The frames are made up of ex. 50x100 (2x4) C24 timber.  We have decided to leave the timber untreated.  As the timber will be well ventilated and sheltered the risk of decay and insect attack seems to be low (it is more prevalent the when the moisture content in the timber increases above 20%).

By definition timber treatments are generally effective by being inhospitable to life, and the treated timber seems to come with long lists of health warnings!  We also felt that the use of harmful chemicals was not appropriate for our healthy eco-build.