Old houses are seen as being a real problem with regards to carbon emissions. Their walls are seen as being really inefficient and difficult to treat. When we use computer programmes to calculate how efficient our homes are we use something called rdSAP (this is Reduced Data Standard Assessment Procedure). With this a U value figure of 2.1 is used. U values are a measure of how thermally efficient a building element is. The lower the U value the better the insulation of the element. New housing is commonly built with a designed U value of 0.3 in the walls. So 2.1 looks really bad. However, research has recently shown that most solid walls actually have a U value of between 0.7 and 1.6, so a lot better than we give them credit for.
Nevertheless, with the need for carbon reduction, we do need to improve the efficiency of this type of housing stock (the old terraces, country cottages, barns etc.) So how best to do this?
Before we can start to look at options we need to understand how walls work and this involves looking at how they were made and what they were made of. For more info on this look at the blog below South Wales’ VictorianTerraces – Home, House or Castle?
So given that we need to preserve breathability what can we use?
There are a number of options here, assuming that your house still has its breathability in tact. If you have cement render on the outside that is trapping in moisture, just by removing this and allowing the walls to dry out you will see a 30% improvement in the thermal performance of your walls (approx.)
The main options you have are based on where you can insulate. Insulation can be placed on the outside or the inside. Ideally you will look at external solutions. This keeps the thermal mass on the inside and also reduces any risk of frost damage to the outside (insulating on the inner walls reduces heat flow and can cause frost to come further into the walls).
External insulation options are:
Woodfibre boards (these are wood boards that are mechanically attached to the wall and then rendered over or overclad with timber / brick slips). Remember that you will need to use a breathable render and paint on the finish. For more info visit some of the suppliers websites. Eco Home Centre can access the best (and oddly the cheapest) system if required.
Lime render with added insulating aggregate. This can be vermiculite, perlite, hemp or just a stone dust. All of these are natural insulants and allow moisture to pass through. They can be applied in varying depths, but should be blown onto a wall rather than troweled. A specialist contractor should be employed for this. Eco Home Centre would recommend Welsh Lime Works or Vale Lime.
Internal insulation options are:
Insowall insulating render / plaster. This is a modern, but breathable, product based on lime and nano-technology insulating material that replaces render. So this is great where space is at a premium. It can be put on up to 40mm thick and will improve the insulation value around 70%. I have used this at home and been impressed by it. However read the instructions on the bag, as some conventional plasterers will try and use it like normal plaster. Can take a clay or lime finishing top coat.
Hemp render / plaster. This is a lime plaster with hemp as an aggregate. A good insulator, but needs to be put on quite thickly to get good values.
Sheeps wool or wood fibre insulation blocks. These are similar to the external renders, but designed for internal use. Mechanically attached to the wall and then rendered over.
Studding out the wall with timber and infilling with a natural insulation material like recycled celloluse, sheep’s wool, hemp or flax. Remember here to use a wood wool board to provide your internal finish for plastering.
PLEASE, please, remember NOT to use conventional dry walling products like insulating plasterboards, phenolic boards etc. These are not breathable and can cause no end of damage to the walls by trapping in moisture and hiding any damp and mould.