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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.

Readers should also see BASEMENT HEAT LOSS, see ENERGY SAVINGS PRIORITIES, and our insulation home page at INSULATION & VENTILATION INSPECTION & IMPROVEMENT. Also, readers considering adding insulation inside or outside a basement foundation wall should BUCKLED FOUNDATIONS due to INSULATION? Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Underground use of Polystyrene Foam Insulating Board

• Polystyrene Underground: Q&A on Underground Use of Polystyrene Foam Insulating Board - PDF-form; use your browser's back button to return to this page

The 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, (see links just above) from Solar Age Magazine and written by Steven Bliss.

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

Question: 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

Answer:

Widely 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:

EPS 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.

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.

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.

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.

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AIR CONDITIONING & HEAT PUMP SYSTEMS
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DEW POINT TABLE - CONDENSATION POINT GUIDE
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FRAMING DETAILS for BETTER INSULATION
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FREEZE-PROOF A BUILDING
HEAT LOSS in BUILDINGS
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Heat Tapes: Use on Roofs for Ice
HEATING COST SAVINGS METHODS
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INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSULATION CHOICES
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INSULATION IDENTIFICATION GUIDE
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INSULATION MATERIAL IDENTIFICATION GUIDE
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  INSULATION R-Values & Properties
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  AIR LEAK MINIMIZATION
  BASEMENT HEAT LOSS
  Blocked Soffit Intake Vents
  BLOWER DOORS & AIR INFILTRATION
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  CATHEDRAL CEILING INSULATION
  ENERGY SAVINGS in BUILDINGS
  ENERGY SAVINGS RETROFIT CASE STUDY
  ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE
  ENERGY SAVINGS RETROFIT OPTIONS
  FRAMING DETAILS for BETTER INSULATION
  FRAMING DETAILS for DOUBLE WALL HOUSES
  Inspect Attics for Moisture or Mold
  Inspect Basements for Moisture or Mold
  Inspect Building Exterior
  Inspect the Ridge Vent System from the Attic
  Inspect the Soffit Vent System from the Attic
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  INSULATION for GREENHOUSE or SOLARIUM
  INSULATION CHOICES
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  INSULATION LOCATION for BASEMENTS
  INSULATION LOCATION for CAPES, CRAWLSPACES
  INSULATION R-Values & Properties
  Insulation Values of Log Home Walls
  POLYISOCYANURATE FOAM INSULATION
  POLYSTYRENE FOAM INSULATION
  RADIANT BARRIERS
  RIGID FOAM USE INDOORS
  ROOF VENTING ENERGY SAVING DETAILS
  ROOF VENTING NEEDED?
  PASSIVE SOLAR DESIGN METHOD
  PASSIVE SOLAR HEAT PERFORMANCE
  PHOTOVOLTAIC POWER SYSTEMS
  SLAB INSULATION, PASSIVE SOLAR
  Urea Formaldehyde Foam Insulation UFFI
  VAPOR BARRIERS & AIR SEALING at BAND JOISTS
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  URETHANE FOAM Deterioration, Outgassing
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LOG HOME GUIDE
MOBILE HOME INSPECTIONS
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Mold Growth Resistance of Foam Insulation
MOLD INFORMATION CENTER
ODORS & SMELLS DIAGNOSIS & CURE
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION
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RADIANT HEAT Floor Mistakes to Avoid
RADIANT SLAB FLOORING CHOICES
RADIANT SLAB TUBING & FLUID CHOICES
ROOF VENTILATION SPECIFICATIONS
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  TERMITE SHIELDS vs TERMITICIDE
ROT, TIMBER FRAME
SAFETY HAZARDS & INSPECTIONS
SEARS KIT HOUSES
SHEATHING, FOIL FACED - VENTS
SOLAR ENERGY SYSTEMS
SOUND CONTROL in BUILDINGS
STAIN DIAGNOSIS
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STUCCO OVER FOAM INSULATION
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SUMP PUMPS GUIDE
THERMAL EXPANSION of MATERIALS
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THERMAL MASS in UPSTAIRS
THERMAL TRACKING Indicates Heat Loss
VAPOR BARRIERS & AIR SEALING at BAND JOISTS
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VAPOR BARRIERS & HOUSEWRAP
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VENTILATION in BUILDINGS
WATER ENTRY in BUILDINGS
WIND TURBINES
WINDOWS & DOORS
WINTERIZE A BUILDING
WOOD Burning Heaters Fireplaces Stoves

• Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years. ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
  
• Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
  Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in BUILDINGS, and other topics.

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