Saturday 25 August 2012

Passive Solar Design and Thermal Mass

Given that the plot I had chosen was off-grid I was very clear that I wished to minimise my energy  and water consumption. This immediately led to me fully researching Passive Solar Design (PSD) as the guiding principle of my house design.

PSD is a method of heating, cooling and lighting a space with least, ideally no, input from "the grid".

The major components of PSD are:

- South facing windows (SE/SSE in my case) to allow low angled winter sun to enter.

- Thermal mass - this is heat absorbing material in floors, walls and ceilings. The function of this thermal mass in winter is to convert the low angle sunlight to heat, radiate that warmth and store excess heat for later use. In summer, when earth sheltered, as in my case, the the thermal mass absorbs heat from the air and, with overnight cooling ventilation, maintains earth temperatures. The ability to absorb and release heat in this way enables buildings with thermal mass to respond naturally to changing weather conditions helping stabilise the internal temperature and provide a largely self regulating environment.
Concrete overhang

Overhangs - to regulate solar gain by controlling how much sun gets into the house by shading walls and windows from the high angled summer sun whilst allowing the low angled winter sun to enter.

- Insulation - The simple rule is for the thermal mass to be located INSIDE the insulated building envelope.

- Good ventilation - necessary to spread the warm or cold air evenly.

- Airtight - build structures that are as airtight as possible. Use sealed entryways (airlocks) to prevent air rushing in and out when main structure's door is opened.

- Sun-free areas - keep some sun-free areas in structure for cool rooms/larders and general storage.

- Have source of back-up heat (eg. wood stoves)

There appear to be no hard and fast rules on how much thermal mass a building should have. From an energy viewpoint it would be difficult to have too much, and generally the more thermal mass the better. However, as with all aspects of design it is necessary to arrive at a workable balance. As a rough guide it seems that the total surface area of the thermal mass should be at least 6x that of the glazing. Therefore as the area of south facing glazing increases more thermal mass is required to maintain a stable summer temperature.

Typically thermal mass is provided by concrete/masonary walls and floors. Surfaces do not have to be dark in colour as any small benefit in heat absorption may well impact on day lighting. However it is important that the surface of such heavyweight walls remain as thermally exposed as possible.

Overhang but non-thermal mass (airbrick) walls
Given that I have always been happiest when living in houses that have been converted from buildings that were built for other purposes ( barns and a granary) and have a long held ambition to live in a minimalist industrial style dwelling the opportunity to use the local skills and building materials (concrete with local recycled aggregate) for my thermal mass was seized with relish. Especially since it is now possible to achieve a high quality fair faced finish although from my observation throughout Crete not as a standard option! From a thermal mass viewpoint such a fair faced finish would be particularly beneficial as heat could pass directly between rooms and the concrete whist at the same time achieving my desired industrial look.

See an excellent detailed explanation of PSD at "Arizona Solar Centre - Passive Solar Design " ( and
"A Simple Design Methodology for Passive Solar Houses" - Dennis Holloway (Architect) at


"The Passive Solar House" - James Kachadorian
"Concrete Design" - daab
"Concrete Architecture" Catherine Croft
"Architectural In-Situ Concrete" - David Bennett