This is a question we are asked pretty regularly. Understandably, if you think that insulation is a one-dimensional issue then why indeed would you spend extra on anything but the cheapest, highest performing insulations available?
As with everything in life it is not that simple. There are several areas to consider in addition to the thermal performance and they are the heat capacity (thermal mass), the ability to store and transport moisture, acoustic performance and finally ease of construction. Natural fibre materials, such as wood fibre insulation perform well in all these areas creating a much more pleasant environment within buildings. They also tend to perform at or beyond the target levels of insulation because they are simple to use, they can be installed simply without barriers or ventilated cavities and they use moisture to actually improve their thermal performance.
Heat capacity or thermal mass is a property which is continually being shown to be beneficial. In the winter months it creates a much more stable environment and so heating systems can work at much steadier (and therefore more efficient) rates. In the summer the mass can not only absorb excess heat but it can delay penetration into the building, again creating much more stable and comfortable conditions inside. This can reduce the amount of heating required by up to 20% in buildings with high thermal mass.
Moisture storage and transport is an increasingly important topic as we begin to internally and externally insulate our existing buildings. We live (in the UK) in a maritime climate with some extreme rainfall but we also produce huge amounts of moisture inside our houses from bathing/showering, cooking and just breathing! In addition to this we generally have poor ventilation rates which combine to allow lots of moisture to flow into our walls. Without insulations that can store and transport moisture mould can grow, timber can rot and structures can decay. Again, natural fibres are used to store and transport moisture in plants and do so as insulation without affecting their thermal performance. In some cases the performance is enhanced by this low level humidity.
As more lightweight buildings are built and more super lightweight insulation is used acoustics get worse. Sound transmission (or lack of it) makes a big difference to our sense of privacy and comfort within our homes and noise is a major source of stress. Wood fibre insulation is one of the densest forms of insulation available and is excellent at absorbing sound, creating a quiet, warm inside space. This is ideal for high density housing or attic spaces. Interestingly, the lower frequency sounds are absorbed more readily than some of the higher ones, meaning you can still hear the birds sing outside!
Finally, ease of installation is vital so that what is designed is actually what is built. The biggest problem with very low conductivity foam insulations is that they are rigid and are very difficult to install correctly on a building site. Unless they are in complete contact with the surface they are insulating they do not work as expected. So much so that studies have shown with a 3mm gap between the board and the insulated surface (inner blockwork leaf or interior of timber frame panel) you can have 150% more heat loss and with a 10mm gap you can have more like 400% heat loss. Looking in to many cavity walls it is rare to see less than a 10mm gap between the inner leaf and the insulation hence the very poor performance of most cavity walled buildings.
With many natural fibre systems the walls are designed to be solid, with no cavities as there is no likelihood of condensation forming (one of the purposes of ventilated cavities is to remove the condensation which forms). Even systems which are used in exposed locations are solid walled but use the correct combination of vapour control and vapour permeable renders to prevent rain penetrating but allow moisture to be released from the structures. Because of the simplicity with which these structures are built the performance of the finished buildings is normally at or beyond required targets.