Insulation is a vital part of your build or renovation project. Approach with due diligence, though. While proper insulation can cut your energy bills by as much as 40%, a badly done job can cause dampness and structural damage to your house. All insulation materials are characterized by thermal conductivity or K-value. It is measured in Watts per metre of thickness per Kelvin degree, W/m.k. Basically, the K-value shows how difficult it is for heat to pass from one side of the material to the other one. The lower the K-value, the better the insulation.
The way to approach eco insulation is to look at the grey energy that goes into producing the material. For example, if you go for a mainstream synthetic mineral wool, better leave the walls empty and turn the heating on. Thus you’ll at least save money on buying the insulation because the amount of CO2 impact of producing mineral wool and the surplus heat you need for an un-insulated house will be pretty much the same. So why bother?
Eco insulation materials with the lowest grey energy are sheep wool and recycled newspaper. The are more expensive than the mineral wool but they will save you money and reduce the environmental impact of a house eventually.