3 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. 4 Bridging details Calculating U-values is easiest when a construction comprises simple, uninterrupted material layers of consistent thickness - but layers often have to be interrupted by other materials. Accounting for bridging is important, especially when it’s an insulation layer and the disparity in thermal performance of the two materials is significant. This section states what percentage of the layer is bridged and by what material. But it might not mean anything to the reader. It’s not obvious what “12.5% timber bridging” represents in reality, but if the layer description says the material is bridged by 50mm wide timber joists or studs at 400mm centres, then the world suddenly makes more sense! 5 Result and corrections And so we reach the headline of the whole document! A figure for the predicted loss of heat energy (in Watts, W), per square metre of the construction fabric, per degree of temperature difference (in Kelvin, K) between inside and outside. But what are the numbers that accompany the result? Upper and lower limit thermal resistances refer to the two methods of calculating heat paths through the construction, as described by the Combined Method. These two values are averaged, then a reciprocal taken, to give the U-value. Within the confines of mathematically modelling construction build ups, certain corrections can be applied to reflect the realities of installing insulation. In any calculation, you may see values associated with the following: Uf - corrections for mechanical fixings penetrating the insulation layer. Ug - corrections for air gaps in the insulation layer Up - correction for compression of insulation in built up metal roofing and cladding. Ur - correction for rainwater cooling on inverted roofs. Urc - corrections for rails and/or brackets supporting rainscreen cladding. 6 Application-specific information In the last issue of Insulate we looked at standards that define how U-values are calculated. The Combined Method is fairly limited in scope, so other standards supplement it with calculation methods for ground floors, anything involving steel components etc. Where these other standards are employed by the calculation software, the relevant information is displayed in this section. One point worth noting for ground floor constructions: while it’s tempting to think that perimeter upstand insulation provided to screeds and slabs would be classed as edge insulation, for the purposes of a U-value calculation it isn’t. It is there to treat the thermal bridge, so could be referenced in the layer description to show it has been thought about (e.g. ‘Reinforced concrete slab with perimeter upstand insulation to address thermal bridging’). In Conclusion Anybody calculating U-values should be armed with as much information as possible about the intended materials, and should ask lots of questions where relevant information is missing. Judging the veracity of calculations is difficult for many, but communication is key. Knowing that a design has been accurately calculated, or that what is being constructed reflects what a calculation shows, can go a long way to helping address performance gap issues. We don’t have the space to delve into these corrections, but if they impact on the calculation sufficiently then they cause the result to be changed. It’s important to ensure that comparable corrections have been included when judging one calculation against another, otherwise the comparison is not a fair one. 24

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