At GoSustainable we install cavity wall insulation for a large number of sustainability projects. While the products we use are innovative, effective and straightforward to fit, understanding how much they reduce heat loss can be less obvious.
One of the most common questions we get in this regard is about ‘U values’ – specifically: what are they? We thought it was time to shed some light on this piece of jargon – and help our readers get ‘under the hood’ of the sustainability revolution…
What is a U Value?
Broadly speaking, a U value is the capacity for heat loss in a building component – most commonly regarding walls, floors, windows and roofs. In some cases ‘U value’ is referred to by the (decidedly less elegant) ‘overall heat transfer co-efficient’ – and is expressed as Watts per metre squared Kelvin (where Kelvin is temperature), or in scientific terms: w/m2k.
To do the maths: if a piece of material has a U value of 1 w/m2k, for every degree of temperature difference between the inside and the outside, 1 Watt of heat from the warmer side would be lost through each square metre of wall. The lower the U value of a building material, the better: it means less heat is being transferred from a warmer interior to the exterior.
Why use U Values?
U values form the basis of any standard of energy or carbon reduction – they are the foundations of the energy efficiency improvements we carry out in homes and businesses. In the modern energy landscape, since almost every single exterior-facing element in a building project has to meet certain standards of thermal efficiency, U values provide a way to calculate and express their performance at a very early stage. Designers drawing up their building plans can test the feasibility of their project using U values – and we use U value calculations to ensure the work we carry out on a project is actually going to provide the owners significant savings.
What Types of U Value Should I Look For?
Obviously, getting U values as close as possible to ‘0’ is most desirable in terms of energy efficiency – but typical values for everyday building materials without insulation measures are as follows:
- Solid brick wall – 2.97 w/m2k
- Cavity wall – 1.37 w/m2k
- Single glazed windows – 4.8 w/m2k
- Pitched roof – 2.51 w/m2K
To comply with enhanced energy efficiency regulations, the government has introduced maximum U value limits for new buildings – which must be met before work can go ahead. Those limits are:
- Wall – 0.2 w/m2k
- Windows – 1/6 w/m2k
- Roof – 0.15 w/m2k
What kind of insulation do I need?
Any kind of insulation on roofs, walls or floors can help your building’s U value – but finding the right kind of insulation is important, both in terms of its thermal benefit and its ability to fit within the space available.
At GoSustainable, we recommend and install the popular Kingspan and Knauf ranges. We use both for internal wall insulation and Knauf exclusively for cavity wall and loft insulation. These are the typical effects of installing them:
Solid walls: Solid walls are less common nowadays, but prior to the 1930s, they were the norm. Insulating a solid wall building is typically very costly – to achieve the regulatory limit of 0.2 w/m2k, in a 225mm solid brick wall, would take 170mm of generic wool insulation. By contrast around 100mm of solid wall insulation would be required to achieve the same U value.
Cavity walls: Uninsulated cavity walls, built prior to 1996 will have U values ranging from around 1.6 to 0.6 w/m2k. Those built prior to 2010 will feature certain insulation measures, but may only reach a U value of 0.45. Using cavity wall insulation measures, we can help your building reach between 0.34 & 0.4 depending on the product used.
Internal walls: in the retrofit market, internal walls have a target of 0.3w/m2k – this is a significant component part of the national carbon reduction target scheme.
Floors: Kingspan offers insulation for draughty floors in the form of Kooltherm floorboards, which offer, like similar products for roofs and walls, U values of 0.02 w/m2k. When fitting floor insulation, additional work may be required – such as perimeter insulation or even the construction of a ‘floating’ floor.
Lofts: Loft insulation is now installed up to 300mm depth, with the Earthwool range from Knauf providing a thermal conductivity of 0.04 w/mK.
Roofs: Conduction through the roof accounts for around 25% of a building’s wasted heat – and an uninsulated roof space will have a U value of around 2.5 w/m2k. Kingspan’s roof insulation boards offer U values as low as 0.02 w/m2k and are only 40mm thick. Spaces between insulation boards should be filled with sealant after boards have been fitted into place.
U values are only a small part of our energy efficiency approach but we hope by shedding some light on this technical corner of the sustainability effort we’ll help people develop a greater understanding of how and why our work takes place. Understanding the science of energy sustainability is a way to make the decision to transform your home that little bit easier!