Posted on: 5th May 2017


At Back to Earth we are fortunate enough to work on some inspiring building projects with some truly innovative partners. One such partner is Carpenter Oak, whom we’ve had the pleasure of working with on a number of new builds.

One of things that attracted us to partner with Carpenter Oak was their genuine interest and open mindedness to insulation systems other than those considered to be conventional at this time.

Modern construction has called for ever lower U-values and ever more airtight buildings. Correspondingly though, there has been a move towards the use of very lightweight, high performance insulation, on the assumption that this will build better buildings.

As well as offering the use of synthetic products Carpenter Oak are looking to offer their clients a variety of choices that would fit well with their natural material ethos and commitment to improve the energy efficiency of their buildings.

The following article goes on to explain the practical reasons why Carpenter Oak are offering their customers a choice in the way they insulate their buildings with the use wood fibre insulation.

Simplicity of construction

The use of rigid foam insulation boards and very lightweight glass wools requires the use of vapour barriers to prevent air leakage from the interior causing condensation in the structure. Rigid insulation boards also require that they are cut very accurately to fit between timber structures and that all of the joints are taped and all of the junctions sealed.  The importance of the seals and vapour control becomes more important as you increase insulation levels as condensation risk also increases.

Externally, these insulation materials require breather membranes, ventilated cavities and render carrier boards to finish the exterior of the building off. Each of the panels requires an allowance for the timber to be able to shrink and move whilst still staying weather and wind tight. Finally, because many insulation materials do not allow the passage of moisture, the panels also have to be kept dry on site, a significant challenge in our climate.

These requirements can amount to quite complex construction making it difficult, if not impossible, to create a building that performs as it is designed to.

Wood fibre insulation simplifies the construction process by allowing a much simpler timber structure to be used. It requires fewer layers and does not require any cavities if forming a rendered facade. Flexible wood fibre insulation is very ‘springy’ and does not slump making it ideal for use around timber as it accommodates any shrinkage without forming gaps.


Increasing robustness and reducing risk

Rather than using easily damaged membranes, tough OSB boards can be used to form the internal lining of a wood fibre insulated wall. The joints in the boards are taped with highly elastic air tightness tapes which ensure very high levels of air tightness, even as the timber frame shrinks.

Flexible wood fibre batts are used between the studs of the timber frame. This is a soft but dense form of wood fibre which can be squashed slightly into cavities but is not so soft that it allows voids to form around itself. It is quite a moisture absorbent material so does get wet if rained on, however it also dries extremely quickly and ensures that any moisture is drawn away from the timber structure. This makes it much safer to use than mineral wool or rigid board insulants as it allows the frame to dry quickly once weathertight, preventing mould formation and rot occurring.

The outer layer of the wood fibre insulation system is a rigid wood fibre board. If the facade is being clad then this is a weather resistant insulation board which does not require an external breather membrane. If a rendered facade is desired then a renderable wood fibre board, such as Diffutherm, is used.

Both types of boards use a tongue and groove profile to ensure weather tightness but also wind tightness. They are also highly vapour permeable, ensuring that no construction moisture gets trapped in the finished building and that the timber frame remains dry from there on.

Small air leaks in the envelope of the building can transport large amounts of moisture into the fabric of the building during the winter months. Wood fibre insulation is not only vapour permeable but very capillary active, meaning it can transport moisture very quickly down each wood fibre. This ensures that the moisture leaking in to the fabric of the building can be quickly dispersed and allowed to evaporate from the exterior of the building.

Increasing simplicity also ensures that the building is built as designed and therefore performs as designed, eliminating the well known ‘performance gap’. This also allows Carpenter Oak’s teams to build faster and smarter with much less waste to landfill.


Optimising the living environment

Wood fibre insulation is slightly less of an insulator in the traditional sense than rigid foam boards. However, because of the simpler construction, significantly lower thermal bridging values and lack of cavities the overall thickness is rarely thicker and may even be thinner.

Where wood fibre really excels is in acoustic insulation and reducing or eliminating overheating risk. A standard wall build-up will typically offer noise reduction of around 50dB, making most common traffic and weather noise inaudible. The most common remark people make when entering buildings with wood fibre insulation is how quiet it is.

Flexible wood fibre insulation stores around 20 times more heat energy than the equivalent volume of glass or mineral wool. Typical wood fibre boards will store around 12 – 15 times more heat energy than foam boards. This ensures that the internal temperature stays very stable all year round and little of the summer sun’s heat passes through the walls to cause over heating.


Wood fibre insulation can create the most comfortable, peaceful, healthy and low energy spaces. Carpenter Oak’s decision to incorporate these materials into their portfolio allows them to continue to create the incredibly beautiful buildings that they always have but also to take the living experience to a new level.

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