It is the kind of question which doesn’t normally get asked until planning permission is out of the way and the client is undertaking the detailed design. But it is important to consider the different insulation options early on in the project, as it could have a big impact upon the wall thicknesses, appearance, as well as the environmental targets for the scheme.
What are the options?
There are a range of products on the market which broadly fit into three categories: Oil based polymers, mineral and natural. Oil based products are normally rigid boards made from polystyrene (EPS or XPS) or polyurethane from suppliers such as Celotex and Kingspan. Mineral products include rock and glass wool. These two catagories have dominated the construction market for many years but are not necessarily the most effective. Recently there has been a resurgence in the use of natural materials available including wood-wool, woodfibre, sheepswool, cork and hemp.
In England, the current building regulations give the performance requirements for insulation in buildings. The unit of measurement given is called the U-value (W/m2K) which is the amount of heat lost through a meter squared area of the walls, floors etc, per 1 degree temperature difference between inside and outside. It sounds complicated! But remember the lower the U-value, the better the performance of the building element. Current regulations state that the maximum U-value for a wall should be 0.28 for an extension and 0.16 for new builds.
Clients, especially self-builders may decide to go for insulation levels which are better than the governments minimum targets, so as to improve the energy efficiency of the building. This will provide long term savings in terms of energy costs, bring better comfort to the space. In my Wellington Lane project we adopted a Passivhaus approach and achieved a U-value of 0.10 for the walls, with a build-up of 300mm mineral wool between timber I-joists clad with 100mm woodfibre external wall insulation. This made the walls very thick but the house super toasty! I will look closer at this project and the materials used in a future post.
Which insulation performs best?
Rigid synthetic board products such as Celotex are, as a rule of thumb, twice as good as mineral wool and natural materials such as sheep’s wool and wood-fibre. But remember that is not the full story. The performance can be bumped up with more insulation so it is worth considering the overall costs of the product, installation and other factors which are important to you.
Which is cheapest?
It used to be that rigid board was the most economical option even though it has always been the most expensive, because much less of the material is required to hit the required U-values. But over the last few years the price has gone up considerably, and now there is much more of a level playing field compared with the eco products which are far better for the environment.
Which is most environmentally friendly?
Whilst synthetic rigid-boards are an effective insulator they come with great environmental cost to the planet. They are manufactured from petrochemicals which cause resource depletion and pollution risks from oil and plastics production, and have a high embodied energy. Natural products such as wood-fibre can lock carbon carbon inside the material reducing the impact on global warming and generally have a lower embodied energy.
Other positives for natural insulation
If you are undertaking renovation work to historic buildings it is important to use a breathable material, such as wood-fibre insulation, which will not lock moisture within the structural fabric of the building. Natural products such as sheep’s wool have proven to reduce problems with asthma, and help to create a healthy internal environment.