Passive Solar Heating Systems – ideas

As I’ve posted previously:

A direct gain system is one where the main means of thermal gain is through direct heating of the thermal mass by solar radiation entering the building through windows and being absorbed by the thermal mass. An indirect gain system is one where solar radiation is captured and stored in a component of the building that has a high thermal mass and from there released to areas within the building that require heating.

That’s all well and good, but what does this mean in practical terms, how do we design our buildings to maximise solar heating? Here are a few tips:

Direct Passive Solar Heating

  • You need plenty of double or triple glazed energy efficient glass that will let the heat in and keep it in. If possible plan for glazing the equivalent of up to 25% of the floor space. Windows should face as close to due south as possible, although anywhere within 30 degrees of this will still provide solar gain.
  • To allow a degree of control over the amount of sunlight entering the building all windows will need curtains, shutters or blinds that can be used to shut out excessive sun-light.
  • Don’t cover stone or concrete floors with carpets, they will inhibit solar energy from reaching and being absorbed by your thermal mass.
  • Dark coloured materials will absorb more thermal energy than light ones.
  • When considering building materials keep the word mass in mind – the more the merrier.
  • To shelter especially exposed elevations, consider planting deciduous trees that will bear leaves and create shade over the summer and help prevent overheating. In the winter the trees will shed the leaves and let what sunshine there is through.

Indirect Passive Solar Heating

Perhaps not well suited to our temperate UK climate were days on uninterrupted sunshine are few and far between, I find the inventiveness of indirect passive solar heating solutions very attractive. For example, the Trombe wall or window box.

To build a Trombe wall. (also known as a solar window), take a south (or north in the southern hemisphere) facing 8 inch or so thick masonry wall, paint it black on the outside then glaze leaving an inch wide gap between the wall and the glass. Sunlight will hit the wall, be absorbed into and radiate through the wall to heat the interior. Heat travels through a masonry wall at about an inch an hour so as the interior cools in the late afternoon, heat that was absorbed by the wall earlier in the day will radiate into and warm the room beyond the wall.

A variation on this is a Window box passive solar heating system. For one of these, build a box beneath a window, glaze it and fill it with stones. Sunlight will heat the stones, heated air will rise up in the box and pass into the building through a vent that passes from the top of the box, through the wall underneath the window and into the interior of the building to be heated. A door over the vent can be used to regulate the interior heating.

Isolated Gain Passive Solar Heating

Not really one for the UK but in sunnier locals sun-spaces (also known as a solar room or solarium) are areas of a home, usually central to the layout, with lots of vertical glazing, minimal horizontal glazing and plenty of thermal mass. Doors into the rooms that lead off the sun-space are used to regulate the passage of heat. Read more at

For anyone interested in indirect passive solar heating solutions then I thoroughly recommend, The Barefoot Architect which has plenty of suggestions accompanied by some (occasionally indistinct, but nevertheless useful) pictures.






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