When is an Insulation Manufacturer not and Insulation Manufacturer headlines the October issue of Insulate Magazine. Possibly the best front cover for an Insulation publication EVER.
Ignoring complexities in a proposed construction for the sake of convenience, and trying to represent them without using numerical modelling, risks making the calculation result unrepresentative of what will be achieved in practice. So what does ISO 6946 allow for? It is best suited to building elements with layers of consistent thickness and uniform thermal properties (“homogenous layers”), but also provides ways of dealing with inhomogenous layers. It can include for the thermal properties of air layers up to 300mm in thickness, but can’t allow for metal components bridging insulation layers. There is a correction factor available for metal fixings, however, as well as one for air gaps in insulation layers; if these corrections total more than 3% of the calculated U-value then the result is amended to reflect the impact on performance. Finally for the purposes of this overview, there is an annex describing how tapered insulation layers can be calculated, providing a means of establishing the thermal transmittance of cut-to-falls roofing schemes. While ISO 6946 describes how to carry out combined method calculations, it doesn’t offer much guidance on what to include in them... BRE Report BR 443 … which is where this free-to-download document comes in, providing essential information appropriate conventions to use when calculating U-values. It should be familiar to anybody who provides a calculation service. 24
www.insulatenetwork.com The report is being revised to reflect new and updated practice, but at the time of writing the 2006 edition remains current. BR 443 is also listed as a ‘source of data’ by the BBA, along with a couple of other standards, current BBA certificates, and recognised declarations by manufacturers. If specific material data is missing from a particular specification, these data sources can provide appropriate assumptions to maintain a level of accuracy in the calculation. ISO 13370 Here is the standard that complements ISO 6946 and provides the means of calculating heat loss through ground floors, or what is described in the standard as the “three-dimensional nature” of heat flow in the ground. It applies to slab-onground constructions and ventilated suspended floors. Heat transfer into the ground is affected by the thermal properties of the ground itself, the area of the floor, and the perimeter of the floor including thermal bridging at its edge. If you request a ground floor calculation you will be asked for measurements of the floor’s area and length of exposed perimeter for this reason. BRE Digest 465 This Digest, developed jointly by the BRE and the Steel Construction Institute, provides a methodology for calculating the thermal transmittance for warm, cold and hybrid steel frame constructions that otherwise fall outside the scope of ISO 6946. Validated by detailed calculations to ISO 10211, it allows a simplified method to be incorporated into common software tools and provides a means of assessing steel frame constructions more widely. SCI Information Sheet P312 This information paper also adapts the procedure in ISO 6946; in this case, to provide a relatively simple means for calculating built-up metal roof and wall cladding constructions featuring rail and bracket spacers. The differences in calculation method account for a compressed insulation layer, as well as the impact of linear and point thermal bridging due to metal components and fixings, all validated by calculation to ISO 10211. BRE Information Paper IP 10/02 Another document dealing with the effect of metal components on insulation layers, this paper offers a method for determining the thermal performance of insulated double skin metal roof and wall systems that incorporate ‘z’ spacers. It accounts for the thermal bridging of the metal connectors between the inner liner and outer sheet. In Conclusion For all the talk of ‘simplified methods’, calculating U-values remains a largely specialist activity. If a calculation says a certain result will be achieved then the majority of readers will assume it to be correct. The quality of a calculation depends as much on the information provided at the outset, and the ability of the person doing the calculation, as it does on the standard used to perform the calculation. Which is why, next month, we will look at some typical calculations to illustrate how they display their data. 25