Thermal Performance describes how well a structure responds to changes in external temperature, during the daily and seasonal cycles.

When you think about thermal, there’s a tendency to automatically assume we’re talking about retaining‘heat’. Now though, a large section of buyers and builders are looking to improve building performance beyond compliance,so natural insulation materials have a new opportunity. Exceeding regulatory requirements for thermal conductivity is easily achieved with wood fibre insulation.

To better understand thermal performance, first, consider a caravan parked outside a stone farmhouse in Cornwall on a summer’s day…

In the caravan, as soon as the outside cladding starts to heat up, the output is recorded within minutes on the inside face.Some heat will quickly transfer through the aluminium and lightweight insulation composite.On the face of the stone wall of the farmhouse, the heat is absorbed by the stone as it progresses slowly from the outside, to be delivered from the inside surface several hours later.

What is different about the caravan and stone house examples is that the heating is not at a steady rate.Heating from the sun varies throughout the day. The variability is known as ‘periodic heat flow’, and, fortunately, for the purposes of building design, it is almost entirely predictable.

This all relates to a massively overlooked principle of phase shift. This is defined as ‘the time it takes for heat generated by the sun, to transfer from the outside to the inside of the building envelope and affect the internal conditions’.


Illustration above: Four roof sections of the same thickness and equal U value but showing how phase shift can be extended simply through the choice of insulation material. The longest phase shift is 16 hours unsurprisingly provided by insulation with the highest density

It doesn’t take much to rapidly heat a building through a lightweight structure such as a roof. As we make better use of our living spaces through loft conversions, it’s not surprising that these spaces are often the hottest areas within a building. The obvious fact is that a roof is a very large solar collector.During warmer and sunnier weather, a building will absorb heat through windows, walls and roofs which, moves to the top of the building as hot air rises. If you are designing a loft living space or vaulted ceiling, then it is important to remember a timber roof is a lightweight material with low thermal storage capacity. So, we often end up with bedrooms and living spaces that get too hot and create uncomfortable sleeping conditions. In most cases, by the time we find this out it’s normally too late. The best time to decide to fix this problem is before you build.

Design in the moderating effect of wood fibre into the roof and walls.If you are using a lightweight structure like timber or steel frame if that is not possible there are a number of options to use wood fibre as a retro fit to achieve the same results. To give you a ‘heads up’ on what a difference material choice can make see the table below.

Wood fibre products offer higher levels of thermal storage capacity. As a building section heats up it absorbs the warmth and acts as a buffer, delaying the heat progress. The internal climate of the building remains comfortable – without expensive air-conditioning. The key is to use this thermal mass to design a building that will delay the heat transfer through the fabric of the building so that peak temperatures would never reach the internal side when it is already cooler outside.