(Also asked in Engineering Stack Exchange)

I'm buying a new place in the northern hemisphere (49° N) with lots of southern (or rather SES) exposure. I'm putting quite a bit of thought (together with an architect) into how to improve its energy performance. I'm particularly interested in passive heating and cooling. I have actually just written a little simulation that lets me see where direct sunlight will fall at a particular date and time: enter image description here

What kind of material do I want on e.g. cabinet doors to absorb direct sunlight during winter days and then give it back as heat in the course of the day or evening? (The doors would be where the sun simply doesn't fall during summer months.) Does a waxed hardwood floor absorb more sunlight than a vitrified one? What should one do during the summer if one doesn't want to just pull the shutters all the way down - use light-colored rugs? (Of what material?)

Climate zone Cfb. Heat waves during the summer are a thing; AC is not. It can be rather cloudy during the winter, so it's unclear to me that I will be able to get much passive heating, but every bit helps.

  • Water is commonly used as a heat sink. It can be passive ( storage tank) , or active with water being pumped between various containers. I have a poor multifunction system with 135 gallons of aquariums in my shed that heat in the day and cool at night. May 8, 2022 at 19:55
  • Asking now at sustainability.stackexchange.com/questions/12167/… May 10, 2022 at 23:23

2 Answers 2


The answer is very clear: since large enough quantities of phase change material with a melting point near the indoor temperatures are expensive, you want to use water. Water isn't a phase change material near indoor temperatures (the phase change is zero degrees Celsius), but water stores a lot of energy per one degree of temperature change.

Water stores 4190 joules per degree Celsius per kilogram, and is cheap as dirt. You won't find many other materials where a degree of temperature change would store as much energy.

I use water for a similar purpose: I want my refrigerators to withstand a long power cut. So I have put large containers of water in the refrigerators.

Water also has the benefit that it's a liquid so it's easy to transport. So you can have a huge container of it, and then pump it through heat exchangers using pipes to actually distribute the heat to indoor air, and you can also construct large solar collectors where water is flowing and collecting heat, to be stored in the huge container.

  • Wait - I was asking what material I can use for wall coverings, bookcases and cabinet walls. May 9, 2022 at 17:37
  • ... that said, I am considering putting solar panelling on the roof. I take your answer implies that I should look into systems that use solar energy captured on the roof to heat water (I have water-based radiators, which will be heated by an air-to-water heatpump otherwise) in the winter, while presumably converting it into energy in the summer. Do such systems exist (for private houses)? May 9, 2022 at 17:42
  • Wall coverings, bookcases and cabinet walls are not effective in storing heat. You really need a water-based system if you want to store significant amounts of heat.
    – juhist
    May 10, 2022 at 17:02
  • I'm not trying to heat my house with cabinet walls. I am trying to see: given that I must have bookcases, walls and cabinets, and that there is a wall that gets direct sunlight (through the windows) during the winter, is there something I can do with said bookcases, walls and cabinets to gain 1 or 2 C, say, at least in the area immediately adjacent to them. May 10, 2022 at 18:22
  • That said, I do have a roof, and of course I am considering using it to heat water (or produce electricity). You are right that that should help much more, but it is complementary to my other question. Or is there some other possibility I am not understanding? May 10, 2022 at 18:23

I would have insisted on Kinetic Step panels. But that might be expensive, even the installation to routing the whole house. But if DIY isn't problem perhaps. Someone could re-engineering the Kinetic Floor Tiles concept into an affordable one

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