Summary
• Wood fibre insulation provides its own “management system” for temperature control.
• “Uncomfortable hours” of excessive heat can be reduced by almost 50%.
• Thermal diffusivity, amplitude dampening and phase shifting are all benefits of wood fibre insulation.

Worldwide, far more energy is used in keeping buildings cool than heating them. In the past this may not have concerned us in the UK. But as our 30°C “tropical days” quadruple, bringing uncomfortable and unhealthy extremes in summer while cold damp winters remain a persistent problem, can we learn something from experts in protection from the heat?

In the UK the focus has always been on winter warmth and even here progress has been painfully slow, with some 27 million existing homes still short of effective insulation.

The new build market, especially the timber frame sector, is responding much more enthusiastically today. Wild fluctuations in energy prices, coupled with the march of global warming, mean everyone knows that energy conservation and cutting emissions are vital.

One material stands out as a natural insulation partner in timber frame construction – and as the leading performer in summer heat protection as well as winter warmth and a healthy living environment.

Temperature control

Wood fibre, with its built-in breathability and higher density than other materials, provides its own ‘management’ system for temperature control, minimising the need for artificial heating, air-conditioning and additional ventilation.

In Continental Europe wood fibre insulation has grown remarkably over the last five years, becoming a lifestyle product increasingly specified by householders and developers alike. Engineers in Germany focus on the fabric of the building as first line of defence against the elements. A one-bedroom house was used to illustrate that “uncomfortable hours” of excessive heat could be reduced by almost half using wood fibre (density 45kg/m³) instead of mineral wool (density 15kg/m³). Using wood fibre sheathing boards (density 150kg/m³) improved the differential with mineral wool to 60-75% (graph 1).

For timber frame specialists, making high performance insulation an integral part of the construction process for roofs, walls and floors can be easily achieved in both new build and refurbishment solutions.

With wood fibre also being used for the web component in some I-joists and insulated wall studs, the timber-based integrity of overall designs can be maintained in parallel with energy- and time-efficient engineered construction throughout the fabric of the building.

But wood fibre has even more to offer the insulation-conscious timber frame builder. Thermal diffusivity, amplitude dampening and phase shifting are all significant benefits from insulating with wood fibre. They are as vital for summer heat protection as the U-value is for thermal insulation in winter.

Low thermal diffusivity

Low thermal diffusivity or a slow temperature transfer time is a benefit of materials with low thermal conductivity, high density and high specific heat capacity. Amplitude dampening shows the strength with which temperature penetration is reduced. Phase shift shows by how many hours the maximum temperature penetration is delayed.

It is particularly important to consider amplitude dampening and phase shift in roof areas where greatest overheating occurs. In summer, areas directly below the roof can reach 80°C, which increases room heat dramatically. With the exception of the roof cladding and the internal plasterboard, the thermal mass of the roof construction is entirely reliant on the insulation. It is therefore vital for amplitude dampening and phase shift that insulation with a low thermal diffusivity is used.

Compare two roofs which both have a U-value of 0.18W/m²K. Using mineral wool with thermal conductivity of 0.035 and density of 20kg/m³, amplitude dampening of 6 and a phase shift of 6.8 hours are achieved. This results in an under roof temperature of 29°C at 8pm and rooms far too hot for comfortable sleeping. With external temperature also at 29°C there is no respite in opening a window.

In contrast, wood fibre insulation, such as Steicoflex, with the same thermal conductivity but a density of 50kg/m³, increases thermal storage mass by five times due to its improved thermal storage capacity. Amplitude dampening rises to 12 and the phase shift to 11 hours. The result is less internal temperature fluctuation and a more comfortable environment. It’s cooler by day yet retains temperature overnight, typically 21°C at 1am when the outside temperature has fallen to a chilly 15°C (graph 2).

Of course, the comparative performance in heat protection also applies in reverse to provide warmth in winter.

Versatility

The versatility of wood fibre is a further benefit to the timber frame builder. A wide range of both flexible and rigid wood fibre products are manufactured specifically for walls, floors and roofs. Insulation can be applied internally or externally and combined with energy-efficient I-joists, wall studs and even breathing external finishes such as lime render. Installation is easy as the material can be cut and shaped on site and panels are often provided with a T&G profile. Overall, wood fibre maximises insulation – whatever the weather.