insight Assessment of thermal bridges is the low hanging fruit lining the path to passive house and low-energy building, according to leading thermal modeller Andy Lundberg of Passivate, who says that taking the time to understand thermal bridging and to minimise and calculate it properly is essential to delivering cost optimal low energy buildings without an Achilles heel. The importance of thermal bridge assessment has never been as prevalent as it is today, and it is about to become even more so with the onset of nZEB (nearly zero energy buildings) and imminent changes in Ireland to Part L for non-domestic buildings. And yet despite tougher building regs and a surge of interest in the passive house standard there hasn’t been any significant palpable change in our knowledge as an industry in terms of what thermal bridging actually is, what the regulations require us to do when it comes to accounting for thermal bridging in buildings, how we minimise thermal bridging effects at design stage, where it sits in the overall context of a BER or Sap assessment and so on. Thermal bridging is a known unknown that gets tossed around like a hot potato. No one can deny its existence and the fact that it can’t be ignored, in fact most professionals involved in building design will attest to its importance, but nobody wants to be the one to do it. And for those who haven’t crossed the threshold of taking an in-depth look into what thermal bridging is, there is a warm fluffy cushion for them to rest on, a place of sheer bliss for which a ticket can be purchased at the price of some extra insulation here and a heat pump there. This place is otherwise known as ‘the default value hotel’. But little do most of the guests know that the hotel is about to enter administration. So before we go on to talk about thermal bridging in the context of nZEB, passive house and Part L compliance, let’s take a step back and look at thermal bridging for what it really is, where it comes from and what the numbers tell us. The first thing to know about thermal bridging is that it’s nothing new. Early cave dwellers, who made excellent use of cliff-face orientation, thermal mass and form to create comfortable environments in all seasons, didn’t crack the thermal bridging issue either. Considering however that we as a human race are now planning a manned mission to Mars, relatively speaking our progress in dealing with thermal bridging has moved at glacial speed by comparison. So what we do know is that all buildings suffer from the effects of thermal bridging, and that is completely unavoidable. What we can do through knowledge, good design and use of finite-element analysis, is reduce its effects insofar as reasonably practicable, sensible and affordable. When it comes to thermal bridge assessment, there are two things we want to look at. One is the additional heat loss attributed to the junction in question, and the second is the risk of any surface condensation – and related subsequent mould growth – occurring. In terms of measuring additional heat loss at junctions, this is important so that we can be sure that
(opposite) Good thermal bridging detailing can be done with conventional build approaches. This Zeno Winkens-designed passive house achieved a negative Psi-value of -0.028 W/(mK) at the ground floor wall junction. (above and right) Psi-therm 3D modelling of a Dublin retrofit which ended up with a line of mould in the kitchen ceiling and upstairs bathroom wall
our buildings are only specified with the amount of insulation and quality of windows etc. that the building actually needs in order to comply with building regulations as well as ensure occupants have an optimal living environment, without unnecessarily forcing self-builders, contractors or developers to put in more than is necessary, just to get green tick boxes in a Deap or Sap compliance check. The same applies to passive house design and PHPP, but the way passive house design is assessed has a high degree of built-in safety when it comes to the inclusion of heat loss from thermal bridges. It’s common knowledge that investment in insulation is a no-brainer for poorly insulated buildings, but that the returns on investment diminish greatly with every additional millimetre installed above a certain level specific to each building. This was a core aspect of the development of passive house back in 1995, and the recast Energy Performance of Buildings Directive also now requires us to build in a cost-optimal way. So only spend what you need to spend in order to create an environment which is affordable to build, affordable to live in, comfortable and hygienic. In order to do that however, we need as much information as possible about what is getting input to the building design. Absolutely detrimental to that goal is the use of default performance values. What may seem like a quick and easy way out of a knowledge-gap issue may inevitably hit you square in the pocket, and that is completely avoidable. When measuring overall building fabric heat loss, we are all extremely comfortable with the term U-value as well as the concept of what it is, right? I know this because in a recent casual poll of 40 building design professionals, I asked if everyone knew what a U-value is,
and they all raised their hands. When I asked if they could readily calculate one, given the right information, less than half kept their hands up. Asked if they could accurately describe the concept to an unknowing stranger, we were down to 12. The elephant in the room there was the fact that there were people who were happy to calculate something that they then couldn’t really describe to a stranger. There is a wider issue there! When further probed about what the units of a U-value were, we were down to eight. And when asked if they could describe well what those units represented and how they were arrived at by calculation, we were down to two. So maybe we’re not as comfortable with the heat loss calculation and U-value concept as we thought. And we’d do a whole lot better to just admit that and start doing something about it. What a U-value does tell us is how much heat energy in Watts is being lost across a given square metre area of a building element, for every one-degree temperature difference between the heated internal space and the outside world. The relationships are linear, so if you double the area of a building element, you double the heat loss. Similarly, if you double the temperature difference you also double the heat loss. The U-value is mainly dependent on the thickness of materials used to build the element, and their thermal conductivity. The thermal conductivity is measured in Watts per metre and Kelvin, where the ‘metre’ describes the heat loss across the thickness of the material from inside to outside, and Kelvin describes the temperature difference between the inside and outside. The incremental difference between 1C and 2C and 1 Kelvin and 2 Kelvin is exactly the same, it’s just that Celsius is used to describe a temperature, and Kelvin is used to describe a temperature difference.
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