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INSULATION INSPECTION & IMPROVEMENT
ACOUSTICAL SEALANT CHOICES
AIR LEAK MINIMIZATION
ASBESTOS IDENTIFICATION IN BUILDINGS
BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BUCKLED FOUNDATIONS due to INSULATION?
CATHEDRAL CEILING INSULATION
CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
DEW POINT TABLE - CONDENSATION POINT
DUCT INSULATION, ASBESTOS PAPER
FIBERGLASS PARTICLE CONTAMINATION
Fiberboard Insulation Sheathing Mold
FIBERGLASS INSULATION MOLD
FIREPROOFING ASBESTOS SPRAY-ON
FRAMING DETAILS for BETTER INSULATION
FRAMING DETAILS for DOUBLE WALL HOUSES
FRAMING METAL STUD PERFORMANCE
FREEZE-PROOF A BUILDING
HEAT LOSS in BUILDINGS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET
ROOF ICE DAM LEAKS
INSULATION AIR & HEAT LEAKS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INSULATION CHOICES & PROPERTIES
INSULATION FACT SHEET- DOE
INSULATION INSPECTION & IMPROVEMENT
INSULATION R-Values & Properties
LEED GREEN BUILDING CERTIFICATION
LOG HOME ENERGY EFFICIENCY
MOLD in FOAM INSULATION, RESISTANCE
MOISTURE CONTROL in BUILDINGS
NOISE / SOUND DIAGNOSIS & CURE
RIGID FOAM USE INDOORS
SHEATHING, FOIL FACED - VENTS
SLAB INSULATION, PASSIVE SOLAR
STAINS on & in BUILDINGS, CAUSES & CURES
STRAW BALE CONSTRUCTION
STUCCO WALL METHODS & INSTALLATION
STUCCO OVER FOAM INSULATION
SWEATING (CONDENSATION) on PIPES, TANKS
Thermal Expansion Cracking of Brick
THERMAL IMAGING, THERMOGRAPHY
THERMAL MASS in BUILDINGS
THERMAL TRACKING Indicates Heat Loss
TRUSS UPLIFT, ROOF
VAPOR BARRIERS & CONDENSATION in BUILDINGS
VENTILATION in BUILDINGS
WALL CONSTRUCTION BARRIER vs CAVITY
WIND WASHING INSULATION At EAVES
WINTERIZE A BUILDING
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
Is Foil-Faced Polyisocyanurate Foam Insulating Board Good to Use Below a Concrete Floor Slab?
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
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.
The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.
Frequently Asked Questions (FAQs) about polyisocyanurate insulation under concrete slabs & floors
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