SLAB INSULATION, PASSIVE SOLAR - CONTENTS: 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
Sub Slab Insulation Amount, Location, & Vapor Barrier Placement for Heated Slabs
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.
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). 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.
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.
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
it's apparent that we are constructing a heat storage system whose heat input is from sun shining onto the surface of the earth box. So you would not want to insulate the top of the box, as doing so would prevent it from gaining solar energy.
In a different design, if for example you were pumping hot water from solar panels through a solar storage medium, then as the heat source is the heated water, not direct sunlight, in that case all sides of the earth box or "earthbox" would be insulated.
Please also see our article series on passive solar designs, beginning at
Athienitis, Andreas K., and Matheos Santamouris. Thermal analysis and design of passive solar buildings. Routledge, 2013.
Claridge, David E. "Design methods for earth-contact heat transfer." In Advances in solar energy, pp. 305-351. Springer US, 1988.
Geraldson, C. M. "The Earth Box: A Containerized Gradient Concept." HortScience 31, no. 4 (1996): 593-593.
Ozturk, H. Huseyin. "Comparison of energy and exergy efficiency for solar box and parabolic cookers." Journal of Energy Engineering 133, no. 1 (2007): 53-62.
Summit Soalr Systems, PO Box 2424, New London NH, 03257, USA, Website: http://www.summitsolarsystems.com/
Excerpt on the company's article on solar system design, where earth box construction is discussed
Once the foundation is completed we can begin to build our "Earth Box". The earth box is a system of its own. It needs to be 16" ~ 24" deep, Insulated at the bottom, and the perimeter with R10 material or better. The best application would be to use the 2" rigid foam board on the bottom, and on the perimeter. The reason I emphasize the perimeter is because we lose heat horizontally, so over insulating the perimeter is always a good thing, and never a waist of material. Once the Earth box insulating is completed, I recommend filling it in 50% full with the material you will be using for your earth box. Either flow fill, or sand. - op. cit. retrieved 2016/03/30
The article continues with adviced about placing the plumbing manifold and domestic zone tubing into the earth box, followed by pouring the slab.
Watch out: be sure to see RADIANT HEAT FLOOR MISTAKES - placing radiant heat tubing for a radiant-heated floor surface too deep into a slab will prevent good radiant floor heating operation. Be sure that your system design objectives are kept clear: storing heat in a passive solar media is not necessarily the same as using an in-floor or in-slab radiant heat system to warm a building's occupied space.
Thirugnanasambandam, Mirunalini, Selvarasan Iniyan, and Ranko Goic. "A review of solar thermal technologies." Renewable and sustainable energy reviews 14, no. 1 (2010): 312-322.
Tiwari, Gopal Nath. Solar energy: fundamentals, design, modelling and applications. Alpha Science Int'l Ltd., 2002.
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Questions & answers or comments about about how to best insulate under a concrete slab where radiant or solar radiant heat will be installed.
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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."
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
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
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
"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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