The external walls will be vital in controlling the thermal efficiency of your new house. Tim Doherty explores the energy performance of key brick and block systems

Tim Doherty

Kingspan Kooltherm K108 Cavity Board is a high performance insulation option with a composite foil facing; it’s used in partially filled cavities

Although there is a great deal of current debate about Building Regulations not changing fast enough to keep up with modern efficiency demands, the rules regarding external wall insulation are an exception. Gone are the days of simply picking an outer brick and an inner block and adding in some arbitrary cavity insulation; today, the process is a fine (and very calculated) art.

THE SIGNIFICANCE OF AIRTIGHTNESS

External walls make an important contribution to airtightness, which is key to overall energy performance. It’s all very well spending time and effort selecting products, but if the interface and detailing is poor then the house could suffer from heat escaping. The Building Regs set an air leakage limit of 10m3/hm2 at a regulated pressure of 50 Pascals – but in practice you’ll probably need to achieve at least 5.0m3/hm2, which is effectively five air changes per hour under testing. By comparison, a property meeting Passivhaus standard would have to achieve 0.6m3/hm2.

Cavity wall principles

Having two layers of brickwork with a space between them is known as cavity wall construction. This method was first introduced in the early 20th century as a way to provide better protection against penetrative damp and to help keep the inside of walls dry. Dense concrete blocks started to be used for inner cavity layers after the Second World War, with lighter weight blocks (with air insulation) introduced in the 1960s and 1970s. A decade later and insulation started to be introduced into the cavities.