At a recent meeting with my architect and engineers my requirement for all round insulation (i.e. under slab, over roof and outside the walls - other than the south facing wall of windows) of the high thermal mass, passive solar, earth sheltered house I am attempting to build in southern Crete was, in the nicest possible way, challenged. I had difficulty getting my head around the arguments that were being put forward for the change. These changes had been suggested after my engineer had talked with a academic friend of his and his findings in running some simulations using a basement model (earth sheltered models not being an option in his modelling package!). Whilst always ready to accept change, being a retired mathematician, I sought a logical explanation for what they were suggesting which was to NOT insulate the external walls.
I therefore decided to revisit the original research I undertook when designing the house. From this and the well documented and publicised examples of earth sheltered housing in the UK eg.
"The Hockerton Housing Project (HHP)" (1)
"The Long Sutton Work Life Project (LSWLP)" (2)
"The AI PassivHaus" (3)
it was apparent that the basic principle when building an earth sheltered house is to wrap a high thermal mass in waterproofing with a good insulation layer on the outside and cover with about 50 - 100 cm of earth.
 |
| The Hockerton Housing Project ( Photo copyright Jenny Pickerill) |
For example an HHP case study by the Energy Saving Trust (1) reports;
"The development consists of a terrace of five ultra-low-energy houses incorporating a number of energy conservation measures that have eliminated the need for space heating. The orientation of the houses allows maximum winter solar gain. A south-facing conservatory runs the full width of each dwelling and all rooms are south facing...
…The building fabric is principally concrete; the roof and slabs are of 300mm reinforced concrete and the back wall of 450 mm concrete… The entire structure has an external surround of 300 mm of expanded polystyrene (covered with 400 mm of earth), providing very high thermal insulation. The earth covering was incorporated to maintain stable internal temperature regardless of variations in air temperature. However, with the very high level of thermal mass and insulation used in the housing, there is,in practice, little functional need for the earth covering."
Similarly a write up of the LSWLP (2) reports;
" The concrete superstructure of the LSWLP buildings is exposed internally. This deliberate exposure guarantees the maximum thermal effectiveness of the buildings and enhances their ability to perform both as a heat sink and as radiators of heat.
….The larger part of this heat (the solar passive heat gains) is collected and stored in the exposed high thermal mass building fabric and then re-emitted back into the rooms as evenly radiated heat from the internally exposed fabric. …
…Indeed, a particular characteristic of high thermal mass buildings of this type is the high degree of thermal comfort which derives from this even distribution of heat…
…This self regulating heating and cooling process applies to a greater or lesser degree to all thermal mass buildings, but is believed to be more effective in earth sheltered buildings. The soil jacket acts as a thermal buffer in which seasonal temperature variations are smoothed in what has been termed the "thermal flywheel effect" i.e. the ability to absorb energy and re-radiate it over a later time….
…With the exception of the glazed southerly elevation, the entire perimeter of the LSWLP buildings (walls, roof and floor slab) is encased in 150mm of extruded polystyrene….By locating the insulation beneath the concrete floors and over the roof and around the solid concrete block external walls, the benefits of their high thermal mass are maximised and their critical role in the process of regulating the ambient air temperature or the interior is secured…
…For the southerly elevation, industry standard sealed double glazed units of 24 mm thickness were specified with low emissivity film applied to the inner face of the outer glass pane."
 |
| AI PassivHaus (Copyright Seymour-Smith) |
And, more recently, a write up of the AI PassivHaus by Seymour-Smith Architects (3) states that;
"It's important to note that PassivHaus does not dictate the design or external appearance of the house. It is simply a method of calculating the amount of insulation, glazing, thermal mass etc. to make the best use of passive solar gain, and ensure that the resulting internal climate will always be comfortable...
… Being dug into the hill to be invisible from the surrounding countryside, this is essentially a stealth house, with absolute minimal visual as well as environmental impact on the landscape.
The house is entirely glazed to the south, and the rest of it being earth-sheltered and therefore highly insulated creates the perfect passive solar design.
The structure of this underground house is entirely concrete, much of which is left exposed internally to exploit the benefits of its thermal mass. It is insulated and waterproofed externally for the same reason...
… Dow are supplying the STRYROFOAM insulation that completely surrounds our house. Extruded polystyrene can cope with being outside the waterproof envelope, and has high compressive strength to sit below the concrete structure, leaving the all important thermal mass exposed internally."
In summary, therefore, in order to control the temperature of the mass fabric of the house insulation must be placed between the home's mass and the earth. In northern climes, as illustrated above, a substantial layer of insulation must completely wrap the below grade portion of the house (concrete floor, walls and ceiling). The high thermal mass of the insulated concrete construction will steadily absorb heat that comes into contact with its surface, conducting it inwardly, and storing it until the surface is exposed to cooler conditions and its temperature begins to drop. When this occurs heat, that has been stored in the insulated mass, will begin to migrate back to the cooler surface and be released. In this way heat moves in a wave like motion alternatively being absorbed and released in response to the variation in day and night time conditions.
Using Passive Solar Design (PSD), see an earlier post, enables concrete and masonry constructed dwellings to exploit their inherent thermal mass on a year round basis. During the summer heat is absorbed on hot days, helping to lower the internal temperature and prevent overheating problems. The stored heat is then removed by overnight ventilation. By allowing cool night air to ventilate the building at night heat that has built up in the fabric is removed. This daily heating and cooling cycle of the thermal mass works relatively well in the UK, for example, as the air temperature at night is typically around 10 degrees less than the peak daytime temperature making it an effective medium for drawing heat out of the fabric. This diuranal temperature variation is rarely less than 5 degrees making night cooling reasonably dependable in the UK.
During the winter the thermal mass will absorb solar gains through south facing windows (which have a well designed overhang to minimise summer sun penetration and maximise winter solar gain) and internal sources of heat (wood stoves, cooking and general living heat) to "charge up" the high thermal mass fabric of the house and to slowly release this heat at night. Without insulation conduction through the massive concrete footings, for example, causes the inner wall and floor surface near the footings to be about the same temperature as the earth at this depth. Any warm moist air created in the house would condense on the cold surface temperature at the base of external walls causing condensation.
However as I was building much further south than the examples quoted above I needed to examine if the insulation strategy should be adjusted. (See next post)
Reference sites:
(1) Hockerton Housing Project (HHP)
(2) Long Sutton Work Life Project (LSWLP)
(3) AI PassivHaus