This is a very practical question; the underlying principles are, I imagine, extremely well-known.

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.)

  • $\begingroup$ We put black tiles on the floor - absorb heat nicely, floor has insulation under so heat is not lost. Also walls can be made to absorb heat, avoid white or reflective surfaces. Easy to work out where the sun will reach to based on architect's drawings. In summer we designed it so the sun does not come in through the window. If you don't let it in then you don't have to work out how to remove it. Simples... $\endgroup$
    – Solar Mike
    May 8, 2022 at 11:42
  • $\begingroup$ Right, that's the general idea. My simulation was based on the architect's drawings (and basic information from Google Maps), together with standard routines for computing azimuth and altitude. Will try to get the coop's authorization for awnings. But see my precise questions above. I suppose I should use dark wood for cabinets that will be exposed to sunlight in winter but not in summer? (I was going to install sone shelving anyhow.) Is one kind of wood better than another? How do you cover your black tile floor during summer (if it gets direct light then)? $\endgroup$ May 8, 2022 at 12:31
  • $\begingroup$ Don’t need to cover the black floor tiles as I said we designed to avoid the sunlight getting in. Said it clearly: « if you don’t let it in then you don’t have to work out how to remove it » . $\endgroup$
    – Solar Mike
    May 8, 2022 at 12:47
  • $\begingroup$ Right. I'll work on that as well, but I don't yet know whether the coop will allow awnings, and at any rate the simulation clearly shows that, while awnings will protect me well around the summer solstice, I'll still get quite a bit of light on the floors in August (the hottest month), unless of course I put the shutters down and sit in the dark. So, I need to think on different fronts. $\endgroup$ May 8, 2022 at 12:56
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    $\begingroup$ You want to store in a way that maximizes the subsequent release of heat into the air inside the house, as opposed to ,e.g., the interior of the walls. Stone walls, fireplaces, etc. do this well. $\endgroup$ May 9, 2022 at 13:10

1 Answer 1


You are looking at the wrong end of the problem. As far as heat collection is concerned, all that matters is the choice of window arrangement and the glazing coatings. Once the sunshine is inside, it doesn't matter what the surfaces are - it is trapped. It isn't getting back out without turning to heat.

The areas of interest are loss minimization and thermal storage (these are the same thing, really), and deploying the stored heat to best advantage in concert with conventional central heat.

Loss minimization and thermal storage require more insulation around the storage volume, so a compact box with high volumetric thermal storage is desirable.

Making best use of the stored heat to assist in providing comfort and a pleasant indoor environment is much more complicated. This is what drives the design of solar-sourced storage systems. The collection system tends to be whatever succeeds in terms of how the storage and distribution is engineered. It usually piggybacks off much of the hardware needed for heat distribution. I usually compare such designs to a solar panel heating a water tank with an electric element. When you compare cost/benefit, footprint, and flexibility, you'll find it quite difficult to make design changes that improve marginal performance more economically than a solar panel or hydronic collector.

So begin by establishing the design of the furnace, ducting and zone controls of the central heating system. Consider point radiant heat and humidity management as cheapish comfort boosters that can save a lot on furnace usage. Optimize the building's insulation based on heating and cooling costs. Then see how solar-sourced supplementary heat can be integrated into the heat distribution system of the conventional central heating system. Then, lastly, figure out what the solar heat collection system needs to look like. Keep a keen eye on the life expectancies and costs of the options.

Trying to design the house to be solar efficient, and then fitting a central heating system to this house is just plain backwards and will only increase the cost of the house more than it should.

  • $\begingroup$ Well, technically, some sunshine will get back out, reflected back from the inside through the windows. Is that negligible? $\endgroup$ May 8, 2022 at 17:12
  • $\begingroup$ I should make clear that this is an old place that I am renovating. We will probably keep the existing radiator-based heating system, replacing the gas heater by a heat pump. What kind of point radiant heat do you have in mind? The walls are not currently insulated, and we are thinking of adding 20cm of insulation (10cm in some places). The architect tells me that hemp-based insulation regulates humidity well - would that be enough? We are also thinking of adding solar panels on the roof. $\endgroup$ May 8, 2022 at 17:18
  • $\begingroup$ If I understand correctly, then, as far as comfort during the summer is concerned, the best solution could be a brise-soleil, which would block light from a high-altitude angle to enter the building to begin with. As far as the light that does get in is concerned, the only hope is for the energy from the sun to be stored and released as heat slowly, rather than be released as heat right away; reflective surfaces just pass the buck to other surfaces - right? Conversely, in winter, once the sun gets in, there would be not much point in trying to trap it, as it is trapped no matter what, correct? $\endgroup$ May 8, 2022 at 17:22

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