Background

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.

Why?

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

Terminology

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)

where,

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 wall 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.

Sources:

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

Carpet Rug
Institute

Tile Council of North America

American Harwood Information Center

World Floor Covering Association

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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. 
 

Click to enlarge

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.

       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.

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