Good Design solutions for passive thermal comfort are based on the following five principles:

1. Orientation

2. Glazing

3. Thermal Mass

4. Insulation

5. Ventilation

1.0 Orientation

A well designed house should be elongated towards a private garden to the north. The shorter eastern and western walls will as a result be exposed to the low angle summer sun in the morning and afternoon.

The main living areas should ideally be orientated to north – allowing sunshine when it is needed for warmth in winter and excluding the sun’s heat when it is not needed in the warmer summer months.

The high angle of the sun (Brisbane 12.00 noon on 22nd December, 85.5 degrees) in the northern sky in summer makes shading the house by a generous roof overhang a sensible option. The longer northern side of the house benefits from the low angle (Brisbane 12.00 noon 22nd June, 39 degrees) of the suns path in winter.

Ideally the roof overhang on the north should allow the sun to shine into the house when warmth is required in the winter months and shade the house from high angled sun in the summer months.

Calculation of roof overhang on the north is determined by making the angle of the line of the base of the glazing and the edge of the eaves the same as the sun at midday of the year when the shading period begin and end – usually late spring to early autumn – (Brisbane 12.00noon late September to late March, 62 degrees).

The house should be sited to gain benefit from desirable prevailing cooling breezes in summer and sheltered from undesirable breezes.

Brisbane desirable breezes are from the south – east in the morning and north – east in the afternoon. The undesirable winds prevail from the south – west and the north – west.

2.0 Glazing

Glazing is an important component of the design for passive heating and cooling of a home. The type, size, position and protection of the glazing should be managed to benefit from winter sun and control room heat loss in winter and to avoid overheating of rooms in summer.

Windows should be well shaded in summer. The most effective form of shading is external. If internal shading is the only means then the use of light coloured blinds and curtains are more effective then dark colours as the dark colours absorb more heat.

3.0 Thermal Mass

The benefits of thermal mass in hot climates works on the premise that in summer the interior of the house should be well shaded. The materials of high thermal mass – e.g. concrete slab on ground, internal walls of face brick or solid concrete block, stone or earth – stay cool and absorb heat from the air by conduction and from our bodies by radiant heat transfer. At night the house can be cooled by opening the windows and drawing in cooler air through the house. This works where difference between day and night temperatures vary between 8 degrees C – 10 degrees C.

During the winter months the sun should be permitted to penetrate the interior of the interior of the house. The sun’s heat is slowly absorbed by the thermal mass elements during the day and in the evening the absorbed heat is slowly released – warming the room by radiation and conduction.

3.1 The benefits of lightweight construction

Lightweight construction such as timber framed or steel framed structures with timber or sheet cladding respond to changes in temperature quickly. Shaded light weight construction is especially suitable in very hot and humid climates where it is hot day and night. In other climates especially a subtropical climates like Brisbane it is useful for bedroom/sleeping and evening common living/dining areas in summer.

4.0 Insulation

Insulation works on the premise that it inhibits the flow or transfer of heat into the house and out of the house. Broadly speaking there are two types of insulation: bulk and reflective.

4.1 Bulk insulation

Bulk insulation reduces flow of heat by conduction and comes in several forms:

• batts and blanket made from rockwool, fibreglass, acrylic fibre, wool and eelgrass;
• loose fill material made from cellulose fibre (usually treated waste paper); and rigid lightweight boards, such as polystyrene, polyurethane and composite boards used for internal lining such as straw/wood fibre mixed with binder
• e.g. woodwool, strawboard and fibreboard

4.2 Reflective insulation

Reflective insulation inhibits the transfer of heat by reflecting radiant heat on the side of the heat source and not emitting much radiant heat to the side to be screened from the heat or cool side.

Reflective insulation usually comes in the form of:

• reflective foil laminate (RFL) made from aluminium foil laminated to a stiffer backing such as building paper. The aluminium foil is usually laminated on both sides e.g. sisilation; and
• reflective coating on the underside of corrugated metal or on the back of plasterboard

4.3 Importance of roof/ceiling insulation

The ceiling roof space is the most important part of the house to insulate as it is the main path of unwanted heat gain and heat loss. Reflective foil insulation is more effective in resisting downward heat flow (heat from the sun) than upward heat flow from the house. Bulk insulation in the form of batts tightly laid directly on top of the ceiling is more effective in reducing heat loss from within the house.

4.4 Wall insulation

Walls should also be insulated. The method of wall insulation varies with the type of wall construction and level and type of heat exposure. Care should be taken in selecting the most appropriate method of insulation.

Options to wall insulation should also be considered include providing shading to the wall – e.g roof eaves overhang, covered verandah or terrace, man-made screens and natural screen vegetation.

5.0 Ventilation

Ventilation is the controlled movement of air flow between the inside of the house and the outside of the house.

Siting, orientation and layout and design of a house dictate the level of ventilation. The house should be positioned to receive prevailing cooling breezes for cross ventilation. Orientation for sun control should take precedence over the direction to the preferable breezes as effective ventilation will still be achieved with the direction of the breeze at angles up to 45 degrees to the direction in which the room faces. Cross-ventilation is important in warm to hot humid areas and is best achieved by positioning windows on the opposite walls of each room in the direction of the preferred prevailing breezes.

On entry, airflow should be directed to the lower part of the room and to areas being used. Smaller openings on the windward side , and a large opening higher up on the wall on the leeward side maximize the air speed in the room for airflow to have a cooling effect on our bodies greater air speed is needed.

The selection of window and door type and operation will contribute to the success of the cross ventilation achieved.

• vertical louvre, casement windows and outward opening hinged glass doors can be used to deflect breeze into and away from the interior depending on their position;
• horizontal louvre windows/central pivot awning sash windows can be used to direct air movement lower into the room, but when partially opened will direct airflow upwards; and

• double hung windows , sliding windows and sliding doors (50% ventilation opening) do not significantly modify the direction of air flow although double hung windows partially opened top and bottom can assist air circulation by convection (cool air entering the lower opening and hot air rising and escaping out the top opening.

Note: Building code regulations and State Government and Local Authority by laws in relation to window design and safety issues will dictate the window type that is appropriate for the circumstance.

5.1 Ventilation of the roof/ceiling space

Whirlybird type roof ventilators (roof mounted wind driven extractor fan) work by expelling/drawing out heated air ‘trapped’ in the roof ceiling space thus cooling the roof space and consequently cooling the ceiling lining.

5.2 Venting through the ceiling

A cowl or wind driven extractor fan can be used more effectively when used to increase airflow within the building. For this to work the ceiling lining will need to be vented to allow the hot air trapped in the room (on the under-side of the ceiling) to be drawn into the roof /ceiling space and expelled via the wind driven extractor fan. This may be problematic in winter when the warm air continues to be expelled into the roof ceiling space but, this is offset by the benefit of a reduction in the amount of condensation of warmer room air forming on the cooler ceiling and walls and reducing the likelihood of mould establishing itself on these surfaces.

Orange Architects is a strong advocate for passive thermal comfort to be considered when designing new or renovating buildings of all types.

Orange undertakes to always consider the five principles mentioned above and where it is deemed that it is in the best interest of their clients, Orange will endeavour to incorporate an appropriate design solution for passive thermal comfort in their projects.

Orange Recommends Reading

Hallo, Nick, Warm House Cool House: inspirational designs for low energy housing, Choice Books, 1st Australian edition, 1995