Passive Design
Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.
Thermal Mass
Dense (heavy) materials such as concrete, concrete masonry, earth, stored water, stone and brick can absorb more heat than lighter materials such as wood or wood products. The ability of a these materials or elements to absorb and store heat is referred to as its ‘thermal mass’.
Uses of thermal mass
When compared to lightweight construction, high thermal mass materials exposed to sunlight may reduce total heating and cooling energy requirements by up to 25%. In cold weather, the thermal mass stores the energy from the sun (or other heat sources) and re-radiates that energy into the room as the temperature drops in the evening. In hot weather, thermal mass may act as a heat sink, lowering internal temperatures and reducing cooling loads.
Thermal mass that has limited exposure to sunlight can draw heat out of the space and increase winter heating requirements (the space heating will have to heat the thermal mass before the air temperature rises).
- How thermal mass evens out temperature fluctuations
By alternately storing and releasing heat, high thermal mass “smooths out” the extremes in daytime and evening temperatures and can be used to provide winter heating.
Typical thermal mass options
Thermal mass can be provided by:
- an exposed concrete floor that can be directly heated by the sun – on sloping sites a suspended concrete floor can be used, or the design may incorporate a concrete upper floor with the underside left exposed
- exposed internal stone, earth, masonry or concrete walls that are directly heated by the sun and also absorb excess heat from the interior
- a specifically designed thermal mass feature such as a Trombe wall where the outside of the wall is heated and the reradiated heat is channelled into the building interior.
A concrete slab floor is the cheapest option to provide thermal mass – other mass such as heavy walls can be added as the budget permits.
Materials outside the insulated framework, such as roofing tiles or brick veneer, or strapped and lined single skin concrete masonry do not typically add to the internal thermal mass of a house because any heat they may store is lost to the outside.
Designing thermal mass
Designers must consider:
- the amount of thermal mass provided
- the need to ensure that it can be heated by the sun (direct gain) or from internal heat sources (indirect gain)
- the thermal lag – that is the time that elapses after the mass is heated for that heat to be re-radiated back into the space or for the heat to be transferred through a material.
A direct gain system using the sun to heat the surface is considered the easiest option. To give sufficient storage and to allow re-radiation when heating is required, a solid wall thickness of 100 to 150 mm is considered ideal.
You will need to explain to your clients that a house with good thermal mass design may take a full heating season for the mass to dry out, absorb heat and to work effectively – it may feel cold initially.
For effective design of thermal mass:
- the building should be oriented and the windows sized and placed so the thermal mass gets direct sunlight – a rule of thumb from the Australian Your Home website is that, where direct heating of a thermal mass is required, the area of thermal mass should be around six times the area of north-facing glass
- the thermal mass should not be isolated from the heat source by insulation materials or finishes – for example, carpet on a concrete floor will isolate the concrete from the heat source
- the building envelope should be effectively insulated to minimise heat losses
- the thermal mass should be located inside the insulated building envelope – single skin concrete masonry external walls can be used to provide thermal mass, as can internal feature internal walls of stone, brick or concrete masonry
- for thermal mass in external walls, the insulation should be on the outside face of the wall
- specify dark-coloured materials and finishes (they absorb more heat) and textured wall surfaces (they have more surface area)
- consider providing a sunspace to harvest (and store) solar gain.
In climates where cooling is the prime requirement and there is little demand for winter heating, thermal mass should be minimised.
There are no definitive rules on using thermal mass. Cost will be a factor, together with practicality and design considerations.
- Thermal mass in winter
In winter, thermal mass will absorb heat from the sun during the day, as well as from supplementary heat sources, and release that heat as temperatures fall at night.
- Thermal mass in summer
In summer, thermal mass absorbs the sun’s heat to keep the house cool. At night, the house can be ventilated to allow the heat to be lost into the cooler night air.
Concrete floors
Generally, concrete slab floors on level or near level sites cost less than suspended timber floors and have a much higher thermal mass.
To increase direct solar gain:
- insulate the outside edge (1 metre) of the slab – the earth under the slab (once warmed) contributes significantly to the total thermal mass of the floor, and the insulation is necessary to prevent heat leakage out through the edges of the slab
- have insulated windows (IGUs) down to the floor so that winter sun falls on the slab
- do not have insulating finishes such as carpet or cork over the area providing thermal mass (i.e. typically the area of slab exposed to direct sunlight); instead use ceramic tiles, thin vinyl sheet or polished concrete
- add a colour to make the concrete darker
- for suspended upper floors, leave the underside exposed and use a profiled permanent form work e.g. profiled permanent steel formwork can increase surface area by 15% to 50%.
- Principle of using the thermal mass of a concrete floor
To increase the direct benefits concrete slab floors offer, insulate under the slab and have insulated windows (IGUs) down to the floor so that winter sun falls on the slab surface. Do not have insulating finishes such as carpet or cork over the floor area providing thermal mass – typically the area of slab exposed to direct sunlight. Use ceramic tiles, thin vinyl sheet or polished concrete over this area.
Trombe walls
One way to utilise solar gain is a Trombe wall, which is a north-facing wall of high thermal mass combined with an air space and insulated glazing. The wall is painted a dark colour and absorbs heat from the sun. Heated air rises and enters the house through vents at the top, and cooler air is drawn by convection from the interior through low level vents. The vents are closed when heat is not required.
- Basic principle of Trombe wall construction
A Trombe wall is a north-facing wall of high thermal mass combined with an air space and insulated glazing. The wall absorbs heat from the sun. Heated air rises and enters the house through vents at the top. It is replaced by cooler air drawn by convection from the interior through low level vents. The vents are closed when heat is not required.
There are many variations and modifications possible such as:
- fans to assist air movement
- moveable or adjustable louvres to shade the collector wall in summer
- an exhaust vent at the top which can be opened in summer so that the wall draws cooling air through the house.
The disadvantage of a Trombe wall is that blank areas of wall are required on the sunny side of the house. A compromise is an insulated, glazed conservatory with windows through to the main house. This room is a habitable solar collector with heat being stored in the floor and wall. Movement of warmed air can be controlled by the windows or a combination of windows and vents.
- Principle of conservatory design
A conservatory with windows through to the main house can act as a habitable solar collector with heat being stored in the floor and wall. Movement of warmed air can be controlled by the windows or a combination of windows and vents. Don’t design a fully glazed roof on a north- or west-facing conservatory – the room is likely to overheat in summer.
Adding thermal mass to an existing home
Thermal mass can be added to an existing building as part of alteration work. Options include:
- incorporating a concrete floor or solid wall exposed to direct sunlight (and increasing window areas where appropriate)
- for existing concrete floors, lifting existing insulating floor coverings such as carpet and replacing with tiles, or grinding and finishing the existing slab
- incorporating a heavy mass wall
- incorporating a sunspace with high thermal mass.
For your clients
To help your clients understand thermal mass, refer them to www.smarterhomes.org.nz/design/using-thermal-mass-for-heating-and-cooling.