Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
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
ICE DAM PREVENTION
INSULATION AIR & HEAT LEAKS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
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 explains the proper insulation amount and proper vapor barrier location below a passive solar heated concrete slab floor.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved.
Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Our page top photograph shows a horrible radiant floor slab insulation scheme installed by a Minnesota contractor - this radiant floor system never worked because the tubing was placed too deep in the concrete slab and the insulation system was incomplete.
Readers constructing an insulated slab with radiant floor heating, whether by passive solar or any other means, should see FLOOR, CONCRETE SLAB CHOICES and should not fail to read RADIANT HEAT Floor Mistakes to Avoid. We discuss whether basement slab insulation is or is not recommended for ordinary basement floor slabs (non-solar, non-radiant heat) at INSULATION LOCATION for BASEMENT FLOORS. 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 concerned about termite damage associated with foam, fiberglass, or other building insulation materials should also see TERMITE SHIELDS vs TERMITICIDE, and Insects & Foam 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 proper insulation amount and proper vapor barrier location below a passive solar heated concrete slab floor, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Questions about a heated slab: What thickness of insulation should I use below a 6-inch slab that has SolaRoll™ (passive solar floor heating system) auxiliary heating installed in the concrete? Where is the vapor barrier placed?
I am in the process of designing a car wash that will use passive solar heating in the self-service bays. In the article "Choices in Underground INsulation," Solar Age, 3/83, you discussed below-grade insulation and waterproofing and recommended the use of extruded polystyrene.
What insulation thickness should I use under a 6-inch concrete slab that has SolaRoll™ auxiliary heating installed in the concrete?
What is the best arrangement and material to use as a vapor barrier under the floor slab?
I have received contradictory advice from solar dealers regarding the placement of the vapor barrier - above and below the sub-slab insulation have both been recommended. - J.W., Gettysburg PA
Our photo (above) shows a well-designed insulated radiant-heated concrete slab with an insulated foundation perimeter being installed in Two Harbors, Minnesota in 2007.
In a heated floor slab, heat losses into the ground and outward through the edges of the slab and foundation wall are quite significant. (See RADIANT HEAT Floor Mistakes to Avoid). Designers generally insulate both directly under the slab and around the perimeter as well. In cold climates it is advisable to use a minimum of 2 inches of extruded polystyrene under the slab.
Many increase the under-slab insulating foam board thickness to 3 or 4 inches at the outer 2 to 4 feet of the slab.
On the perimeter foundation wall, 2 inches of solid foam insulation are commonly used, again increasing the thickness toward the ground's surface. Our photo (left) shows insulation placed at the perimeter of a heated garage floor slab in Minneapolis, MN in 2007.
In any case, careful attention should be paid to insulating the slab edge, where heat loss is greatest. Details that thermally isolate the slab edge from the foundation wall and outdoors appear to work the best.
For a vapor barrier below a concrete floor slab, most choose 6- to 10-mil polyethylene. because extruded polystyrene insulating board will absorb little moisture, the vapor barrier can be placed above or below the slab insulation. Some builders prefer to place the vapor barrier below the insulation or even below the gravel bed because they find it easier to work on preparing the slab itself without destroying the vapor barrier membrane.
If you want to pour the concrete directly onto the plastic vapor barrier, you might protect it first with a layer of heavy felt building paper. If migration or moisture into the building is not a concern, as we suspect not in a car wash bay, you could eliminate the vapor barrier altogether.
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. Also see these radiant heat flooring articles:
RADIANT HEAT - usage guide, strategies for using radiant heat
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.
"Insulating a Slab" - links to the original article in PDF form immediately below are preceded by an expanded/updated online version of this article
Frequently Asked Questions (FAQs) about insulation beneath poured concrete floors & slabs: insulation choices, R-values, installation, effectiveness
Questions & answers or comments about about how to best insulate under a concrete slab where radiant or solar radiant heat will be installed.
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.