Why do we insulate?
More than two-thirds of the heat loss from an unheated home can be directly tackled by installing insulation, so it’s important to encourage greater confidence and understanding about how insulation works and how all-important U-values are calculated.
Insulation manufacturers can't set fuel prices or specify heating systems, but they can produce quality products that contribute to efficient building fabric. Asking why we insulate sounds like an obvious question, but aside from mandatory Building Regulation targets there is also thermal comfort, environmental impact and running costs to think about.
What do we insulate with? can be a harder question to answer. Different types and brands of insulation have their own characteristics and price-points, so how to distinguish between them to ensure the right product is used?
Where building fabric is concerned, we are mainly interested in how easily (or otherwise) materials conduct heat. Manufacturers declare a figure for this ability: thermal conductivity, or lambda value, expressed in the units W/mK.
Steel, for example, is an excellent conductor, which is why detailing it properly is vital to avoid thermal bridging. Its thermal conductivity is 50 W/mK, compared to dense blockwork at 1.20 W/mK, and a typical PIR insulation at just 0.022 W/mK.
The thermal conductivity of a material remains the same regardless of its thickness, so how do we state the performance of different thicknesses of the same material? Or find equivalent-performing thicknesses of different materials? Thermal resistance is the ability to resist heat transfer; a bit like the tog rating for duvets.
Resistance is useful
Dividing a material's thickness by its thermal conductivity gives the thermal resistance, with the units m2K/W. That calculation means two materials with the same thermal conductivity will offer the same thermal resistance when used in equal thicknesses.
Two insulation boards of equal thermal conductivity at, say, 100 mm thick will give the same thermal resistance, but it would take some six metres of dense blockwork to obtain the equivalent performance!
Few people refer to thermal transmittance by that name – most of us know it better as the U-value (units W/m2K). It measures heat loss through a complete building element, including material layers, air layers, continuous and bridged layers, fixings, and air gaps. Thermal resistances for each layer are added up and a reciprocal taken to give the U-value.
It is in everybody's interests that calculated U-values accurately represent what has been designed – and, more importantly, what is likely to be built.
What use is there in seeming to comply with regulations, only to pay more in fuel and heating costs when the finished building doesn't come up to standard? Calculations aren't about manipulating variables to offer the thinnest possible solution, regardless of whether it is achievable in reality.
The values for thermal conductivity and resistance used in a calculation should be accurate, based on reliable sources like European standards and manufacturer declarations, because consistency and quality of information - from specifier to manufacturer to contractor - should be the real values we prioritise.