InspectAPedia ^®

Concrete Slab Insulation for (Solar) Radiant Heat InspectAPedia®  -     What insulation thickness is needed under a passive-solar heated slab floor? Where should floor slab insulation be placed to avoid heat loss? Where does the vapor barrier go under an insulated, heated floor slab? Heated floors for car wash bays Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices

This article discusses the proper insulation amount and proper vapor barrier location below a passive solar heated concrete slab floor. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Readers constructing an insulated slab with radiant floor heating, whether by passive solar or any other means, 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 BASEMENTS. 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.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Sub Slab Insulation Amount, Location, & Vapor Barrier Placement for Heated Slabs

"Insulating a Slab" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article

• Insulation Specifications for Passive Solar Concrete Floor Slabs - PDF form, use your browser's back button to return to this page

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, (see links just above) 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.

Answer:

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.

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.

Share this Article      

...

Technical Reviewers & References

Search InspectAPedia

• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• InspectAPedia Bookstore lists recommended books, organized by topic & available for purchase. Most of our articles also include a list of recommended books for the specific article topic as well as other references, and information sources.
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

INSULATION & VENTILATION INSPECTION & IMPROVEMENT
  INSULATION R-Values & Properties
  Air Bypass Leaks, Thermal Tracking
  AIR LEAK DETECTION TOOLS
  AIR LEAK MINIMIZATION
  AIR SEAL STRATEGIES
  ATTIC CONDENSATION CAUSE & CURE
  BASEMENT HEAT LOSS
  Blocked Soffit Intake Vents
  BLOWER DOORS & AIR INFILTRATION
  BRICK VENEER WALL INSULATION
  CATHEDRAL CEILING INSULATION
  ENERGY SAVINGS in BUILDINGS
  ENERGY SAVINGS RETROFIT CASE STUDY
  ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE
  ENERGY SAVINGS RETROFIT OPTIONS
  FRAMING DETAILS for BETTER INSULATION
  FRAMING DETAILS for DOUBLE WALL HOUSES
  Inspect Attics for Moisture or Mold
  Inspect Basements for Moisture or Mold
  Inspect Building Exterior
  Inspect the Ridge Vent System from the Attic
  Inspect the Soffit Vent System from the Attic
  Insulation Air & Heat Leaks
  INSULATION for GREENHOUSE or SOLARIUM
  INSULATION CHOICES
  INSULATION FACT SHEET- DOE
  INSULATION LOCATION for BASEMENTS
  INSULATION LOCATION for CAPES, CRAWLSPACES
  INSULATION R-Values & Properties
  Insulation Values of Log Home Walls
  POLYISOCYANURATE FOAM INSULATION
  POLYSTYRENE FOAM INSULATION
  RADIANT BARRIERS
  RIGID FOAM USE INDOORS
  ROOF VENTING ENERGY SAVING DETAILS
  ROOF VENTING NEEDED?
  SLAB INSULATION, PASSIVE SOLAR
  Urea Formaldehyde Foam Insulation UFFI
  VAPOR BARRIERS & AIR SEALING at BAND JOISTS
  VAPOR BARRIERS & CONDENSATION in BUILDINGS
  VAPOR BARRIERS & HOUSEWRAP
  VAPOR CONDENSATION & BUILDING SHEATHING
  Vermiculite Insulation

RADIANT BARRIERS
RADIANT HEAT
RADIANT HEAT Floor Mistakes to Avoid
RADIANT SLAB FLOORING CHOICES
RADIANT SLAB TUBING & FLUID CHOICES

SOLAR ENERGY SYSTEMS
  PASSIVE SOLAR DESIGN METHOD
  PASSIVE SOLAR HEAT PERFORMANCE
  PHOTOVOLTAIC POWER SYSTEMS
  SLAB INSULATION, PASSIVE SOLAR
  SOLAR COLLECTOR AIR or GAS COLLECTION
  SOLAR COLLECTOR FILMS
  SOLAR COLLECTOR WOOD HOUSINGS
  SOLAR HEATING SYSTEMS
  SOLAR HOT WATER HEATERS
  SOLAR HOUSE EVALUATION

• 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.
• Portland Cement Association: www.concretethinker.com/Papers.aspx?DocId=8 indicates that
  - tubing for radiant heat in a concrete slab is installed UP TO two inches below the surface of the slab
  - the slab is insulated from the ground at all sides to direct heat upwards to the living space [this is our preferred design for a cold northern climate]
• The Radiant Panel Association: www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=1 offers design guidelines at http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=115 including these insulation R-value and coverage details:
  Application#, Minimum R-Value, and Insulation Coverage
  The following insulation alternatives are given for Slab on Grade construction:
  Alternate #1 [(Ti-To)x0.125)=R-value, with coverage from perimeter to below frost line ["Ti-To" means we calculate the necessary R-value as (Ratio of indoor to outdoor temperature) x 0.125]
  Alternate #2 R-value=5, with coverage 4' horizontal or vertical at perimeter
  Alternate #3 R-value=5, with coverage under entire slab and slab edge [this is our preferred design for a cold northern climate]
  The Radiant Panel Association offers education and publications in radiant heat design. see radiantpanelassociation.org
• Takagi radiant heat systems: Takagi offers pre-assembled radiant heating system installation packages including for do-it-yourself'ers, and including systems that combine radiant heat flooring with domestic hot water production using a gas-fired tankless water heater. See takagi.com for more information. "The T-KJr model (gas inputs up to 140,000 BTU per hour) is the smallest unit in the Takagi line-up. The T-KJr is perfect for light residential (i.e. small apartment units) and radiant heating applications." Also see Tankless Water Heaters.
• 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

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• ...

More Information on Building Diagnostic Inspections and Repairs

...

InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

Building Inspection, Problem Diagnosis, Forensic Investigation & Testing, Repair Consulting

Contact Daniel Friedman for website content suggestions or for fee-paid consulting