The Significance of U-Values in Buildings and Construction.
What is a U-value?
The second law of thermodynamics says that heat will move from a hotter body to a cooler body, as explained by Flanders and Swann in 1964:
“Heat cannot of itself pass from one body to a hotter body…
…the second law of thermodynamics, which lays down:
That you can’t pass heat from a cooler to a hotter
Try it if you like but you far better not-er
‘Cause the cold in the cooler will get hotter as a rule-r
‘Cause the hotter body’s heat will pass to the cooler”
So, when you heat a building, the heat will escape. The U-value is a measure of how fast the heat will escape through each building element – walls, roof, floor etc. The greater the U-value, the faster the heat escapes.
The unit is W/m2K, meaning the number of watts of heat that will escape through one square metre of the element for a one degree temperature difference either side. (What is K? K stands for Kelvin, which is the international standard unit of absolute temperature, equal in size to degrees Celsius – absolute zero being zero Kelvin or −273.15 °C)
How is a U-value calculated?
Building elements are complicated. There are multiple layers, and some layers are made up of more than one material. However, there is a key document that sets out conventions to allow simplification while still achieving realistic results for the construction industry, and that is the recently updated Conventions for U-value calculations (BR443 2019).
ERS Consultants can calculate U-values for your particular element build-ups, and many insulation manufacturers provide free online calculators for typical constructions using their own products, including Knauf (for Dritherm), Kingspan, Recticel, Ecotherm, Celotex, British Gypsum etc.
The calculation takes into account the thermal conductivity – aka lambda (λ) value, or k-value – of all the component materials, measured in W/mK (watts of heat that will pass in a straight line through a one metre thickness for a one degree temperature difference either end).
The calculation also accounts for heat transferred via radiation as well as conduction. Some elements will have air gaps in them, such as a partial fill cavity insulation or the gap between rafters from insulation to breather membrane. The air in the gap has its own resistance to conduction (compromised by convection, which BR443 also deals with) but many insulation products have a ‘foil facer’, and this reduces the heat lost by radiation. The foil surface has ‘low emissivity’ (or low-e) and only works in this way if there is an air gap next to it (needed on both sides for multi-foil products). BR443 confers extra thermal resistance (= lower thermal conductivity) to the adjacent air gap to account for this phenomenon.
U-values in context
The smaller the building, the larger the surface area is in relation to its volume – the surface area to volume ratio increases as volume decreases. Dwellings are (mostly) smaller than commercial buildings, which is why good fabric energy efficiency is more critical for Part L compliance in dwellings. Low U-values are a major contributor to fabric energy efficiency, but so too are linear thermal bridging values. These are the straight line junctions between different elements e.g. the wall to roof junction. Good U-values are wasted if continuity of insulation at these junctions is neglected. Linear thermal bridging became more critical when Part L was last upgraded in 2014 and U-values had to get better.
U-values and linear thermal bridging details are passive design measures for the energy efficient building. They determine the energy demand required to keep the inside warm. The better it is, the smaller and cheaper the boiler or heat pump.
U-values in 2022
Things are getting tougher from 15 June 2022 when the new Part L takes effect. We are guessing that 125mm cavities might become the norm. There may be harder decisions for builders on thermal performance versus cost of materials. Material substitutions will become more risky. Photographic evidence of insulation will be required.
ERS Consultants are here to help you, get in touch to book in a CPD to help prepare yourself and the team for the future. Contact us on info@erscltd.co.uk