Insulation Basics – ‘R’ and ‘U’ Value
R Value
R1.5 Batts, R2.0 Batts……. all the way up to R6.0 Batts, but what does it all mean?
The R value is a measure of the thermal resistance of the component of the material. In other word how hard it is for heat to pass through that component.
U Value
Once you have the R value you can calculate the Heat Transfer Rate, the U Value.
The calculation is U = 1/R (The units are watts/degree C for each square m)
To get the R value of a structural element, for example a ceiling, you add the total of all the R values of each of the components.
The following table shows the effect on the value of ‘U’ for various levels of Insulation for a ceiling.
Table 1. Ceiling ‘R’ and ‘U’ values
Batts |
Total R value |
U value |
No insulation |
0.36 |
2.78 |
R1.5 |
1.86 |
0.54 |
R2.0 |
2.36 |
0.42 |
R4.0 |
4.36 |
0.23 |
Examples
So how do you use these figures?
The following two examples are for a house of 150m2, which you want to keep at 22 degrees C
- On a summers day the temperature in the roof space is 50 degrees C (not unusual in Australian summers) and you want to cool it to 22 degrees C, a difference of 28 degrees C.
Heat transfer through ceiling = 150 x 28 degrees x ‘U’
- On a winters day the temperature in the roof space is 5 degrees C and you want to heat the house to 22 degrees C, a difference of 17 degrees C.
Heat lost through the ceiling = 150 x 17 degrees x ‘U’
The results of the heat gains and losses for the various R levels of ceiling insulations are shown in Table 2 below.
Table 2. Heat Gain / Heat Loss Through Ceiling.
Insulation |
Summer Heat Gain = Cooling Required |
Winter Heat Loss = Heating Required |
No insulation |
11.6kw |
7.1kw |
R1.5 Batts |
2.3kw |
1.37kw |
R2.0 Batts |
1.8kw |
1.1kw |
R4.0 Batts |
0.97kw |
0.6kw |
You can see from the above table that by providing insulation you will need considerably less cooling in summer and less heating in winter.