(C) Daniel Friedman 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?

Heated floor slab insulation detail (C) Daniel FriedmanQuestion:

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


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

  1. the 2-foot square samples used had proportionally far more edge area than full sheets (edges are torn cells that are open to moisture)
  2. 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. --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

Polystyrene foam insulation below concrete slab (C) Daniel FriedmanFor 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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