Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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ndoor Environmental Quality: particulate matter


Floor Coverings: thermal and thermo-optical properties (see also the radiant design guide).

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Be sure to visit our comprehensive bibliography on radiant cooling and heating.


Let there be no mistake, when using interior surface finishes as part of an on-site fabricated radiant heat exchanger, one needs to understand the thermal and thermo-optical properties of the materials specified by the interior design profession.


Because interior finishes affect thermal comfort, the efficiency of the radiant based HVAC system, combustion and compression efficiency, temperature sensation of floors and indoor air quality.

Obtain your copy of the Radiant Flooring Guide



Conductivity: heat flow per unit area per unit thickness per unit temperature

In IP units the thickness is given as per inch or per foot - some manuals indicate in the denominator ft2 as the cross sectional area of the specimen. In SI units the thickness is per meter - some manuals indicate in the denominator m2 as the cross sectional area of the specimen.

k = Conductivity, Btu·in/h·ft2·°F (W/m·K)

Conductance: heat flow per unit area and temperature

C = Conductance = k/L = Btu/h·ft2·°F ((W/m2·K)


L = Thickness, inches, in. (meters, m)

Note: L is typically shown as l or d, but we use L here so as not to confuse the letter l with the number 1 (one)

Resistance: resistance to heat flow of a material or layer or composite assembly measured from surface to surface

R = Resistance = 1/C = h·ft2·°F/Btu (RSI = m2·K/W), RSI to R =  RSI / 0.1761 = R

* re: bare concrete, this is a fictitious resistance - in reality if there is no covering it should be zero but there is no zero value on radiant design nomographs.


  1. 2008 ASHRAE Handbook—HVAC Systems and Equipment, Panel Heating and Cooling, Chapter 6

  2. 2009 ASHRAE Handbook—Fundamentals, Physical Properties of Materials, Chapter 33

  3. 2009 ASHRAE Handbook—Fundamentals, Heat, Air, and Moisture Control in Building Assemblies, Material Properties, Chapter 26

  4. 2010 Radiant Flooring Guide, BNP Media for the Radiant Professional Association (IAPMO)

  5. Straube, J., Heat Flow Basics, Arch264, University of Waterloo, 2003

  6. Physical Properties and Moisture Relations of Wood, Forest Products Laboratory. 1999. Wood handbook—Wood as an engineering material

  7. Thermal Insulating Properties, Dow Chemical, Technical Publication, 2006

Virtually all flooring association publish documents, guidance and advice relating to radiant heating systems

Carpet Rug

Tile Council of North America

American Harwood Information Center

World Floor Covering Association

Ever hear radiant floor heating ruins hardwood floors? This is by far the longest running myth in the industry - consider this...100% of all hardwood flooring problem in homes conditioned exclusively with forced air did not have radiant floor heating to blame. Learn more about heated hardwood floors.

Discussion on flooring

Flooring is the most misunderstood and underestimated element in designing in-floor cooling and heating systems. Consider that flooring characteristics not only effect indoor environmental quality but also heat transfer via conductivity, resistivity, emissivity and absorptivity. For example, floors that are less conductive can be operated at a lower surface temperatures because they are less effective at drawing heat out and away from the foot. But being less conductive is also being more resistive and so even though they could operate at a lower temperature, all things being equal the fluid in the system must also operate hotter in heating or colder in cooling which destroys heating and cooling plant efficiency. You can experience the effects of conductivity and thermal sensation when holding a steel rod in one hand and a wooden dowel in the other. Both are at the same room temperature but the steel will feel cooler due to its conductive properties.

Flooring also plays a major role in indoor air quality due to the potential for volatile organic compound (VOC's) emissions. Highly conductive masonry type floors such as tile, slate, concrete and terrazzo enable low fluid temperatures in heating and high fluid temperatures in cooling which promotes boiler, chiller and heat pump efficiency; and they have low very VOC emissions which is good for indoor air quality. But to the average person, unless heated, low VOC conductive masonry floors tend to feel cooler as in the steel rod and wooden dowel experiment.

Additionally, floors used for heating all have very similar emissivities (>0.85) which means they all make good radiators (see below), however floors used for cooling don't always have similar absorptivities which is important for absorbing radiant energy. So in addition to looking at emissivity, resistivity, and conductivity we also need to consider in cooling, color and absorptivity which is an thermo-optical property of the floor.

Now you know why I have been saying for years that HVAC can not operate in isolation from the interior design professional.

If you want to use floors for both heating and cooling to enable high boiler/chiller/heat pump efficiencies and promote good indoor air quality then pick a flooring that has both good conductivity, emissivity, good absorptivity and low VOC's emissions.

For those who wish to study the thermo-optical properties of materials which relate to radiant heating and cooling visit the Hyper Physics site operated by Georgia State University, Department of Physics and Astronomy.

Emissivity, absorptivity and reflectivity

Emissivity is a measure of a material’s radiating efficiency.  An emissivity of 1.00 implies that the material is 100% efficient at radiating energy - aka the perfect black body.  An emissivity of 0.20 implies that the material radiates only 20% of that which it is capable of radiating. Virtually all flooring, wall and ceiling finishes make very good radiators as they have very high emissivities. It helps some people to think of emissivity as water surface tension where rough surfaces have less tension in comparison to smooth surfaces. Incidentally the emissivity of the human skin is about 0.98 making the human body an ideal emitter and absorber of radiant energy. Note: all values below are approximate but close enough for general assessment.

There is generally an inverse relationship between emissivity and reflectivity where a material having a low emissivity will have a high reflectivity such as mirrors and polished metals or low-E coatings. But this is not a pure relationship and the values of each material should be checked against industry handbooks.

The thermal absorptance represents the fraction of incident radiation that is absorbed by the material or is the proportion of radiation absorbed vs reflected at each wavelength.  An absorptivity of 1.00 implies that the material is 100% efficient at absorbing radiant energy.  An absorptivity of 0.20 implies that the material absorbs only 20% of that which it is capable of absorbing with the balance being reflected.

There is a corresponding relationship between emissivity and absorptivity in long wave radiation typical of room
temperatures where materials having high emissivities also have high absorptivities; but it's not a perfect
relationship because unlike long wave radiant emissions in heating where colour is irrelevant, color is very
important in absorptivity of short wave (solar) radiation for radiant cooling systems, i.e. darker colours will have
higher absorptivity than lighter colors.

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Radiation: Incident ray (electromagnetic energy, shortwave - shown as solar gain) is delivered to a surfaces where it is converted to heat; transmitted through as back losses or gains to floor below; and reflected or re-radiated (emitted ) as long wave electromagnetic energy back into the space.


Without removing the solar heat gain in the slab, a function of its absorptivity, the mean radiant temperature will rise leading to thermal discomfort. By using high absorptivity flooring with radiant cooling, the back losses or gains to floor below are reduced as is the re-radiated (emitted) into the space which keeps the mean radiant temperature lower in summer months.


Reflectance and absorptance calculator developed by Dr. A. Marsh: This still shot of a flash file is a useful educational tool for illustrating the influence of color on absorptance of electromagnetic energy. Generally speaking the darker the color the higher the absorptance which is important when deciding on flooring for radiant cooling systems; likewise the use of lighter colors for reflecting solar energy on exterior surfaces.

Message: When it comes to heating and cooling - color matters to the indoor climate engineer and why it's necessary to work with the interior and exterior design professions.

Contact coefficient


One method of describing the influence of flooring on bare feet is by using the contact coefficient (b) which integrates flooring characteristics into a numerical value written as;


       b = √k ·р· c


       k = conductivity

       p =density

       c = specific heat


Note how the higher the contact coefficient of the floor the more effective it will be at drawing heat out of the feet. Likewise the higher the contact coefficient the lower the fluid temperature required for heating and cooling all other elements being equal.

Contact coefficient for various floor coverings based on conductivity, density and specific heat


Contact coefficient, b ( kCal/m2 hr0.5 °C)







Oak wood


Pine wood




source: Fanger, P.O., Thermal Comfort: Analysis and Applications in Environmental Engineering, McGraw-Hill Book Company, 1970


Using the terminology

Function, process, act or action within, from, on or through the specimen

A material property of the specimen in general

Specific ability of the insitu specimen

conduct / conduction / conducted



resist / resisted



absorb / absorption



reflect / reflection / reflected



emit / emission / emitted



transmit / transmission / transmitted



permeate / permeable



adsorb / adsorption




  1. Rigg, J.C., Visser, B.F., Lehmann, H.P., Nomenclature of Derived Quantities, Pure & Appl. Chem., Vol. 63, No. 9, pp. 1307-131 1, 1991.

  2. Martinez, I., Heat Transfer and Thermal Radiation Modelling, Departamento de Motopropulsión y Termofluidodinámica de la Escuela Técnica Superior de Ingenieros Aeronáuticos de la Universidad Politécnica de Madrid

  3. Martinez, I., Thermo-optical properties, Departamento de Motopropulsión y Termofluidodinámica de la Escuela Técnica Superior de Ingenieros Aeronáuticos de la Universidad Politécnica de MadridBreuch,

  4. Breuch, R., Handbook of optical properties for thermal control surfaces, volume III Final report, NASA-CR-87484, LMSC-A847882, Jun 25, 1967

  5. Pedersen, C.O., Fishers, D., Lindstrom, P.C., Impact of surface characteristics on radiant panel output, ASHRAE RP-876, 1996

2014 Radiant Flooring Guide

The Radiant Flooring Guide is the intellectual property of the Radiant Professionals Alliance, Published by BNP Media

Order hardcopy or view online version

2014 Radiant Flooring Guide

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