Remember the adage, all models are wrong but some are useful? Well, they are more likely to be useful when we feed in the most accurate inputs. When creating energy models, you should use the external dimensions of your building for all energy and Code calculations.
This figure shows at a glance the difference between three different ways of measuring a building’s area. Keep reading to find out why choosing the right one is important. (I know this isn’t a super-sexy topic: I’ll keep it brief. Remember, I only ever write about things it’s worth my readers knowing.)
Figure 1: Area of the thermal envelope is measured using one of the following three systems normally used for component dimensioning: internal dimensions, internal overall dimensions or external dimensions. NZBC uses internal or internal overall depending on the project. The area is largest when using external areas and this area measurement is required for use in PHPP and the NZGBC V5 ECCHO tool based on PHPP. Illustration by Sustainable Engineering Ltd based on Berggren et al 2018.
This is simple but at the same time, very important. Using internal dimensions produces inaccurate results for energy calculations and totally distorts the economic consequences of insulating parts of a building that need it.
In New Zealand, it’s been usual practice to use the internal dimensions when measuring a building’s area for energy efficiency. This ignores heat loss at uninsulated midfloors and other corners and junctions in buildings. Now of course heat is streaming out of these thermal bridges: the future occupants’ cold feet and mouldy carpet will be likely real world consequences.
So we have a model that deviates from reality and doesn’t predict the negative impact on building performance and durability and occupant health and comfort. That’s bad enough, right? Worse is that such a model purportedly proves that insulating a concrete midfloor edge on a multi-storey apartment building, for example, doesn’t produce any energy savings. It will cost more; but there’s no benefit! That is clearly garbage, which is what you get out of a model when you put garbage in.
The New Zealand Building Code previously notionally used internal dimensions for H1 calculations by referencing NZS 4218: 2009 “Thermal insulation – Housing and small buildings”. Happily, this has been removed in all H1 Version 5 documents, the change that came into effect last November.
The industry needs to start using external dimensions, all the time. Such a move would make it possible to accurately compare the benefits of insulating midfloor edges.
Another significant benefit is external dimensions make it easier to do the maths on thermal bridges. For instance, the window-to-wall fraction becomes smaller if you use external dimensions for measurement of the building areas. This allows the use of the schedule method (H1/AS1) for more buildings and I’m hoping this will be sufficient to encourage the use of external dimensions.
Using external dimensions produces the largest area for heat loss calculations and so the calculated building heat loss number needs minimal correction to be sufficiently accurate. Note that Passive House methodology exclusively uses external dimensions. In comparison, using either of the internal dimensions grossly underestimates the actual heat loss through thermal bridges, so necessitates application of a larger correction value to better fit reality.
When using external dimensions, most thermal bridge junctions in timber-framed buildings can be neglected as they will be small or negative—so long as the regularly repeating timber content is accounted for in the construction R-value and the junctions are actually insulated. (Negative thermal bridges are common using external dimensions. This is because you are using slightly larger areas to estimate the heat loss from the building. When you calculate the actual heat loss at the junction you can find the thermal bridge value is negative. This simply means that your estimated heat loss is slightly too high and the thermal bridge calculation corrects this downwards.
If internal dimensions are used, most junctions will show up as a positive thermal bridge and would therefore need to be individually calculated. Given you have an alternative, there’s no purpose in making work for yourself and creating extra expense for your client.