Diary Of An Eco-Builder

Oak bark sounds like the appropriate insulation for tree house, but we need something better

I've been dreaming of insulation for months, but last week we finally fitted the first pieces into Tree House. Grabbing the opportunity of a dry day between the storms, all hands were on deck - the roof deck - to build the thick layer of insulation between the roof's plywood sheathing and the plastic rainscreen that will support our solar power station. By the end of the day the job was done, and the roof was covered in a bright blue tarpaulin just in time for the next downpour.

Our super-insulated roof is not graced with any of the materials alluded to in my dream: hemp, sheep's wool, cork and recycled newspapers. These are all excellent insulants, but - although cork (the bark of the oak Quercus suber) would perhaps be the most fitting choice for Tree House - our roof and floor are in fact being packed with a synthetic product, Kingspan Insulation rigid phenolic boards (www.insulation.kingspan.com).

The key issue with all insulation is to select the most appropriate material for the task. Bark may be all that a tree needs to protect its living tissues from the extremes of the elements, but our "zero carbon" house needs something more substantial. We have chosen rigid phenolic boards for the roof simply because they keep the heat in (or out) better than any other material on the market.

The most important characteristic of any insulant is its thermal conductivity, so always check the value of this when making comparisons. The thermal conductivity of a material is the rate at which heat flows through it when the temperature on one side is greater than the other. This is standardised as the rate of energy flow (in watts) through one metre of material for one degree difference between inside and outside (in degrees Kelvin - the same scale as Centigrade, but starting at absolute zero). Typical values are 0.034 W/mK for mineral wool, 0.037 W/mK for sheep's wool and 0.022 W/mK for phenolic board. The lower the conductivity, the more effective the insulation.

Beyond this bottom line, there are lots of other issues to consider in choosing insulation, such as the size and shape of the space; moisture and fire risks; and the potential for poor installation to undermine effectiveness. For example, we will be spraying Warmcel 500 recycled newspaper (www.excelfibre.com) into our walls because the nooks and crannies of our "I beam" wall studs would not be adequately filled using only rigid boards.

Synthetic insulation materials have often been dismissed by eco-builders because of the blowing agents used in their manufacture. These were originally ozone-depleting CFCs, subsequently replaced by marginally less nasty HCFCs and HFCs. Happily, Kingspan Insulation now produces all its insulants using pentane, which has minimal impact on either the ozone layer or global warming, and any energy used in manufacture is rapidly off-set by its energy-saving performance once installed.

For advice about the insulation in your walls, wall cavities, floors, loft or roof, see the Energy Saving Trust website (www.est.org.uk) or phone 0845 727 7200.

In years to come, our super-insulated roof will keep us cool in hot summers as well as warm in cold winters, so we should sleep soundly beneath it. With any luck, my dreams will be more peaceful too, with all those giggling right-wing sheep long since chased into the wilderness.