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 Therm will allow you to keep a 300mm wall thickness (more on this later). Blocks are bought from builder’s merchants and each retailer generally has their own favourite supplier. Shop around until you get the product you want from the trader that has the best supply arrangements – 100mm-thick lightweight blocks bought in a bulk order are likely to cost in the region of £15-£20 per m 2 .
A neat way to improve performance is to go for thin-joint. The blocks used are basically the same (albeit larger). The main difference is that a thin layer of adhesive grout is used to bond the units together instead of the standard 10mm mortar bed.
Normally, the sand and cement mortar paste makes up roughly 7.5% of the wall; and as it’s a highly conductive material, it reduces thermal efficiency. By comparison, the thin joint option makes up closer to 1% of the wall face, meaning a more insulated and airtight system.
Porotherm blocks might suit any self-builders keen on a more natural product. Made from clay, the hollow core of these units mean they can be used as either inner or outer skins; alternatively, they can form a single, monolithic solid wall. While these unit’s credentials may be more ecological, Porotherm blocks are more conductive than concrete units (with a K-value of 0.29 W/m 2 K in comparison to 0.11 W/m 2 K).
The main choices are large rigid-foam PIR (polyisocyanurate) boards; EPS (expanded polystyrene) sheets; or mineral wool – the generic term for rolls or semi-rigid batts of fibreglass or rockwool.
EPS and mineral wool have similar K-values, whereas PIR is significantly better. Insulation can be added to the wall cavity in two ways. One option is to partially fill the gap, leaving at least 50mm clear.
The other is to fully fill the void; however, this isn’t recommended for very exposed locations (on the coast, for example) and any facing brickwork wouldn’t be allowed to have recessed mortar joints, which could affect your choice of external finish.
You can fully fill the cavity with a mineral wool product or with PIR Cavity Therm, which has a 5mm factory-installed plastic protective outer layer.
Kingspan has developed a product that reduces the retained gap to 10mm. Generally, however, PIR isn’t allowed as a full-fill option as it could react with the outer skin’s damp mortar. Let’s assume you want to achieve the Building Regs notional dwelling U-value target of 0.18 W/m 2 K. You could achieve this within a 300mm build-up with 100mm lightweight blocks and 100mm of Cavity Therm insulation
Alternatively, you could increase the overall wall thickness to 350mm and use 100mm PIR in the gap with a 50mm cavity between the insulation and brickwork.
This option also works with no cavity gap by using 150mm mineral wool as a full fill between the block and brick. In terms of cost comparison, 100mm thick PIR comes in around £15 per m², whereas 150mm of semi rigid mineral wool insulation would cost £9 per m².
Another option is to add insulation to the inside face of the blockwork by opting for laminated plasterboard. With this product, varying thicknesses of PIR insulation can be glued to the boards in the factory to achieve the spec you want. The result will be a dry-lined internal finish, secured to the walls with wet plaster dabs and then skimmed or taped and jointed as required.
This technique means the 300mm core wall can be retained, with a 100mm full or partially filled cavity, plus the laminated plasterboard hanging on the inside of the block skin. So it will increase the overall wall thickness, but won’t change the bearing on the foundations.
There are also opportunities to introduce conduits for services with this method, which would make them easier to install later on.