Building Insulation and R Values

How good are Building Insulation materials?

Building Insulation and R Values

Insulation is a subject we are going to hear more about in the future as the world tries to reduce the amount of energy we are using.

It is estimated that air conditioning currently uses around 10% of the worlds electricity consumption and the number of air conditioners in the world is expected to triple to 5.6 billion units in the next 30 years.

Things are changing rapidly and more and more people are realising that insulation can make our buildings far more comfortable to live and work in while dramatically reducing our energy costs.

Of course, in cold climates insulation has long been widely used to reduce heating costs where, in may places, the difference in temperature between the inside and outside of houses is often 10, 20 or even 30 degrees.

In hot climates where we are trying to keep heat out, temperature differences between the outdoor air temperature and what we find comfortable are usually much less. As a result few people have cared much about insulation even though they may be paying a lot of money for air conditioning which often makes up the majority of their electricity bills.

But increasingly we are finding that there are days when it is just too hot and we start looking for respite in air conditioned spaces.

This is particularly the case in Australia where, after years of drought and increasing summer temperatures, people are starting to wake up to the fact that the majority of Australian houses were never designed with any thought of insulation in mind.

Effective ventilation through clever building design can go a long way to reducing the problem. This is a specialist area of knowledge that is best applied at the building design stage.

Whether we are using ventilation and/or air conditioning for our cooling, by incorporating well designed and installed insulation into our buildings we can dramatically improve our ability to keep them cool.

Heat Transfer

To insulate a house we look at three ways buildings get hot.

1 Radiation

Heating by radiation is caused by infra red rays (sunshine or other heat sources) hitting our building and heating it up. We can stop this by shading the walls with overhanging roofs, trees and awnings or we can reflect the heat by painting the walls white or using reflective materials such as aluminium foil under our rooftiles.

2 Conduction

Conduction is when a surface gets hot and the heat is conducted (transferred) through the walls, windows, doors, floors and roofs into the building. We deal with this by using building materials that reduce the amount of heat transfer, we install insulation materials and we use cavity walls.

3 Convection

Convection is when a flow of air carries heat into or out of a building. We deal with this by sealing the building up to prevent air flows, we use impervious materials, seal up cracks and pipe entry points, put draft excluders on our doors and rubber seals around our windows.

Building Principles

Modern building design principles address all of these issues and are being covered in design guidelines such as Germany’s “Passiv Haus” and Indonesia’s Green Building Council.

Managing the amount of heating or cooling a building requires has developed into a sophisticated science which allows us to calculate exactly how much heat will be transferred into or out of a building. Central to these initiatives is understanding how effective different insulating materials are.

What Are Insulation R Values?

The R value of a material is its thermal resistance, its ability to prevent heat to pass through it, its ability to provide insulation.

Insulation R values are something we are going to hear a lot more about. R values are easy to understand and very useful for anyone who wished to insulate a building.

Basically an R value tells us how insulating an inch (25.4 mm) thick piece of a material is, the higher the R value the lower the amount of heat that will be transferred through the material.

For example an inch thick piece of glass has an R value of around 0.14 per inch (that is very poor) while rockwool batts have an R value of around 3 to 3.85 per inch (that is very good).

R Values of Common Materials

Below we have a list of common materials and their R values. When comparing these materials it is important to bear in mind the thickness. As we have said these value are for a 25.4 mm (1 inch) thickness of the material. Typical window glass is usually 5 or 6 mm thick so the 25 mm R value of 0.14 comes down to only R 0.03. This very low value is why our widows are often responsible for the greatest heat transfer into buildings.

Aerated Concrete blocks are being increasing used for wall building in thicknesses of 100mm to 200mm giving us R values of 4 to 8 which are far better than other common materials such as brick.

R value Calculation

To calculate the R value of a wall or other building component we add the R values together.

Example: A brick cavity wall with the cavity filled with rockwool and a layer of plasterboard on the inside so we have 4 inches of brick, 4 inches rockwool in the cavity, 4 inches of brick then 1/2 inch of plasterboard.

R = (0.2 x 4) + (3 x 4) + (0.2 x 4) + (0.5 x 0.9) = 14.05

R Values of Common Materials

U values and how we calculate R values

The R value is a material's ability to resist the transfer of heat and is calculated by using the inverse of what is called the U value.

The U value for a material is its ability to transfer heat and is found by measuring how much heat in watts is transferred through a 1 square meter of the material. The higher the U value, the better it is at transmitting heat.

Calculating a Building's standard of insulation

We can calculate the R value for a material, for a part of a building such as a wall, roof or floor and we can calculate the R value for a whole building

We do this by adding the R values for the different components as shown in the example above and then multiplying these figures by the surface areas of each.

R Value Targets

In some countries government recommendations are made for building R values according to the climate. For example in a hot climate desirable R values are R30 to R50 for roof spaces, R12 to R20 for walls and R10 to R15 for floors.