The product is most commonly available as boardstock of fixed size and thickness. Manufacturers often tint XPS a primary color for brand recognition.

EPS insulation absorbs significantly less moisture than does XPS insulation according to studies of real-world installations.

EPS insulation absorbs significantly less moisture than does XPS insulation according to studies of real-world installations.

Moisture absorption and thermal performance
There is much confusion in the marketplace regarding whether EPS or XPS insulation resists moisture better. This is a key point, as wet insulation has lower thermal performance. While manufacturers of both insulation types tout that their products have lower moisture absorption, in-situ tests indicate that EPS performs better in this regard.

For example, in 2008, Stork Twin City Testing – an accredited independent testing laboratory – examined sheets of EPS and XPS removed from a side-by-side installation after 15 years in service on a below-grade foundation in St. Paul, Minnesota. The XPS was significantly wetter on extraction, with 18.9 percent moisture content by volume compared to 4.8 percent for the EPS. After 30 days of drying, the XPS still had elevated moisture of 15.7 percent, while the EPS had dried to 0.7 percent.

The U.S. Department of Energy’s Oak Ridge National Laboratory also reports high moisture absorption levels for XPS. In a 2012 study, the lab reported “all samples of XPS insulation gained much more moisture during the 15 years of contact with soil moisture.” The resulting loss of energy savings performance was 10 percent for a full basement (“deep basement”) and 44 percent for a slab-on-grade installation.

By comparison, the U.S. Army Cold Regions Research and Engineering Laboratory found EPS buried in wetted soil for 1,000 days absorbed only 1.7 percent moisture by volume, which is substantially lower than the XPS rates noted above.

Installing rigid foam insulation below grade
On building foundations, the insulation (whether EPS or XPS) is installed over the damp/waterproofing, after that layer has adequately cured. Crews can use mechanical fasteners or polystyrene-compatible adhesive to attach the insulation. Applying a bead of polystyrene-compatible caulk or mastic to the top of the insulation board minimizes water infiltration behind it.

For under-slab applications, the rigid foam insulation typically should be installed over a gravel base, with a poly vapor diffusion retarder between the gravel and insulation. Additional insulation is applied along the edges of the slab, because that is a primary surface for heat loss. To avoid damage to the insulation, it is necessary to ensure removal of any jagged surfaces or irregularities in the substrate before installing the rigid foam panels.

In either case, it is important to confirm all details with the insulation manufacturer and local building department, and to ensure appropriate construction techniques to drain water away from the building.

In addition to its lower moisture absorption and better long-term thermal performance, EPS has the highest R-value per dollar among rigid insulations. As such, it provides a cost-effective way to insulate building foundations, and under slabs.

Ram Mayilvahanan is the product marketing manager for Insulfoam, which offers below-grade insulation under the Insulfoam and R-Tech brand names. For more information, visit www.insulfoam.com.

 

Ram Mayilvahanan

Ram Mayilvahanan

Contact Ram Mayilvahanan, Insulfoam’s Product-Marketing Manager

Ram.Mayilvahanan@insulfoam.com

Connect with Ram on LinkedIn  |  Follow Insulfoam on LinkedIn

Read more on Insulfoam.com

 

Below Grade and Under-Slab Building Insulation

Originally posted in Modern Contractor Solutions Magazine, June 2014 issue

Modern Contractor Solutions Magazine, June 2014 Digital Issue

MC_june 2014

Modern Contractor Solutions, June 2014

Three tips for choosing the right material for your project

Which insulation is best for use on buried building foundations and under concrete slabs? Sales reps, naturally, will tell you that their company’s product is best. But, what does independent testing and research say?

These three tips will help your firm select a cost-effective and high-performance rigid foam insulation type for your next below-grade insulation job.

1.  CONFIRM LONG-TERM THERMAL PERFORMANCE

Two of the rigid foam insulations most commonly used below grade and under slabs are expanded polystyrene (EPS) and extruded polystyrene (XPS). Although both are closed cell insulations, they perform very differently over the long term.

XPS has a higher initial insulating R-value than does a similar thickness and density of EPS, but the R-value of XPS degrades over time. EPS does not experience such “thermal drift,” and the reported R-value remains the same throughout years of installed service.

This is a crucial point when selecting insulation, as a decreasing R-value means lower thermal performance over time, and thus increased heating and cooling energy and costs for the building owner.

Below Grade

Rigid foam insulation is increasingly common in below-grade and under-slab applications.

A simple way to confirm an insulation’s long-term thermal performance is to review the warranty. Established EPS manufacturers typically warrant 100 percent of the published R-value for 20 years. By comparison, most XPS warranties typically cover only up to 90 percent of the published R-value, to account for the R-value degradation that occurs in the field.

2.  ENSURE MINIMAL LONG-TERM MOISTURE ABSORPTION

In addition to R-value stability, rigid foam insulations differ in their rates of moisture absorption and their ability to dry. Wetted insulation provides lower thermal resistance and can degrade over time. Since insulation installed below grade frequently contacts wetted soil, rates of moisture absorption and the ability to dry is key in these applications.

Independent laboratories have conducted extensive tests of moisture absorption rates for both EPS and XPS. Although XPS often rates better in laboratory short term, fully submerged tests, real-world long term tests show that EPS performs much better. The reason is that EPS has the ability to dry much faster than XPS. This ability to dry at a fast rate helps EPS remain drier during conditions of repeated exposure to moisture.

A 15-year in-situ test of EPS and XPS dramatically demonstrated this point. Stork Twin City Testing evaluated the moisture content of EPS and XPS buried side-by-side for 15 years on a building foundation in St. Paul, Minnesota. At the time the insulations were removed, the EPS was much drier than the XPS—the EPS had only 4.8 percent moisture by volume, compared to 18.9 percent moisture content for the XPS. After 30 days of drying time, the EPS had only 0.7 percent moisture by volume, while the XPS still contained 15.7 percent moisture.

The high moisture absorption rate of XPS in real-world settings is further seen in a 2012 report from the U.S. Dept. of Energy’s Oak Ridge National Laboratory (ORNL). Their researchers found that XPS insulation installed below grade for 15 years had absorbed 67 percent or more moisture.

3.  TARGET AN APPROPRIATE COMPRESSIVE STRENGTH

EPS insulation resists moisture better than XPS, including on buried foundations where it is in regular contact with wetted soil.

EPS insulation resists moisture better than XPS, including on buried foundations where it is in regular contact with wetted soil.

One of the best ways to save money on rigid foam insulation installed under concrete slabs is to ensure the material is not over-engineered. A common design assumption leads to specification of rigid foam strengths that are many orders of magnitude higher than necessary, which can double the insulation material cost.

Without getting into extensive technical details and mathematical formulas, the problem is engineers often use an overly conservative approach for insulation under concrete slabs. Many designers assume that point loads applied to a slab, such as those from the wheels of a forklift, transfer to the insulation in a triangular load path. Yet, concrete slabs distribute loads more uniformly than this, which means the insulation does not need as high of a compressive resistance as when one assumes a concentrated triangular load path.

An overly conservative design approach often results in specification of a high compressive resistance XPS product, when a more cost-effective EPS would offer sufficient strength. Since XPS typically costs more per inch than EPS, this is wasted money that comes off the contractor’s bottom line.

A simple solution for contractors is to ask the designers if they are using a formula from the Theory of Plates on Elastic Foundations, which take into account how slabs and insulation behave together. A resource to point them to for example calculations is the article “Right-sizing Under-slab Insulation,” in the April 2014 issue of Structure magazine.

CONCLUSION

With building owners’ growing desire to save money on heating and cooling costs, and increasingly stringent energy codes, contractors will be installing below-grade and under-slab insulation on more of their projects. EPS insulation out-performs XPS for both long-term thermal resistance and long-term moisture absorption, and EPS comes in a variety of compressive strengths suitable for nearly all building projects. With the highest R-value per dollar, EPS is the cost effective insulation choice.

Modern Contractor Solutions Magazine, June 2014 Digital Issue

EPS QUESTIONS?

 

Ram Mayilvahanan

Ram Mayilvahanan

Contact Ram Mayilvahanan, Insulfoam’s Product-Marketing Manager

Ram.Mayilvahanan@insulfoam.com

Connect with Ram on LinkedIn  |  Follow Insulfoam on LinkedIn

Read more on Insulfoam.com