21 Mansell Street

Killarney Gardens

087 711 0229

Sales / Support

Mon - Fri: 8:00-16:30

Shop / Admin

Roof Insulation

Understanding R-value

Importance Of Understanding R-value

In reality, the importance of understanding R-value is the cornerstone of having a comfortable, energy-efficient home. For this reason, we will break down the benefits of installing roof insulation with the correct R-value.

Importance Of Understanding R-value

Essentially understanding the importance of R-value will give homeowners the ability to select the correct roof insulation to suit the required benefit.

What is roof insulation R-value?

R-value is the ability of specific insulation to reduces the heat flow up or down in the roof space. Of course, the higher the R-value, the better the insulation’s performance in the roof space. For example, insulation of 100mm thick will have a lower R-value than insulation 135mm thick.

So a higher R-value is essentially better insulation properties. For this reason, it is important to install roof insulation with a higher R-value. However, there is also a limit.

To Understand R-value, We Look At Both Down and Up R-value

Up R-value: The up r-value is essentially the winter benefit. In winter, the air is cold at the same time our homes seem to be freezing. An insulated home will not lose warm air to the roof space. As we all know, warm air rises; however, the roof insulation will not allow this warm air to be lost to the roof space. In the event the home has under-insulated, the reduction in warm air loss will be increased.

Down R-value: The down r-value is essentially the summer benefit. As we know, the hot air build-up in roof space heats the ceiling boards resulting in the indoor temperature increasing substantially. If the roof space is insulated, the roof’s heat will be blocked from entering the home.

Importance Of Understanding R-value In The Roof Space

As a result of the R-value allocated for sisalation foils or roof waterproofing, there is some confusion regarding the down r-value.

So how does Sisalation Foils R-value Work

Importantly the sisalation foils must be fitted following the roof pitch? However, there must be at least a 38mm gap between the sisalation foil and the roof. Because of this, an R-value of 1.2 is given on the reflective benefits of Sisalation foil. The problem is that this application is fine for summer, but it will leave your home compromised in winter.

Quick Look At How The R-value With Insulation and Sisalation Foil Works

Required R-value for the Western Cape: The insulation must achieve an R-value of 3.70 down as well as up.

Insulation R-value 3.70

A home with no Sisalation foil must be insulated with insulation achieving the R-value of 3.70. For this reason, the roof insulation will be fitted directly on the ceiling between the beams. And because the insulation is fitted directly on the ceiling, the roof space’s heat will not enter the home in summer. Resulting in the optimum benefit.

Likewise, in winter, the same will apply. As with the summer benefit, the insulation will block the home’s warm air from escaping into the roof space. As the warm air in the home rises, the result is the insulation will block this loss. Thereby resulting in a warmer home in winter.

This is achieved with an R-value of 3.70, allowing a winter R-value and a summer R-value of 3.70.

Insulation and Sisaltion Foil R-value 3.70 Combined

  • 100 Insulation R-value 2.5
  • Sisalation Foil R-value 1.2
  • Roof Space R-value 0.40
  • Installed R-value 4.10

Winter Insulation Benefit: In my experience, the winter R-value is compromised. Let us look at why the winter R-value is compromised.

We know our homes are colder in winter. As a result, the warm air rises. Now, this is where the problem comes in. The roof insulation has an R-value of 2.5, resulting in a lower winter R-value as we cant use foils for up R-values as they are radiant barriers, not insulation. In conclusion, the home will not achieve the full benefit of 3.70.

Summer Insulation Benefit: Once again, in summer, the suns heat builds up in the roof space.  However, the foil will act as a radiant shield barrier. Reducing the heat in the roof space by a few degrees. As a consequence of the radiant barrier, the roof insulation of 100m thick with an R-value of 250 is sufficient. In conclusion, the Summer benefit is perfect; however, the winter benefit is compromised.

The Importance Of Understanding R-value Is Essential For Both Up And Down R-values

Significantly, the importance of understanding the R-value is that we need to calculate the up and down R-values for the best result. Understanding R-value and selecting the best insulation for your application will give years of home comfort and energy saving. In particular, an insulated home will recover the cost of insulation in 3 years on energy saving.

For The Record, Not All Insulation Has The Same R-value

A classic example is Aerolite Insulation and Isotherm Insulation.

135mm Aerolite has an R-value of 3.38 with and installed R-value of 3.78.
100mm Aerolite has an R-value of 2.50 with an installed R-value of 2.90

135mm Isotherm Insulation is way under specification with an R-value of 3.14 and an installed R-value of 3.54
100mm Isotherm Insulation has an R-value of 1.57 and an installed R-value of 1.97

As we can see, we must compare apples with apples. Isotherm Insulation products are no longer SABS approved, so, in my opinion, this can be an even bigger problem with Isotherm Insulation.