Insulation – U values vs decrement values

There are a range of issues with insulation and a major overlooked factor is decrement.

Insulation values are normally measured in U (the lower the U value the better the insulative properties). This measures the thermal resistance of products from one side to another. However this does mean that it does not take into account the time lag / decrement that certain insulations can give. So light weight insulations can be good at insulating, but they do not ‘store’ the heat within themselves. So a good U value does not necessarily mean a comfortable internal environment. A classic form of this is can be found in loft conversions. Often these get really warm in the sun despite lots of insulation.

Good decrement values can be found in heavier weight materials as these effectively store up the heat as well as providing resistance to heat loss. So the time delay in the heat passing through can become very important. Decrement is measured in hours, so to have a system in place that can delay heat passing through by 12 hours will even out the diurnal differences between night and day. This is especially important where you have a large difference in temperatures between day and night. In the UK this difference is not so pronounced and so we can do with a shorter time lag.

Heavy weight materials are not always very insulating though, despite having a high decrement value, so having load of heavy blocks is not the way to go either as these will not keep heat as well in the winter. This can be seen with older buildings where they have solid walls. The heavy stone and brick helps keep the building cool in the summer, but they are also cold in the winter and require higher amounts of heat to keep the warmth to a modern day acceptable level.

So the best solution is a heavyweight insulator. Does one exist?

Thankfully yes.

One of the best product types is wood fibre boards. These have an insulation value similar to sheep’s wool, fibreglass, mineral wool, hemp etc,  but also have a decrement value of 1 hour per cm of thickness. So a 80cm board will give you an 8 hour decrement value. So in the high heat of summer(!) it will take 8 hours for the highest peak temperature on the outside to be transmitted to form the peak temperature on the inside. This delay by absorption of heat can really help to balance out the temperature fluctuations in a house and certainly in areas like loft conversions.

Due to the insulation values associated with wood fibre boards as well they act as great insulation in the winter.

Another advantage of using wood fibre boards is that they are breathable and hence can be used in conjunction with older solid walled buildings to preserve the overall breathability of the structure. Fibre boards can be used as internal or external insulation and work really well with lime render / plaster.

There are several manufacturers in Europe (but none in the UK). Some of the better known brands include:





Eco Home Centre can access Hofatex boards if required.