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.
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, so you can keep your funiture like your wooden shelf or best rated mattress dried without worrying them getting wet.
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.
Today, thermal (air-concrete) blocks are made from pulverised fuel ash (PFA) and sand, plus a cementitious binder (lime/cement) and raising agent, which allows the mix to rise in its mould. They are lightweight and easy-to-handle, plus there’s minimal conductivity (which makes them relatively good insulators). Different thicknesses and strengths can be produced.
Increasing strength usually also ups the density, which reduces conductivity (so it’s less insulating, but offers better thermal mass). Good performance can be achieved with all these blocks; it’s about choosing the right overall wall makeup. Achieving the threshold U-value of 0.30 W/m 2 K is straightforward. Most big developers would do this with 300mm-thick walls, consisting of a 100mm external facing brick, a 100mm internal thermal block and a choice of insulations in the cavity gap.
This sits comfortably on a 600mm foundation (with 150mm safety margin on either side). In practice, you’ll want better U-values, which means you’ll need to increase the insulation thickness and/or opt for a more substantial internal block skin.
Alternatively, using a specialist insulation known as Cavity … Read the rest of article