Passive use of Thermal Mass

In passive houses the solar heat gain is very important to the overall heating load of the house.

In the northern hemisphere the amount of sun to shine in the winter will depend on the geographical location of the house.

The suns energy falls as solar radiation.

When it hits a window some is reflected and some travels through the window and strikes walls and floors and is absorbed and stored in the form of heat.

The amount of storage is determined by the type of material is strikes.

Concrete or masonry walls have a greater thermal mass than say a timber stud wall.

The use of thermal mass walls ans floors are therefore an essential part of the passive heating cycle.

Thermal Mass in temperature delays

On a cold but sunny day the sun will still give very useful energy.

As its lower in the sky it will shine through the windows to the inner walls and some of this energy will be stored.

The walls ability to absorb heat is measured by its specific heat, which is the amount of heat it takes to raise the temperature of 1 kilogram of the material by 1°C (or 1K or Kelvin).

By using larger glazing to the south side of the house the amount of sunlight entering the building will be greater.

In passive designs there is a trade off between the window sizes and the amount of thermal mass in the house.

If there is too little thermal mass than the rooms inside the glazed areas will overheat.

By using thermal mass walls the heat is soaked into the walls during the sunlight hours.

Thermal mass wall take time to heat up and this time lag is very important.

As the wall heats up the temperature balance is maintained as the heat from the sun is absorbed in an even fashion.

When evening comes the sun will have set and the heat gains the wall will have soaked up will now be radiated from the wall.

Again this is a slow release of heat and as the inner house temperatures are greater than the external temperatures the wall will release heat as it flows from hot to cold.

This heat transfer within the house will be crucial to maintaining a stable heat pattern in the house. The position of the absorption area is very important to harvest as much energy as possible.

The thermal wall should be in direct line of the sun and the windows should be unobstructed.

Now walls and floors will also benefit from indirect solar radiation by reflection in certain rooms. The clever use of thermal mass walls is to get a larger solar heating fraction.

The suns energy is free so it should be used wisely to extract the most out of it.

Types of Thermal Mass

There are two ways to absorb the sun, floors and walls.Floors carpeted or timber floored are less effective than say ceramic tiling.

Walls such as block walls are more effective because of there large areas. In more temperate climates the heating cycle should be reasonably quick to release and absorb heat.