Everything you’ve always wanted to know about U-values - yes, really!

Aware that thermal measurement is not top of anyone’s list of fascinating topics, insulation specialist Actis has created a handy guide to U-values – complete with reassurance that building control officers can take a sympathetic view on targets when it comes to older or listed building refurbishment projects.

Dan Anson-Hart, the company’s national specification manager explains.

A U-value is the measure of the rate of heat flow through a wall, floor or roof. The lower the number the slower the heat flow and the more efficient it is.

U-values are expressed as W/m2K – the rate in watts at which heat transfers through a square metre of the surface of an element when there is a temperature difference of one Kelvin between the inside and outside of the element (that is, a wall, floor or roof).

A U-value doesn’t apply to any individual material used in the build-up, but to each of these elements as a whole. So a brick won’t have a U-value, but a build-up consisting of brick, block, mortar, insulation and so forth does have a U-value.  What each material component does have though is an R-value – or thermal resistance, which is the resistance to the transfer of heat across the material.

While U-values need to be as low as possible for maximum efficiency, R-values have to be as high as possible. Think of water attempting to get out of a narrow tap compared with it just cascading over the edge of a weir. The latter is a much easier route and the number of gallons per minute achievable is far higher in the latter scenario – the aperture of the tap gives far greater resistance than the edge of the weir. R-values are shown as m2K/W.

U-values don’t have to be as mind boggling as they seem. And we’ve tried to make them a little more palatable by drawing up tables, creating graphics and devising an online U-value simulator (whoever thought those words would all be used in one sentence!)

Part L of the building regulations governs the conservation of fuel and power (aka energy efficiency) of dwellings – with Part L1a looking at new builds and Part L1b focusing on refurbs and extensions including loft conversions.

We’re going to be concentrating on roofs on this occasion – partly because loft extensions are probably the number one home improvement in the UK, making materials required for these projects a high priority for merchants.

While builders are doubtless aware that the U-value requirement for a pitched roof in a loft conversion is 0.18, what we’re finding is less well known is that some builders are giving themselves a hard time trying to achieve that target when it is simply not physically attainable. And here’s the get out of jail free card - in some circumstances it doesn’t have to be this way.

In cases where ceiling height is limited and thus won’t be able to accommodate sufficient insulation – often in an older or listed buildings – building control officers can be prepared to relax the U-values to 0.35 – making that seemingly impossible loft conversion project feasible after all.

POS how to guides, which are also downloadable, can come in handy when merchants are advising customers. Online videos and an online U-value simulator are among other tools available to help. There’s even a CPD module offering training to all construction professionals (and that includes merchants of course).

One of the printed and downloadable guides, Hybrid Solutions and Systems, offers at-a-glance technical information about which products to use in particular scenarios.

It offers charts, diagrams and advice on how and where to use each of Actis’ four LABC, LABSS and NHBC compliant Hybrid products - honeycomb style insulation Hybris, insulating breather membranes BoostR Hybrid Wall and Boost R Hybrid Roof and insulating vapour control layer HControl Hybrid. It also shows the U-values achievable, accompanied by detailed graphics and build ups.