Of The Two Main Solar Passive Design Strategies DIRECT GAIN – Is the Most Popular.

By far the most common form of solar passive design uses a strategy of DIRECT GAIN

Direct solar gain is incredibly simple. Solar radiation enters a building, strikes a material of high thermal mass like stone, brick or concrete, is converted from shortwave radiant solar energy to heat. This heat is stored for a period of time  within the material of high thermal mass (depending upon it’s type and thickness) and then it is re-radiated into the adjoining airspace.

In ancient buildings this was done without the use of glass to contain the airspace inside the building and magnify the solar radiation so the effect f solar passive would have been limited. Nevertheless the heat that the sun could provide was valued and used in the time before cola, oil and gas as it was one of just two fuels available – wood and solar energy. In some cases like at Mezhirich, where dwellings of mammoth bones have been excavated dating from 24,000 BC, skins would have been drawn over the opening facing the sun at night to hold in the air warmed by the sun and by a central fire. In the case of the Mezhirich dwellings all of their openings faced South to the sun.

With the advent of glass, solar passive direct gain has been supercharged. A glass wall covering the opening pointing towards the sun magnifies solar radiation and also contains the air within a building resulting in a greenhouse effect. This is where the internal air gets continually warmed by the solar radiation absorbed and re-radiated by the material of high thermal mass.

Thermal mass: An interesting couple of things to note about the above illustration. Firstly, direct gain solar passive does not work without materials of high thermal mass. So, if one were to take a timber framed building and point it at the sun there would be little gain (excuse the pun) because there would be nothing to convert the solar radiation to heat and then store it. Wood does, to be fair, convert solar radiation to heat, but it is not nearly as effective as materials of high thermal mass. Oddly, the best material to use for converting and storing solar radiation is water which is much more effective even than stone, brick or concrete, though it has certain practical limitations.

It is a rather sobering thought that the reason the earth has not already turned into an arid desert as the result of global warming is because the oceans have absorbed most of the additional heat generated by the greenhouse effect carbon dioxide and other greenhouse gasses have created. It is the high thermal mass (the ability to receive and store heat) that water has that has saved us from frying on a global scale, and which makes water a very suitable receptor material for solar energy.

Cylinders of water in a house used to collect solar energy.

There are many differing opinions about the amount of thermal mass appropriate to any given solar passive application – as many opinions as there are pertaining to the amount of glass to use in the ‘solar wall’. 

Thermal mass balance: Thermal mass has a really interesting quality that affects its ability to heat and cool a building. High thermal mass in the sun acts to convert and store solar energy, but high thermal mass in shade acts to cool. This is probably best illustrated by your own experience. On a hot summer day you walk into an old church or cottage with thick stone walls. You immediately detect a drop in temperature – quite a steep one. The sun is excluded by the thick stone walls, yes, but the temperature inside is still much cooler than it is in the shade outside. So, to cut a long story short, if one has a theoretically unlimited amount of high thermal mass inside a building some of it is actin to heat the atmosphere inside (the part that is in the sun) and part is acting to cool the air inside. In daytime this is fine because we are active but at night the amount of cooling thermal mass may outweigh the warming amount of thermal mass and the expected passive heating may be missing because it is counterbalanced by the cooling thermal mass. And that is why some designers say one should not have too much thermal mass in a solar passive direct gain building. 

Outside-Insulation-Thermal mass-Inside: A second thing to note from the top diagram. In any eco building regardless of where the heat comes from the insulation should be OUTSIDE the thermal mass. Thermal mass is often used even when a building is not a solar building. This is on account of its ability to store heat, and provide passive cooling. If one does not have thermal mass then internal heat gradually dissipates through the structure because a timber structure does not hold/store heat. Even if the building is highly insulated but not solar passive thermal mass is used to hold heat within the building as well as helping to cool the building in hot weather. Here is a detail section through one of our structures showing the insulation outside the thermal mass of both the walls and the floor.

The above is a section of a garden office structure. The insulation thickness for a normal house would be at least double the thickness shown here.

Getting it wrong: This is in stark contrast to the way most UK builders construct domestic buildings. They still build houses with an outer skin of brickwork and an inner skin of timber frame. A number of years ago there was a major problem with buildings built of timber frame by Barratt Homes. Rot was occurring in the timber frame that was hidden from view by the masonry. You can imagine how hard it is to repair a building that is rotting from the inside out. This is the problem they made for themselves, in addition to flying in the face of common sense in terms of energy retention. The builders were  trying to make their buildings look more ‘traditional’ for marketing purposes. Housebuilders even construct fake chimney stacks and fake dovecots to try to sell their ticky-tacky boxes to the public as ‘traditional’. Thy are not traditional, they hide poor quality workmanship, and they are inherently thermally inefficient.

 Here is an excerpt of a newspaper article of the time:

‘As it is, the bricklayer’s role on many new housing developments is now limited to disguising flimsy timber frames by constructing a thin veneer of brickwork around them. This has the effect of making many buyers of new homes think that they have actually bought a brick house. On one large site that I visited recently, even the salesman in the show house was unaware that the houses he was selling were timber-framed’. (Jeff Howell – The Telegraph)

The way UK builders build buildings is completely wrong and inside out, or it is outside in? Solar passive buildings by ourselves and other architects are built with the masonry skin on the INSIDE and a timber frame holding the insulation on the OUTSIDE. This is the way all Passivhauses are constructed also.

There are a great number of design issues bound up and forming a part of a direct gain solar passive design strategy. Hopefully you will stay with us as we go through these. Some topics to be dealt with are:

  • Basic orientation and form issues in direct gain solar houses.
  • The astrophysics of solar design.
  • Calculating the size of the solar wall.
  • How much thermal mass? Calculation methods.
  • Orientation – how far can I go from the solar direction?
  • Maximising the usefulness of solar heated air – internal design.
  • Insulation.
  • Solar shading.
  • Heating and cooling a house naturally.
  • Natural cooling methods.
  • Using natural building materials – how this compliments solar passive.
  • Solar passive and zero carbon/offgrid houses. How possible is it?
  • Solar passive and global warming/climate change.
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