8 months ago

Insulate Magazine Issue 12 - November 2017

The 1st Birthday issue of Insulate magazine titled "Round 12 with Recticel" features an exclusive interview with Recticel's commercial Director Kevin Bohea. If that wasn't enough we have a great exclusive inside the BBA, featuring an interview with BBA Chef Executive Richard Beale.

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The only independent insulation industry trade magazine Insulate Exclusive What is in a U-value Calculation? U-values are an important indicator of building performance. So why is there such limited understanding of what calculations show? By Insulate Magazine. Mainly because the standards describing how to calculate U-values are complex and inaccessible. For the majority, there is no reason to try and understand them - but the knock-on effect of that is assumptions. Assumptions that any calculation committed to paper has been done accurately; that the piece of paper is automatically correct. If the result is the right number then all is well. If it shows a higher number then suddenly there are problems to solve. There are a variety of software packages for calculating U-values. In theory, they are all created equal as they are based on the same standards and calculation methods, but their user- and reader-friendliness can vary. The images accompanying this article are from one widely used program and illustrate what a good level of information looks like. Other software will display it differently, but the important thing is knowing what to look for. 1 Project details and construction type In an ideal world, every U-value calculation should be for a particular project. Of course, specifications get repeated from one job to the next, so it’s easy to understand why generic calculations might be relied upon, but relating the calculation to a project leaves no room for doubt as to what has been specified and what should be constructed on site. Stating the application or type of construction also makes clear to the reader what exactly is being calculated - just in case a lack of detail or confusing terminology elsewhere means it is not entirely obvious. 2 Material layers This section provides the greatest scope for ensuring the proposed construction can be well understood. The build up is listed layer by layer, with a description of the material and/or a product reference where necessary. If proprietary products are not being used then the description should reference a formal standard from which acceptable material data has been obtained. 3 Material data Although insulation materials contribute the bulk of thermal performance in any calculation, it’s still necessary to take account of every layer in the build up. The calculation also has to factor in the direction of heat flow (i.e. downwards, horizontal, upwards) for the purposes of establishing surface and airspace thermal resistances. Working out the thermal resistances of materials requires their thickness and thermal conductivity. Sometimes a calculation doesn’t display these values - such as when the layer is a very thin membrane that has no meaningful thickness and no declared thermal conductivity. The order in which materials are listed is not important for calculating the U-value. It does matter, however, for the accuracy of any accompanying condensation risk analysis - and for aiding the readability of the calculation. For anybody comparing the calculation to a design specification or on-site construction, it is common sense and good practice that calculations reflect the correct sequencing of layers. 22

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