Issue 9, Vol 5 Out Now
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Featured Article
| Steep decline |
The world has learned the hard way that our political leaders lacked the judgement and resolve to identify and address the problems which led to the recession. Richard Douthwaite argues that a similarly flawed judgement is evident in the assumption that the economy will recover, and advises on how to prepare for a future of global economic contraction.
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Official magazine of Easca  |
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Breaking the mould - part III |
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Thursday, 17 September 2009 |
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Page 3 of 10
Figure 1: front page of the ParexLahabra
Agrément certificate showing a house in
the ‘Ard na Mara’ estate of Malahide
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At this point the guidance seems to divide on two paths. The more onerous path closely reflects the new focus on thermal bridging in new-builds. Clause 2.1.3.2 refers to table D1 (8) of appendix D which lists thermal bridge (or Psi) values, and the tranche of ‘Acceptable Construction Details’ (ACD) (9) each of which is given a fixed Psi-value. Using these allows a good practice 'Y-factor' of 0.08 W/m2K, which is ahead of the minimum standard for new-builds of 0.15 W/m2K. At this level of performance minimization of thermal bridging goes beyond basic condensation control to playing a big role in reducing the overall heat loss of the building. See figure 1 of the introduction to the ACDs for further information. Even though the Psi-values listed represent good practice we see two problems with this clause:
A) The ACDs are for new-build, they have questionable relevance to many conditions that occur in refurbishment. For instance there is no ACD for a solid brick wall or a concrete eaves or the weights-box of a sliding sash window.
B) The Psi-values listed in table D1 become harder and harder to achieve as the U-value of the plane element increases (in both new-build and refurbs). Thus a Psi-value of 0.04 W/mK for a sill where the wall is 0.6 W/m2K is much easier to achieve than with 0.27 W/m2K. But anyone who uses the ACDs in designing building fabric to a super-insulated level (in other words 0.15 W/m2K) will get much poorer values than those listed in table D1. In fact those designers shouldn’t be allowed use the 0.08 Y-factor referred to in appendix D, as it bears little relation to the real value that would result!
Figure 4: diagram 2 from TGD L (2007)
showing ‘thermal bridge-free’ details
for refurbishment
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Effectively the current ACDs are only relevant to one level of thermal performance, and are only obliquely relevant to refurbishment at all.
If that is the first, let’s say higher path, the lower path is set-out in clauses 2.1.3.3 and diagram 2. In this author’s view these are in direct conflict with the clauses before them and are a clear hangover from the new-build standard of previous editions of TGD L. Diagram 2 in fact dates back to TGD L (1997) and is dreadfully out-of-date. Its ‘thermal bridge-free’ details are shot through with thermal bridges (see figure 5 below). In turn clause 2.1.3.3 refers vaguely to 15mm of 0.04 W/mK insulation as being ‘generally adequate.’
So should the sill be refurbished to a good performance-based standard of 0.04 W/mK (Psi-value) or a poor prescriptive-based standard of 15mm of mineral wool wrapped around its back? The weaker standard is about preventing the worst impacts of thermal bridging (condensation and mould) rather than avoiding excessive heat loss, but relates only to jambs, sills and lintels. It is clear in light of the current move towards low carbon housing that this standard is inadequate and incomplete. The more onerous standard, on the other hand, relates to all junctions. Presumably most builders will choose to follow the weaker standard where possible, but what about other junctions? It's clear there's a lot of room for confusion.
Figure 5: excerpts from diagram 2 from TGD L (2007) with peach arrows added (see figure 4). The details on the right are meant to contrast with that on the left as having no thermal bridges, but they clearly do have.
The result of all this is that it is genuinely hard to say what the minimum enforceable standard for thermal bridging in refurbishments is. It is reasonable to deduce that there may have been a rush in drafting the document or that an oversight occurred, but it is probably fair to say that the effect of thermal bridging in refurbishment has just not been considered that important on all sides of the construction industry until now. The economic slowdown and the HES scheme have certainly caused a shift in focus. Coherent guidance on minimum, enforceable standards for thermal bridging in refurbishment, as a clear step towards dwellings with ‘zero-carbon emissions’, would be a most valuable outcome of the review of TGD L (2007) that starts in September 2009.
The author's suggestion for this element of that review is twofold. Firstly, that the revised document could state clearly that all refurbishment should at least meet the same 0.15 Y-factor as new-build. Given that thermal bridging is about extra-over heat loss, this is not unreasonable but would bring considerable clarity. Secondly, that a new table of Psi-values be produced to sit alongside Table D1, for use in both refurbishment and new-build, that would show how each junction contributes to the overall 0.15 Y-factor.
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