The following email was received from Joseph Little, Architect and environmental consultant on 10th October 2014. We have formatted it into a post format. If you are unsure as to terminology used please see Introduction to thermal bridging: The basics of thermal bridge heat loss (courtesy of Joseph Little Architects).
IF you are a Certifier and you don’t know that the following conversation means, or if you don’t know what a temperature factor (fRsi) is, you probably should not be certifying Part L compliance.
At the moment, under 184.108.40.206 (iii) of TGD L(2011), ‘reasonable provision with regard to limitation of thermal bridging’ for non-ACD details can be proven by a manufacturer applying to NSAI for agrément certification by ANY thermal modeller (whose calcs are then vetted thoroughly by NSAI), or by someone on an approved thermal modellers scheme (the only scheme we know of worldwide is the NSAI scheme). An architect in private practice must use the latter approach in establishing psi-values for his/her own (non-Accredited Construction Details, non-agrément) designs. Obviously the UK Accredited and Enhanced Construction Details are equal to the Irish ACDs.
Please note that there are clear limits on the use of ACDs that seem to be frequently ignored, e.g. you may have a detail similar to an ACD but sufficiently different in adjoining U-values and/or geometry that the ACD value may not be used.
220.127.116.11 (iv) focuses not so much on limiting thermal bridging but on surface condensation based on the hugely onerous statement in 18.104.22.168 ‘Any thermal bridge should not pose a risk of surface or interstitial condensation’. As a member of the Building Fabric team of DECLG told me in a telcon some time ago one cannot ‘know’ this unless one calculates it… If calculation is indeed needed for every potentially vulnerable junction of every building it would constitute a vast number of calculations. I would argue this requirement does not require all thermal bridges to be calculated, only the ones that may be vulnerable… but who decides which junction is in which of these groups?
22.214.171.124 (iv) states one may ‘use alternative details which limit the risk of mould growth and surface condensation to an acceptable level as set out in paragraph D.2 of Appendix D for all junctions’. This is proven by establishing that the temperature factor (fRsi) of the surface or junction is above an accepted threshold or that building type (0.75 for dwellings). This little calculation is very simple and any design professional could do it, however they must use surface temperature information that can only be established in two ways:
(a) by thermal modelling at design stage (it does not explicitly state that a modeller on NSAI scheme must carry out this do this but I think it would be hard to prove who else would be clearly acceptable given the clarity of the clause above it); and
(b) after the building is completed, by having a thermographic survey carried out under IS EN 13187. Only a trained thermographer (trained in an internationally recognised training programme, such as ‘ITC Level 1 Thermographer’) is likely to be able to do that. Unless you are allowed to certify design after the building is complete and have a survey carried it (in accordance with specific conditions and manner in IS EN 13187) it does seem to push one back to the desktop thermal modelling by someone on the NSAI Scheme again.
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