Ever told a client that they could save 200 kWh/m²/year in heating for their 180 m² home? That’s a normal statement from a Passive House designer looking into a deep retrofit option. Do you know how crazy that sounds?
Seriously, even with a COP of 3 heat pump, that’s 12,000 kWh/year of heating electricity (200*180/3) or roughly $3,600 annually just for the heat pump. My folks paid $4,000 or more annually for heating in Massachusetts but that is not typical for a New Zealander. Why? Why does that sound crazy?
Because most people in thermally inefficient houses aren’t using that much energy in the first place. They are cold.
This leads to a phenomenon that researchers Minna Sunikka-Blank and Ray Galvin have dubbed the “prebound effect”. In a fantastic 2012 paper, they analysed data from 3400 German homes and found that, on average, the occupants were using 30% less heating energy than the home’s official energy performance rating (EPR) calculated they should be using.
The worse the house was, the bigger this gap became. For instance, a house with a terrible rating of 300 kWh/m²/year was likely to have an actual consumption 40% below that calculated value. Why? Because people aren’t stupid. They aren’t going to spend a fortune to keep a leaky sieve at a balmy 22°C. They put on a jumper, heat only one room, or just put up with being cold. They are “pre-saving” energy that our models assume they are using.
This is the “prebound effect”: the energy that was never actually consumed before the retrofit.
Then there’s the flip side: the well-known “rebound effect”. After a deep retrofit, when the house is cosy and cheap to run, occupants tend to consume a bit more energy than the new, improved rating would predict. They heat to 22°C and not the 20°C the model assumes (or keep the door ajar for the dog and leave the heating on … really). They take some of the savings back as increased comfort. Can’t blame them.
The combination of these two effects means the actual energy savings are often squeezed from both ends, as this schematic from the paper neatly illustrates:
Figure 4 from Sunikka-Blank & Galvin (2012) shows how the actual energy saving is reduced from the theoretical by both the prebound and rebound effects.
So when we calculate a huge potential “saving”, we’re comparing the theoretical use of a bad building with the theoretical use of a good one. The reality is that the occupant’s starting point is much lower (the prebound effect) and their finishing point is a little higher (the rebound effect).
This isn’t just a German quirk. The paper notes similar findings in Dutch, Belgian, French, and UK studies. It’s a human behaviour thing.
The big takeaway for us as designers is that using a building’s energy rating to predict fuel and CO₂ savings will likely overestimate the actual savings and underestimate the real payback time. We aren’t just making a building more efficient; we’re often moving a household from a state of fuel poverty and thermal discomfort to one of health and comfort. The energy reduction is real, but it’s the gap between their actual pre-retrofit use and their actual post-retrofit use. Let’s be honest with our clients and ourselves about what we are actually saving: money, yes, but more importantly, we are delivering comfort and health that was never there before.
In our work we often neglect the prebound and rebound effect and point out that the comparisons are theoretical with equal comfort level. This makes sense if a client is designing a Passive House as it wouldn’t make sense to compare their warm comfortable home with the old villa they froze in but it is important that they understand it is theoretical and assuming equal comfort!
Abstract
German regulations for the thermal renovation of existing homes demand high thermal standards, which the government claims are technically and economically feasible. This paper examines existing data on 3400 German homes; their calculated energy performance ratings (EPR) are then plotted against the actual measured consumption. The results indicate that occupants consume, on average, 30% less heating energy than the calculated rating. This phenomenon is identified as the ‘prebound’ effect and increases with the calculated rating. The opposite phenomenon, the rebound effect, tends to occur for low-energy dwellings, where occupants consume more than the rating. A similar phenomenon has been recognized in recent Dutch, Belgian, French and UK studies, suggesting policy implications in two directions. Firstly, using a dwelling’s energy rating to predict fuel and CO2 savings through retrofits tends to overestimate savings, underestimate the payback time and possibly discourage cost-effective, incremental improvements. Secondly, the potential fuel and CO2 savings through non-technical measures such as occupant behaviour may well be far larger than is generally assumed in policies so policy-makers need a better understanding of what drives or inhibits occupants’ decisions.
Reference
Sunikka-Blank, M., & Galvin, R. (2012). Introducing the prebound effect: the gap between performance and actual energy consumption. Building Research & Information, 40(3), 260-273. https://doi.org/10.1080/09613218.2012.690952