Choosing Insulation

The table below summarises most of what you need to know about insulation. There are three or four different elements within every house that you have to consider, these being the ground floor, the walls and the roof, which can be either a flat ceiling, a sloping roof or sometimes both. These elements each have a column in the table.

ABOVE: The wool-types of insulation sold in rolls are the cheapest option, but they are only useful when laid flat, typically above ceilings.

Next you have to consider how much insulation is required in each element. Here you must reference the Building Regulations which lay down minimum requirements. There is no reason why you shouldn't exceed these levels and indeed many self-builders choose to do so, but the current standards make a convenient point of reference to enable comparisons between different insulation. The current U value requirements for each element are indicated in the top row of the table.

The U value (W/m²K) is a theoretical measurement of the insulation capability of your wall, roof or ground floor. The lower the U value, the more insulation is required. So when you see a U value of 0.16 or 0.27, you can work out from reference tables exactly how much insulation is required to meet these standards. Every building material you use contributes something towards reducing heat loss, but bricks, blocks and timber are not fantastic insulators in their own right and the dedicated insulation materials tend to do around 90% of the work in this respect.

Most commercial insulation is based on the principle of trapping air and these tend to perform to a pretty similar standard. This group includes all the wool-type insulations, such as mineral wool, sheeps wool and blown fibres, and also expanded polystyrene.

Insulation options for new dwellings

POLYURETHANES: There are also a number of plastic foam, polyurethane-based materials which are gas-based - the best known names are Celotex and Kingspan - and these are considerably better, in insulating terms. These appear in the table as polyurethane-type boards. Whilst they tend to be expensive, when compared with the more basic air-based materials like mineral wool and expanded polystyrene, they are increasingly popular where space is at a premium, such as in sloping roofs.

EXTRUDED POLYSTYRENE: Another widely used insulation is made from extruded polystyrene. This is denser than the expanded version and is widely specified under floors and where there is contact with the ground, such as around basements. Leading suppliers include Dow Styrofoam and Knauf Polyfoam.

BOARDS, ROLLS OR SLABS? These are the three common formats insulation is sold in. The wooltype insulations are cheapest sold in rolls, but this is only useful when laid flat, typically above ceilings. The polyurethanes and polystyrenes are almost exclusively sold as boards: these can be used in any application, but are particularly good under floors where rigidity is important. Boards are often fitted in sloping roofs but this is slow work, requiring a lot of cutting. Walls can be insulated with either boards or slabs, a semi-rigid format which stiffens the wool-based insulation.

ABOVE: Sheeps' wool is a wonderful material to use in place of synthetic wools, such as those made of glass fibre and mineral wool.

BLOWN-IN: Insulation doesn't have to be dry fixed. Wool, fibres and plastics can all be blown into position via a nozzle. This is a widely used technique for cavity wall insulation and under roofs. Warmcel is one popular form of insulation, used widely in timber frame houses and lofts. Its made of cellulose fibre, made from recycled newspaper and summarised in the table as blown fibre.

BUILT-IN: A recent development is to see insulation built into the fabric itself, rather than being added later. Insulated Concrete Formwork (ICFs) supplies polystyrene wall panels, which are assembled on site as moulds for readymixed concrete. Structural Insulated Panels (SIPs) consist of wall and roof elements made out of insulation and boards combined into a load-bearing components. And pre-cast floors and now available using polystyrene formworks.

MULTIFOILS:Multifoils are a form of insulation based using thin rolls of aluminium foil alternated with layers of foam padding. They promise a very good insulation level, way in excess of their 25mm width, and have become a popular way of insulating sloping roofs. However, the powers-that-be in the building industry remain sceptical as to just how good multifoils are and currently advising builders not to use them on their own. Leading manufacturers Actis, YBS, Thinsulex.

NATURAL MATERIALS: The use of more natural insulation materials is becoming popular. Sheeps Wool is wonderful material to use in place of the synthetic wools, made of glass fibre and mineral wool which, whilst being very cheap, are not pleasant to work with, requiring face masks and overalls. Look out for Thermafleece. We are also beginning to see wood fibres and hemp being used in both roll and slab format, useful for insulating timber walls and roofspaces. Watch for products such as Homatherm and Thermo-Hemp.

Further reading:

• Solutions for insulating walls
• Installing loft insulation
• What will it cost? - Insulation