Polystyrene foam insulation (C) Daniel FriedmanFoundation Insulation Options
Guide to Underground & Aboveground Use of Polystyrene Foam Board

  • POLYSTYRENE FOAM INSULATION - CONTENTS: Foundation Insulation Options for buildings - using polystyrene
    • Insulating below grade with polystyrene foam insulation - choices
    • Performance comparison of expanded polystyrene and extruded polystyrene foam insulating panels for underground foundation insulation
    • Dow Chemical Study Reports insulating performance of extruded and expanded foam insulation used below grade after 6 and 18-month exposures
    • Recommended protective coatings for foam insulating board used against building foundations
  • Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about foundation insulation choices & methods

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This article discusses the question: which is the best foundation insulation to use below-ground. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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Underground use of Polystyrene Foam Insulating Board

Polystyrene foam insulation below concrete slab (C) Daniel FriedmanThe question-and-answer article about the below-grade use of expanded versus extruded polystyrene insulating foam boards, found just board, below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Is Expanded or Extruded Polystyrene Foam Insulation Better to Use for Underground Foundation Insulation?


I am building an earth-sheltered home and discovered that I could save a great deal of money by insulating below grade with expanded rather than extruded polystyrene foam insulation

. Is it unwise to use the expanded polystyrene board, given its tendency to absorb moisture below grade? -- D.S., San Jose, CA

Our photo (above left) shows polystyrene foam insulating board below a concrete slab being poured in Two Harbors, MN.


Basement heat loss (C) Daniel FriedmanWidely reported research by Dow Chemical Co. shows that their extruded polystyrene foam insulation, on average, outperforms expanded polystyrene (EPS) in below-grade insulation applications. Due to the study's limitations, however, Dow cautions against using the findings to predict long-term performance of polystyrene foam insulation board in this application.

Interestingly, the foam board insulation testing found that the most common expanded EPS - 2-inch thick, low density (1 lb./ft3), material - held up about as well as extruded stock in both vertical and horizontal insulation board applications. Both showed 2- to 13- percent increases in temperature conductivity after 6- and 18- month exposures.

Dow speculates that the poor performance of the high-density DPS was due to high void content and poor bead fusion. EPS manufacturers agree that good bead fusion is harder to achieve in high-density foam insulating board stock.

Here is a simple test of foam insulating board quality:

Polystyrene insulating foam board indoors (C) Daniel FriedmanEPS should break through the beads, not around them. Because over 175 small companies make EPS foam insulating board products, quality varies considerably. Our photograph of white polystyrene foam board insulation (below) shows broken foam boards at an indoor foam board insulation retrofit project.

Photo at left: styrofoam insulating board used indoors.

With well-drained soil, a low water table, [protection also from roof drainage spillage against the foundation wall], and a waterproof outer membrane (e.g. polyethylene), a good-quality EPS foam insulation board at least 2 inches thick should perform adequately.

To play it safe, some underground builders use EPS toward the inside and extruded insulating board facing the backfill soil around the building foundation.

Readers considering adding insulation inside or outside a basement foundation wall should also take a look at BUCKLED FOUNDATIONS due to INSULATION?

See BASEMENT HEAT LOSS for a discussion of foundation and basement insulation methods.

See BUCKLED FOUNDATIONS due to INSULATION? for a discussion of insulation's role in foundation damage from freezing soils.

See TERMITE SHIELDS vs TERMITICIDE for a discussion of avoiding insect damage when foam insulating board is used below or at ground level.

See WET BASEMENT PREVENTION for other exterior treatments of building foundation walls to combine foundation insulation with basement waterproofing membranes and drainage systems.


Foundation Foam Insulation Protection Methods & Advice

Question: What finishes are available to protect Styrofoam insulating board used for foundation insulation?

I will be insulating a foundation with Styrofoam. I understand there are masonry finishes available that can be applied directly to the foam or to an intermediate mesh, and that such a surface will hold up to weather exposure. What finishes are available to protect my foundation insulation? - Thomas F. Harter, Oakland CA

Reply: Several Ways to Protect Rigid Foam Foundation Boards

There are several ways to protect rigid foam foundation insulation boards, ranging from pressure-treated plywood to factory-applied stucco finishes.

One pre-finished foundation insulation panel is Thermboard, a 3/4-inch Styrofoam panel coated with a fiber-resin compound that has a stucco-like appearance. It costs [1980's prices] around $`.47 per square foot in the Wisconsin area where it is made. Thermboard is distributed by Georgia Pacific Corp.

Another pre-finished foundation insulation product is Styrofoam FP Panels, distributed by Dow Chemical and available at many lumber yards. The finish is a cementious coating that looks something like concrete.

For a field-applied finish to protect foundation insulation boards, you can trowel on your own stucco to the foam over expanded metal lath, or brush on a finish designed for this purpose, such as Styrofoam Foundation Brush-on Coating (also available from Dow Chemical). It comes in a kit consisting of a dry polymer cement mixture, liquid additive, and fiberglass reinforcing tape [use at seams between foundation insulation boards].

You mix the components with water and apply directly to the foam foundation insulating board. The insulating board is scratched-up to promote good adhesion. It dries to resemble concrete. The cost of this foundation insulation board coating is around 40 cents per square foot. [1986 prices].

[Solar Age 8/86]

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article


Continue reading at SUPER HI-R INSULATION or select a topic from the More Reading links shown below.


Readers should also see FLOOR, CONCRETE SLAB CHOICES for a discussion of choice of finish floor materials to preserve use of thermal mass of a concrete floor slab.

Readers interested in basement insulation improvements should also see INSULATION LOCATION for BASEMENT WALLS and BASEMENT HEAT LOSS.

For more general energy savings advice see ENERGY SAVINGS PRIORITIES, and our insulation home page at INSULATION INSPECTION & IMPROVEMENT.

If you are going to insulate under a basement or grade-level concrete slab floor, be sure to read SLAB INSULATION, PASSIVE SOLAR.

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POLYSTYRENE FOAM INSULATION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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