Posts Tagged ‘effusivity’

Thermal Mass

April 15th, 2009 by

In our climate, even on the hottest days of summer, the outdoor nighttime temperature drops below the indoor temperature. Using massive materials inside the insulated envelope, we can take advantage of that diurnal temperature swing to reduce the amplitude of the indoor temperature swings. The mass absorbs heat during the day and radiates it back at night.  If we do a good job of keeping that mass shaded during the summer there’s no need for mechanical cooling and the dog has a nice cool floor to lie on.

Thermal Inertia
Talking about thermal mass in more detail gets a little more complicated. I’m afraid we’re going to need a few definitions:

  • Heat capacity is the ability of a material to absorb heat.
  • Diffusivity is a measure of the speed heat moves thru a material.
  • Effusivity describes the ability of a material to exchange heat with it’s surroundings. It is similar to emissivity (as in low-e or low emissivity windows).

Good materials for thermal storage have high thermal inertia.  They have a high heat capacity, but low diffusivity and effusivity.  Metals don’t work well for thermal storage.  They can take on a lot of heat because they have a high heat capacity, but they can’t store it very long because they also have high diffusivity and effusivity.  Metal heats up quickly, but it gives it right back. Clay is much better for storing heat in a timeframe that’s useful for conditioning houses.  It has a high heat capacity, but low diffusivity and effusivity. It’ll take all day to heat up a cold earthen floor sitting in a room with a warm air temperature, but it will take all night for it to radiate that heat back. That’s what we’re looking for.


Even when the two items are identical in temperature, the metal feels colder. Why? Wood is not a good conductor of heat, so it is slow to absorb the heat from your hand. Metal has higher thermal effusivity, so the heat from your hand flows into the metal quickly – creating the sensation of it feeling cold.

Mathis Instruments

In our climate massive construction is awesome in the summer. The downside is that dense materials like tile, concrete, and compressed earth block also feel cool to the touch during the winter. That’s why European stone castle walls are covered with tapestries.

Mean Radiant Temperature
To derive your Mean Radiant Temperature, look around you and take the temperature of every surface you see. You are exchanging heat with all of those surfaces. Surfaces warmer than you radiate heat to you and all the other colder surfaces. You’re just another room surface exchanging heat with all the others. To be comfortable all the surfaces around you need to be within a few degrees of each other (and you), and in a well insulated house with good windows they will be. However, believe it or not, our skin does not have good temperature sensors. Instead, we have excellent heat flux sensors. All of the surfaces in a room can be exactly the same temperature, and some will still feel colder than others when we touch them. The surfaces that feel colder are the ones with higher effusivity. The castle tapestries have low effusivity so they feel warmer than high effusivity stone.

Radiant Heating
In a typical (minimally insulated and drafty) house, radiant floor heating feels great because the mass is heated up to about 80 deg F. The floor radiates heat up to other surfaces, and brings the mean radiant temperature up so we’re nice and toasty. The problems are:

  1. radiating 80 deg F from the entire floor is a lot of heat.  A house that needs that much heat is wasting a lot of energy, and it should be insulated better.
  2. you lose a lot of ability for a slab to absorb free heat coming in the windows from the sun if the mass has already been heated by radiant tubing.

In an efficient well sealed house, a conventional concrete radiant floor heating slab won’t have to rise above about 73 deg F to meet the heating load (assuming the entire floor is heated). You will wonder if the heat is really on because it won’t feel warm. Even though the floor slab is adding heat to the house and the mean radiant temperature is high enough that we aren’t radiating much heat to the other surfaces, concrete has a relatively high effusivity. It exchanges heat with us pretty easily and feels cool even with a slight temperature difference. In a passive solar house, high effusivity materials located in areas that get direct solar gain will feel tactically warm on sunny days, but those same materials in northern rooms without solar exposure or in southern rooms on cloudy days won’t.

If you use radiant heat, insulate the house well enough that a small area of radiant will heat the entire house. Locate it in northern rooms (especially bathrooms) that can’t be heated by the sun.

Concentrate high heat capacity materials in the south rooms where they will do the most good. Use low-medium effusivity materials to store heat. Assuming no radiant heat, a north bath or kitchen would be better off with low effusivity wood or cork floors and wood countertops, but the same room located on the south would benefit from medium effusivity concrete countertops and tile floors.  Likewise, soft earthen plasters will feel warmer than hard venetian lime plasters, and soft lime and gypsum plasters will feel warmer than harder cement based plasters.

This spreadsheet shows the effusivity and interface temperature (how warm the surface feels when you touch it) for a few typical materials. I assumed an 85 degree hand surface temperature and all other surfaces at 70 degrees, but you can go to the spreadsheet and change those values as you see fit: Google Docs | Thermal Effusivity.