Use of Polyisocyanurate Foam Insulation Below Concrete Slabs InspectAPedia® -
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This article discusses the use of polyisocyanurate foam board insulation below concrete slabs. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Sketch at page top is courtesy of Carson Dunlop.
The link to the original Q&A article in PDF form immediately below is followed 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
The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article, (see links just above) 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.
The question-and-answer article about use of polyisocyanurate foam insulating board below concrete slabs, quotes-from, updates, and comments an original article, (see links just above) from Solar Age Magazine and written by Steven Bliss.
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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.
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
"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-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 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.
"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.
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