TFA or CFA? The difference matters Passive House and Homestar calculate energy use differently

18 September 2023 by Jason Quinn

Anyone who notices there are fewer biscuits in a packet that is outwardly the same size has encountered shrinkflation. Same price, but nine chocolate biccies instead of 11 means 18% less treats for your money. It’s not lying but it is confusing.

We’ve got to watch out for shrinkflation in relation to building performance. A Passive House designer recently claimed a house they designed against Homestar metrics, which had a heating demand of less than 15 kWh/m2/year, met Passive House performance levels. I had to wade in to correct that. Despite being below the magic number, that house wouldn’t meet the Passive House target and it was misleading to claim otherwise. Homestar defines the floor area in a different way so the results from each system don’t map exactly. Here’s an example to illustrate what’s at issue.

Here we have a single level detached family home. On the left, in yellow, is the Treated Floor Area (TFA) as defined by the Passive House standard. In the middle is the New Zealand Green Building Council Conditioned Floor Area (CFA), which is used by the Homestar rating. On the right, I’ve overlaid the two illustrations: you’ll see that using CFA, the floor area is five per cent larger (188m2 vs 178m2 TFA). 

With the reference area being larger, the energy per that area is reduced. So for the same building, using CFA as the basis for calculating normalised energy consumption will give a better (lower) result compared to using the TFA. Gross floor area would give an even lower result.

For smaller multi-level apartments or buildings with low ceiling storage spaces, the area differences can be greater. In a few cases we’ve seen, the difference was quite significant. 

There is no issue with using a different reference area—unless you are comparing energy rating systems. In that case, it’s important to realise that the numbers you are comparing mean different things. As an industry, we’re moving to using external surface areas rather than internal surface areas. This is an attempt to simplify energy modelling. In the past, industry standard has been to use gross floor area—and there hasn’t even been clarity about whether that includes the garage or not, or any other unconditioned spaces.

Comments 1

  1. I little part of me had hoped you’d built a 10% margin in to ECCHO to compensate. 🙂

    Question: both PHPP and ECCHO have been validated against ASHRAE 140, but my experience with TFA and CFA gives markedly different results as you allude to in your article. Is the deviation in ASHRAE 140 too loose?


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