Insulation Basics – ‘R’ and ‘U’ 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.

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

 

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

  1. 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’

  1. 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.

 

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