R-value of insulation is the most important guiding factor on the ability of the insulation to reduce heat flow up or down. As a result, the higher the R-value, the better the insulation properties for roof insulation. What this means is by increasing or decreasing the thickness of the insulation, one can change the effectiveness.

Importantly to remember that compressing insulation will reduce the value of the compressed thickness of the insulation. With regards, You value the lower, the better. A lower U-value equals a higher R-value. All Roof Insulation and Ceiling Insulation products will always be quoted as R-value.

Therefore the R-value of Insulation is the standard measure of Roof Insulation and Ceiling Insulation ability to reduce heat flow. Having said that, not all insulation products are equal. In all cases, the thicker the insulation, the higher the Insulation R-value will be.

Some Roof Insulation products will achieve a lower value on thinner Roof insulation. The higher the R-value, the better the Thermal Insulation Properties will be.

How to Find The Correct Insulation R-value For Your Home

All roof insulation products in South Africa must be SANS compliant. In other words, the insulation for roofs and ceilings must achieve a specific R-value for your region. The entire country is divided into zones. Each zone depending on the weather patterns will have to have insulation installed to achieve the desired goal of reducing the need for heating and cooling.

R-value of Roof Insulation

Some insulation products will deliver a higher R-value on the exact thickness than other Roof insulation products. Sometimes, the Roof Insulation density can be affected as some manufacturers will have a lower density and a higher thickness.

Other manufacturers will have a higher density and a lower thickness. What is better, you may ask? Well, nothing really because the R-value will be the same.

However, in my opinion, I would instead opt for the achieved R-value of the thickest insulation. Roof Insulation products like Aerolite Insulation and Isotherm Insulation achieve the recommended R-value.

So how would we calculate the thermal properties of roof insulation of the specific roof insulation material using K-value? In fact, this is quite simple. Check the roof insulation thickness by about one meter and multiply this by the K value of the roof insulation. So if the roof insulation is 145mm thick, you will multiply .145 ÷ K value= R-value.

  • This is simple terms R-value is measured in meters squared Kelvin per Watt ( m2K/W)
  • For example, we have Aerolite insulation that is 135mm thick, and the insulation has a K-value of 0.04.
  • 135 divided by K value 0.04 = R-value 3.37. This would be SANS compliant for the Western Cape region.
  • Remember, R-value only takes conduction into account and not convection or radiation.
  • There are times when you would only take U-value into account. On the building of walls and window options etc.


The U-value of a home is a combination of the opposite of the total thermal resistance of this element. This is the measure of heat loss through the thickness of insulation. However, this includes all three heat loss factors—namely, Conduction, Convection, and Radiation.

Remember, the ambient temperature outside the home plays an essential role in the calculation of the U-value. Importantly remember the lower the U-value, the more effective the insulation will be.

This same formula can be used when calculating brickwork for sidewalls. Once you have factored the value of the walls with the cavity space, you can then work out what thickness insulation to fit between the walls. Insulating the walls with polystyrene or Isoboard will give a considerable R-value. This, in turn, will make the home far more energy-efficient.

Calculating U-value

Calculating U-value is a complicated exercise and is better done on a U-value calculator.

  • We are giving an example of the value of standard brick walls with cavity gap approximate U value 1.6 W/m2K approximately.
  • Another example of drywall partitioning with cavity space value 1.5 W/m2K with the correct thickness U value 0.18 W/m2K. This is almost six times more efficient than drywalling.
  • A solid wall will give a U value of 2 W/m2K approximately.
  • However, remember when considering R-value or thermal resistance, the higher numbers are better.

As energy has become an essential factor in building all new homes, the U-value is crucial when building houses. The lower the U-value of windows, doors and walls, the better the energy saving. When it comes to the roof space, R-value is King and the higher the R-value, the more efficient the Roof Insulation properties.

Some examples of U value in Buildings

  • Solid brick wall 2 W/m2K
  • Cavity space wall without insulation 1.5 W/m2K
  • Insulated Cavity space wall 0.18 W/m2Single pane glass windows 4.5 to 5.5 W/mK
  • A Double glazed glass windows 1.2 to 3.2 dependent on the type and thickness of the glass.
  • Triple Glazed glass windows 0.9 W/m2K
  • Solid wooden Doors 3 W/m2K
  • As we can see, some options have a rather significant U value in effect. This is not effective in energy conservation.

The 145mm Isotherm will achieve an R-value of 3.70 when installed. This takes into account the R-value of the roof and ceiling at R-value 0.40. For more specifications on Isotherm, read this.

The 135mm Aerolite will achieve an R-value of 3.70 when installed. This is also including the Roof and Ceiling at R0.40. For more technical specifications on Aerolite, read this.

What is R-Value

All products have Thermal Resistance, and the R-Value defines the speed at which any material or product will lose heat. It is common sense for insulating material to retain heat well, and the R-value is a measure of how good a product or material is at doing so. The R-Value is proportional to the thickness of the products or material. Likewise, the value is inversely proportional to the thermal conductivity (k-value) of the product. Read the specifications of the Isotherm as well as the specifications of the Aerolite. In fact, read the specifications of any insulation that you would need to purchase. This will give you a good idea of selecting the correct insulation. In simple terms, the best insulation material has a high R-Value because it is generally thicker and denser and has a high level of thermal conductivity. Thermal conductivity is the measure of heat transfer which in insulation refers to the amount of heat lost and how fast.

There are many different types of insulation products on the market, each claiming to be better than the other. The truth lies in the R-Value and the K-Value. All insulation products are sent for testing to ascertain the K-value after manufacture. Having the R and the K values helps you determine the most suitable insulation product for your building. Installing the correct Thermal insulation is critical.

Thermal Performance

You keep your home warm in winter through the Thermal Performance of its thermal envelope. It can be achieved by varying factors, such as insulation material and thickness or how well it insulates against outside elements like wind chill conditions that may cause freezing temperatures in the evening.
For example, solid brick walls have an average U value of around 2-3, whereas cavity walls usually register less than 1/2.

Conduction Convection and Radiation

Heat transfer occurs when two objects come into contact with each other, and the temperature difference causes heat to flow from one object into another. The process can also happen through fluids, solids or radiation. Without involving particles in between them; however, it’s most notable for taking place via direct contact because that way, there is no need for any materialization process like convection (which includes moving molecules around) – just let these little guys do their job!

The Thicker Insulation, The Better The R-value

The thicker the insulation, the better the R-value. If you compress the insulation in your roof space, We will reduce the R-value to the compressed thickness R-value. For this reason, we mustn’t store boxes on our insulation as this will reduce the Thermal Insulation R-value.

Thermal Resistance

Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance.
Thermal resistance, measured in 
 is the length of the material (measured on a path parallel to the heat flow) (m)
 is the thermal conductivity of the material (W/(Km))
 is the cross-sectional area (perpendicular to the path of heat flow) (m)


Resistant To Heat Flow

One of the most useful and widespread thermally resistive materials is the concrete that is used during the construction of buildings and homes.