Use of Polyisocyanurate Foam Insulation Below Concrete Slabs
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This article discusses the use of polyisocyanurate foam board insulation below concrete slabs. We describe research on the effectiveness of polyisocyanurate foam boards under a concrete building floor slab, and we compare using Polyisocyanurate Sub Slab Insulation with Polystyrene Foam Insulation Below a Concrete Slab.
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Properties & uses of Polyisocyanurate Insulation & Using Polyisocyanurate Foam Insulation Underneath a Concrete Slab
The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Is Foil-Faced Polyisocyanurate Foam Insulating Board Good to Use Below a Concrete Floor Slab?
Question:
The articles I read concerning the insulation of concrete slabs refer to extruded polystyrene foam insulating board for use underneath the slab and for perimeter treatment. (See photo at left).
Except for Owens-Corning literature that recommends use of its Energy Shield sheathing, foil-faced polyisocyanurate foam insulation use below slabs seemed to be ignored in the 1980's - why? -- Steve Scheller, Falls Church VA
Answer:
In 1984 the only published research to date on underground applications of rigid foam insulation was a study by Dow Chemical, in which samples of insulation were buried for extended periods and later unearthed.
In those tests, foil-faced polyisocyanurate foam insulating board did not fare so well, absorbing on average 5 percent water by volume and losing 40 percent of its R-value.
Manufacturers of the foil-faced products argued that the tests were not valid because
- the 2-foot square samples used had proportionally far more edge area than full sheets (edges are torn cells that are open to moisture)
- the applications did not resemble a typical application, but a worst-case scenario.
The major manufacturers of foil-faced polyisocyanurate foam insulating board, Owens-Corning and Celotex, both recommend its use below grade for foundation wall and underslab applications, but in general they recommend taking extra care while backfilling so as not to puncture the foil facing.
[Additional foil facing punctures would be caused, for example, by a radiant heat floor contractor who installs the tubing by stapling it or nailing it to the foil-faced sub-slab insulation before the concrete is poured. See RADIANT HEAT Floor Mistakes to Avoid. --DF.]
They also recommend good drainage around the building foundation so the insulating polyisocyanurate foam board panels are never immersed in water.
Specifically, Celotex recommends that the insulating foam board panels be protected during backfilling with a rigid material such as fiberboard sheathing.
Owens-Corning recommends that all polyisocyanurate foam board joints and edges below grade be covered with aluminum tape.
Comparing Polyisocyanurate Sub Slab Insulation with Polystyrene Foam Insulation Below a Concrete Slab
For comparison's sake, extruded polystyrene (see POLYSTYRENE FOAM INSULATION) has higher compressive strength than polyisocyanurate, but both have similar water absorption properties as measured by standard tests. Both are over 90-percent closed-cell foams.
Our photo (left) shows polystyrene foam insulating board below a concrete slab being poured in Two Harbors, MN.
In 1984 Owens Corning was currently monitoring samples buried over the summer and was expected to produce data results by 1985 when the samples were dug up. But in the mid 1980's builders had to rely on scanty published data and on manufacturer's recommendations.
Also remember that each foil-faced insulating board product has its own chemical makeup and facing material. See INSULATION R-Values & Properties for details about specific insulating board properties.
Readers should also see POLYISOCYANURATE FOAM INSULATION, see POLYSTYRENE FOAM INSULATION and INSULATION LOCATION for PASSIVE SOLAR FLOOR SLAB and also RIGID FOAM USE INDOORS. 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. Finally, readers considering adding insulation inside or outside a basement foundation wall should see POLYSTYRENE FOAM INSULATION as well as 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.
The question-and-answer article about use of polyisocyanurate foam insulating board below concrete slabs, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Orignal article
The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.
- Q&A on Below-Grade Polyisocyanurate use of solid foam insulation below concrete slabs - PDF version, use your browser's back button to return to this page
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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."
- Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: info@buildingadvisor.com
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
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
- John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
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- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
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- Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
- Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
- Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
- The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the InspectAPedia Bookstore
- "Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
- "Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
- "Heat Exchangers for Solar Water Heating Systems", U.S. DOE describes the types of solar water heater heat exchange methods between the sun and the building's hot water supply
- "Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
- "Solar Water Heating System Maintenance and Repair", U.S. DOE
- "Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
- "Scaling and Corrosion in Solar Water Heating Systems", U.S. DOE
- www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 is the base U.S. DOE website for these articles
- "Active Solar Heating Systems", U.S. Department of Energy, including
- "Radiant Heating Systems" U.S. DOE
- "Absorption Heat Pumps & Coolers", U.S. DOE
- "Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
- "Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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