Managing my low-energy house: I. Features needing no attention

Photo of sunlit house interior

July sun heats the house

This post, and the companion post “II. Features needing attention” were posted originally to a forum of the Alternative Technology Association (See Note below.)

My low-energy house at Manilla, NSW, maintains year-round comfort in a climate of daily and seasonal extremes. In the climate classification of the Building Code of Australia, it is in Zone 4: “Hot dry summer, cool winter”, along with Tamworth, Mildura and Kalgoorlie.
This house differs from most houses in relying on the design of the house to achieve comfort, with hardly any energy needed for heaters or coolers.
There is little artificial control: the “home automation system” consists only of timers set twice a year. Some of the comfort features call for daily action in certain seasons. However, these simple daily chores could have been avoided by small changes in the design. [See “Note added 2016” below.]

The success of the house in maintaining comfort in all seasons is shown by scatter-plots of daily indoor and outdoor maximum and minimum temperatures over a period of three years.

I. Features needing no attention

Heat transfer to and from the heat bank

The mass of concrete, bricks and rubble under the concrete floor slab is edge-insulated with foam to a depth of half a metre to prevent heat leaking sideways to and from the surrounding soil and subsoil. This 150 tonne edge-insulated under-floor mass is a “heat bank” which absorbs and yields heat so slowly that it holds the same temperature (at 750 mm depth) within a degree for weeks at a time.
Double-brick walls (17 tonnes) inside the house, and the floor slab itself (28 tonnes), are also parts of the heat bank. Their exposed surfaces (See photo.) absorb heat from sunshine (and yield heat to cool flows of air) so as to spread heat (or coolness) around the rooms. Within each day they conduct heat to and from the rooms of the house, and from room to room. They then conduct heat slowly to and from the under-floor mass.
In the absence of the house, the under-floor mass would have the same temperature as the subsoil of the area. A thermometer at 750 mm in the subsoil near the house shows a 14.6° yearly temperature range, from 12.9° to 27.5°. Even an ordinary light-weight, poorly-insulated house built on a concrete slab on the ground here would be made more comfortable by these stable subsoil temperatures. Midsummer and midwinter temperatures in such a house (next door) are plotted here and here.
It is clear that temperatures in that conventional house vary much less than outdoor temperatures, and remain close to that of the subsoil. The heat bank under my solar-passive house has an even more stable temperature than that of the surrounding subsoil. (There is a graph showing one year of heat bank and subsoil temperatures here.)

Insulation

Thermal insulation reduces the flow of heat in and out of the house. With sufficient insulation, the heat of the day is replaced by the cool of night before the house becomes too warm. Insulation improves comfort permanently.
This house, built in 1998, is insulated to a standard that has since become normal: the walls have foil and R = 2 batts, and the roof foil and R = 3 batts. As mentioned, foam insulation is also placed around the edge of the floor slab to reduce heat flow below ground. Windows are double-glazed to reduce heat flow, and they are fitted with heavy curtains with pelmets. All door and window openings are well sealed against draughts. Entrance rooms to the house form air-locks, limiting the loss of warm or cool air.

Solar heat gain

Most windows face north to catch winter sun and are sheltered by eaves to exclude summer sun. Much of the trapped sunshine falls on internal brick walls or floor tiles that store the heat and conduct it through the house at room temperature.

Shading

The house is well shaded in summer to reduce heating by the sun, and hardly shaded at all in winter, so as to catch the sun’s warmth. This is done by the design of the house, including the roof eaves, and by planted vegetation. In summer, the short, almost-windowless east and west walls are shaded by a veranda, porches, a garage, water tanks, trellis, vines, shrubs and trees.

Continue to Part II. Features needing attention


Note added 2016.

Admission of cold night air through an automated louvre.
I have described the installation of a low-level louvre window controlled by a programmable time clock in the post “Louvre window for summer nights”.


Note.

The original ATA forum posts in the thread “Low Energy Houses” are here and here. These two posts were in answer to a question by Jeff Bloggs in the ATA forum thread “Highly insulated home with little or no thermal mass – thoughts anyone?” Jeff asked “How do you manage your thermal mass?” I realized that I had never explained how I do it.

The house was described earlier in the “Low Energy Houses” thread, and pictured here.

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