Avoid
- Cement / gypsum plaster
- Vapour barriers
- Closed-cell insulation
- Spray foam insulation
The science
Insulation needs to completely cover an area to be effective; any gaps in or around the insulation will dramatically reduce its performance. Most modern insulation is marketed focusing on the U-value, but there are other things to consider when choosing an insulator. Thermal mass: the ability of a material to absorb, store and release heat. K-value: thermal conductivity, or the ability to conduct heat. U-value: thermal transmittance, the rate of transfer of heat through matter.
Insulated lime render and plaster
Render is the best method to insulate an older property, as it forms a breathable and insulated envelope around the property. Plaster keeps heat inside and stores it, so your home stays warm long after the heating is turned off. Your solid wall being insulated will reduce the amount of condensation forming within the wall, so it can escape more easily as water vapour.
Closed-cell insulation
Used mostly in modern construction, and often used on period properties despite its flaws. This insulation isn't breathable and doesn't actually store heat: once the heating is turned off the heat will quickly pass through it and escape through any gaps, leaving you sat in the cold in no time. In summer this magnifies the heat and is often unbearable to live with.
Spray foam insulation
This can be extremely damaging to your property as it traps moisture in your roof. There have been many cases of it causing the trusses to rot, leading to roof collapse if not dealt with. It is difficult to remove and can often lead to your property being difficult or impossible to mortgage, sell or insure. It should be avoided like the plague.
Choosing an insulator
Closed-cell insulation looks great when they show you the U-value; that's how they market their product. It has no place in old buildings, as it isn't breathable. Anyone who lives in a modern home insulated with these products also knows that the house soon gets cold once the heating is turned off. That's because it does not store heat; these properties also get uncomfortably hot in the summer. Thick, solid walls are best for keeping cool. When choosing an insulator, it's important to look at all the factors involved, not just U-values: lambda (k-value) thermal conductivity, where the lower the value the better; thermal mass, a material's ability to absorb, store and release heat, where the higher the mass the better; acoustic absorption on a scale of 0 to 1, the higher the better; and vapour permeability (breathability), the ability for water vapour to pass through a material. To reach desired U-values, you must take all aspects of the walls into account. You can simply increase the U-value by increasing the thickness of the insulation used. Insulation works best when air cannot pass through or around it; this simply proves that certain retrofit methods and cavity walls with air gaps are not always the best methods for insulating buildings.
Key points
- Choose a breathable insulation method for a solid-walled property
- Insulated lime render forms a breathable, insulated envelope
- Look beyond U-values: thermal mass, conductivity, acoustics and vapour permeability all matter
- Spray foam can rot roof timbers and make a property hard to mortgage, sell or insure