U-valuesIt is a common misconception that heat rises. What it actually does is flow from warm areas to cold ones. So if your home is warmer than the temperature outside it will always lose heat (through the walls and roof and doors and so on). And the colder it is outside the faster it will happen. You can’t entirely stop this heat loss but you can slow the rate at which it happens. That’s what insulation does; it slows heat escape and keeps it in your home for longer.

A typical uninsulated cavity wall has a U-value of 1.5W/m2K. That is to say, for every 1° difference in temperature (in Kelvin) between inside and outside, every square metre of wall will lose 1.5 Watts of energy (Energy Saving Trust 2011). Insulation which fills the cavity will reduce the heat loss to a value of around 0.5W/m2K. In other words, the insulated wall loses 33% less heat than the uninsulated one.

When choosing insulating material neither the thickness nor the weight matter (as far as thermal resistance is concerned). Two materials with the same U-value should insulate equally well regardless of any difference in thickness or type of material. For example: 100mm of mineral wool could have the same U-value as 12mm of calcium silicate sheet.

Remember: The lower the U-value the better.

R-values are a measure of the thermal resistance of a material, or how well they insulate and are likely to be displayed on the packaging of insulating material. Materials with a high R-value are poor conductors of heat and so make for good insulators. The higher the R-value the better.

°Kelvin Vs. °Centigrade

1°Kelvin is the same as 1°Centigrade but the difference is that Kelvin starts at absolute zero (0°K or -273°C) and is recorded in positive numbers only e.g. 20°K or 200°K; whereas Centigrade takes the freezing point of water as its start point so it can have negative (-20°C) or positive numbers (20°C).

To convert Kelvin to Centigrade subtract 273: for example, 273°K – 273=0°C. Another example: 373°K – 273 = 100°C. And to convert Centigrade to Kelvin just add 273.