Movement of Heat

The heat energy of the sun is both harnessed and controlled within passive solar design

Many different elements of passive solar design work together.  Passive solar design harnesses and utilizes the heat energy of the sun.  The building is designed and constructed in a way that allows it to capture or absorb the sun’s heat energy within the building when it is desired to be warm.

The materials that are chosen and used in a passive solar building should be chosen for the object’s inherent ability to conduct and radiate heat.  Many passive solar houses also use natural convective movements within the air or water to better control the flow of heat.  As such, it is important to understand how heat moves.

Heat is in constant motion.

Heat moves to constantly try to reach equilibrium and will constantly move from warm to colder areas. When the door is opened to a cold winter evening, heat will move out of the house while the cold sweeps in. Similarly, when reaching for the ice cream and the freezer door is open, heat moves in, as cold air rushes out. In regard to both the building and the freezer, the presence of insulation functions to keep areas warm or cool areas stable.  In buildings, it is desired to live within a constant, comfortable temperature, regardless of the temperature outside of the building.


Heat Movement, Courtesy ORNL

The economical solution to a warmer house in the winter and a cooler house in the summer is to insulate it well, and have a basic understanding of the movement of heat.

Heat moves from warmer to colder areas through these main types of movement.


Conduction takes place when heat energy travels from molecule to molecule

Conduction – Conduction is the movement of heat as it travels through solid objects.   Heat energy travels as it is transferred from molecule to molecule. The more dense an object, the faster heat moves. Conduction is illustrated in the process that takes place when heat travels from an electrical burner, through a pan, to heat the water inside of the pan.

In passive solar design, heat is absorbed in the building’s thermal mass.  The object with thermal mass is usually a dense material like a cement, tile, stone or water. Objects with this ability to conduct heat are usually located in the floor or walls of the building. In the winter, the sun’s is heat energy is absorbed, then spreads through the thermal mass by conduction.

Convection – Convection is the movement of heat circulating through an air (gas) or liquid. Warm molecules rise. Convection is illustrated within a building as warm air rises, the second story of the building will often be warmer than the basement, located at the bottom of a building. Likewise, in a liquid, warmer water will circulate toward the surface. Some passive solar buildings use strategically placed windows or fans to circulate warm and cool currents of air throughout the house.

Radiation – The sun’s radiant heat energy moves through space as an electromagnetic wave. When it strikes a large object like the Earth, the radiant heat is absorbed in the mass of the large object. Although the space that radiant heat travels through can be quite cold, the heat is not absorbed until it hits a dense object. A microwave operates on a similar principle: The contents in the middle of the microwave absorbs the radiant heat and gets hot, while the surrounding air in an of itself does not. When something in the microwave is hot, the heat is coming from the heated object, not from the air.

After the heat is absorbed, with will release the heat energy back into the surrounding environment.  Food that has been heated in the microwave is radiating the heat that it previously absorbed.  The cement floor that has been hit by sunlight all day will radiate the heat that it absorbed.  The difference between the food and the cement floor is the density of the materials.  The food with quickly get cold, while the floor, because of its mass and density, will continue to release its stored heat for hours.  Passive solar design relies on the use of materials with thermal mass within the building that can radiate heat.  It also relies on materials that allow the heat energy to be absorbed.


The sun room absorbs the heat energy, from there, it can radiate out to warm other parts of the house

Glass is a material that allows the radiant heat energy to enter the house and be stored in a dense object with thermal mass.  In the picture to the right, the sun room has a concrete floor (thermal mass) that absorbs the heat energy from the sun.  The sun room is recessed into the house and has openings that allow the heat to radiate to other parts of the house.

In the winter, warmth is generally desired.  When materials with thermal mass are well placed so that the sun’s energy and heat can shine on them, they will absorb the sun’s heat and act much like a large rechargeable (heat) battery. The heat stored up through the day is released into the building to warm it through the evening and night.

Because of the planned placement and design of the building to its surrounding environmental elements, a well designed passive solar house will be warm in the winter and cool in the summer. If a house, regardless of its aesthetic design, is not well insulated and allows heat to escape through various cracks in walls, windows, doors, or other vents, the house will reach an equilibrium with the temperature that exists outside of the house much more quickly.


14 Responses

  1. Chelsea says:

    The molecules vibrate a little since the glass is a solid. The molecules absored the heat. And some of the energy they absored are puy back into the environment.

    Mrs. Jones Science Class 1st period

  2. Chase says:

    They build more heat.

  3. Lillie Cantin says:

    when the sun heats the molecules up they slightly vibrate because the the glass is a solid and it does not melt easily so they don’t move like they would if it was a liquid. when the heat is absorbed into the molecules they”consume” the energy and they let some off into the enviroment.

  4. Matthew Burkhart says:

    Mrs. Jones, when the sun heats the molecules up they will vibrate faster. The effect of the energy is, that the (warm) molecules will travel faster.

  5. Destiny says:

    The glass lets the warmth get into the house, so I am guessing that since warm molecules move more that the glass would be trying to move but since it is a solid it can’t. So because of this it might be doing something, but you wont see it happening. I am Destiny from Zebulon Middle school.

  6. dave says:

    glass is a kind of solid and in a solid , molecules are tightly joined together, since glass is a bad conductor, and heat is not any form of matter , the outside of the glass, i think , will be heated and the molecules on the inside will stay joined together.

  7. kathy irvin says:

    To the human eye you cant see it the molecules are vibrating .

  8. Lori Jones says:

    My question to my students is this. Based on this article and what we have learned in class about molecules. When the sun heats the molecules up, what are their reaction. What is the effect of the energy? Please explain after reading the article and making the connection.
    Mrs. Jones
    Zebulon Middle School

  9. Cindi says:

    I am constructing a small structure in the mountains of California. The structure is 18 ft by 20 ft and will have a cement floor and walls. I am planning a south wall (18 ft side) to have a 4 ft high cement wall and 4 ft of double paned polycarbonate “windows”. I also have plans to hang thermal roman blinds to cover this window bank at night. Also a secondary “trombe” wall will be placed about 5 ft away from this south facing wall, it is to be constructed out of glass blocks filled with a fluid to hold the heat. This trombe wall will run 9 ft so the sun will still penetrate the structure and hit the cement floor and walls on the other 9 ft side of the back wall.

    What I would like to ask you is this…. do you think that this structure will hold a constant temperature throughout the winter with this set up or if not – what modifications should I make?

    FYI: This will be a work shop and will have items stored in it that I wouldn’t want to be exposed to wide temperature swings. Also, even though it is in Southern California we get down to 0 to 10 degrees during the winter.

    Thank You for your time and consideration.


    • Keya Lea says:

      Hi Cindi,

      Let me think on this and I’ll get back to you when I have more brain power. (I am now a teacher and spent all day in the classroom.)

      In the meantime, here is an article about an earthship built on the top of a mountain to power a radio transmitter. If a building has enough thermal mass to capture the sun’s heat and can store it through the evening and night, the temperature can stay fairly constant. The passive solar structure houses sensitive electronic equipment and does not have an alternate source of heat, yet it works.

  10. Brian says:

    I am doing an owner builder project on my own house and want to get the suns warm into our house for winter, but people tell me that any heat gain during the day is lost at night when having big windows. Is this true.

    • Keya Lea says:


      When building a passive solar home, there are a few elements that will need to work together. When building with windows on the south side, make sure that are well-sealed, double or triple paned windows are used. A friend of mine built a really efficient passive solar home with windows on the south side, and used both thermal mass with tripled paned windows. If materials with thermal mass are used, the solar heat is absorbed throughout a sunny day, then slowly releases the heat through the evening. Pretty much all the homes in the residential section have windows on the south, materials with thermal mass near the windows, and are well insulated.

      So, to answer your question, if you have a combination of elements: well built windows, with thermal mass, within a well insulated home, then the heat from the sun will stay instide the home to heat it through the evening and night.

      Take a look at the Overview of Passive Solar to see the different elements working together.

      If you used only one of these elements, then it won’t work out so well. If you only had windows on the south side, without making the house well insulated, then the heat will be quickly lost – out the windows. For example, if you used crappy single paned windows, and used no other materials with thermal mass in a place within the home where the sun’s heat can be absorbed, for example, if you only had carpet on wood, then yes, the heat gained through the day would be lost fairly quickly.

      Hope that helps. Let me know if you have other questions.

  11. Joe Moe says:

    team solar,
    You did a very good job with your explanation…it did make sense. Would apprecciate any links you may have to purchase a set of plans for a 1800-2000 sq ft / 2 br / 2 bath / large great room.
    A Job well done…keep up the work,
    thanks, Joe

    • Keya Lea says:

      Thanks for your comment and kind words about this little mag. I find passive solar fascinating, so I write about them, but I do not have any specific plans for houses.

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