Insulation might not be a sexy topic but it’s a vital part of sustainable house design and key to achieving the number one goal of building a passive solar house: to maintain an even, comfortable temperature throughout the house.
Planning the right eco friendly insulation for your home will be fundamental to reducing your reliance on artificial heating and cooling and thereby your carbon footprint.
The type of insulation you choose depends on a number of factors including the climate where you live.
In this article we’ll cover:
What is insulation?
Insulation is simply a material that slows the transfer of heat and noise through walls, floors and ceilings.
Without insulation, the interior of your house would quickly become the same temperature as outside.
Insulation is scientifically designed to work with the other building materials in the house to form a ‘thermal envelope’ to trap heat inside and keep the cold out (or vice versa in a hot climate).
How does insulation work?
Bulky types of insulation, like batts and polystyrene, work by trapping air into millions of tiny air pockets. The trapped air inhibits the transfer of heat out of the house in cold weather or into the house in warm weather.
Another type of insulation is reflective foil, often used in hot climates to reflect heat before it gets into the house.
It provides a radiant barrier between a hot surface such as the roof and the ceiling cavity underneath.
As with bulk forms of insulation, air is used to help prevent heat transfer with an air gap of 2-3cm being required between the foil and the roof in order for the foil insulation to work effectively.
When installed correctly, reflective foil can reflect up to 97% of radiant heat.
Hybrid or composite forms of insulation are also available, combining foil with batts or foil with polystyrene.
Composite insulation works well for areas where the temperature is hot in summer (the foil reflects the heat away from the house) but cool in winter (the bulk insulation retains heat in winter).
Structural insulated panels (SIP’s) are a relatively new product which are an alternative to conventional frames and come as a sandwich with insulation in the centre and provide excellent insulation and airtightness. They are often used to build cabins and tiny houses looking for eco friendly insulation built into the structure, but SIP’s are also used to build larger homes.
How to choose the right eco friendly insulation
In Australia and many other countries, a minimum level of insulation is required under the building code.
“R-values” are used as the measurement of an insulation’s resistance to heat flow. The higher the number, the greater the insulating value.
Climate is the biggest factor in choosing insulation as it will vary according to whether your climate:
- Has daily temperature fluctuations, or
- Is almost always cold, or
- Is almost always hot
All else being equal a fluctuating temperature will use batts or composite insulation, a hot climate will use reflective foil and a cold climate will use the thicker, higher r-value batts to keep warmth in.
Insulation can’t be chosen on climate alone, however, as the rest of the “thermal envelope” such as building materials and the size and positioning of windows also play a role.
Some would argue the higher the r-value the better and buy the highest you can afford but a higher r-value doesn’t automatically result in the higher thermal performance of the building.
Our house is a good example of how windows played a part in the choice of insulation.
We have mild winters and medium to hot summers where we are, so the house was designed with a full bank of north-facing, floor to ceiling windows for maximum solar gain in winter.
With such large panes of glass in the building envelope, higher r-value insulation in the walls or ceiling would have been a waste of money – like having a well insulated fridge but leaving the door open!
Another important consideration when choosing insulation for a sustainable house is thermal bridging.
Thermal bridging occurs when heat conductors like timber and steel frames and aluminium window frames absorb heat from outside into the house, completely bypassing your insulation and diminishing its overall effectiveness.
A good builder or an insulation expert can help you design an eco friendly insulation plan which takes into account all the factors and help avoid or minimise these ‘thermal bridges’.
What are the most eco friendly types of insulation?
The good news is most forms of man-made insulation are made from recycled materials.
The two cheapest and most popular insulation batts are both made from recycled materials – Glass Wool is made from recycled glass and Rockwool is made from slag castoffs from iron ore processing. Both have traditionally contained a toxic gas formaldehyde but there are formaldehyde-free options available.
The the greenest, most eco friendly insulation is 100% natural, renewable and biodegradable.
Natural insulation is made from natural fibres and materials such as sheeps wool, hemp, cellulose (paper) and cork. We will be covering these options in more detail in future articles.
Sheeps wool insulation has a higher r-value than glasswool and rockwool and is sustainably produced using offcuts from sheep shearing. It also is naturally fire-resistant with minimal or no chemicals added and superior moisture resistance compared to man-made insulation.
Alternative eco friendly insulation forms come as part of the construction – rammed earth, straw bales, mud bricks, green roofs and earth covered homes being very effective natural forms of insulation.
Does an eco home need to be super-insulated?
A sustainable house should have a super thermal envelope which doesn’t necessarily mean it should be super-insulated unless it’s in a cold climate.
By all means if going above minimum standard makes sense with the rest of the build and will result in better heat retention, cooler summers and less need for artificial heating and cooling, then go for it!