The use of Structural Insulated panels is a construction approach that is seeing more abundant use and is becoming a widely available method. Preformed units are usually a composite structure which often include a range of bio-based materials such as timber, wool or straw. Traditional laboratory based wood decay tests do not take into account this combination of biobased materials and it may be possible that non wood materials such as plant or animal fibre insulation could influence the durability of adjacent wood elements by synergistic mechanisms. One method may involve the insulation material acting as a moisture reservoir or moisture buffer for wooden structural elements with a consequential effect on their durability. This paper details an experimental approach to determine if there could be such an effect, using a modified form of existing wood decay testing methods. The test utilises an agar jar approach with a pad of fibrous insulation e.g. wool or hemp placed between the inoculated agar and a wood block. Results show the effects of the type of insulation used on the moisture content and decay of the test blocks. It is shown that moisture was able to pass through the insulation to varying degrees to be absorbed by the wood blocks. Fungal mycelium was then able to grow through some of the insulation materials to attack the wood. The test show that wool for example appeared to hold water away from the wood samples and reduced subsequent decay. Hemp however, did not prevent water access to the wood blocks and in fact enhanced decay of the blocks.

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