Not a 1 house-retrofit focus, but a stock focus: hybrid solution

Since quit a few years I am involved in mass retrofit projects for 0-energy performance, mainly with prefab panel based solutions. In the Netherlands, but also in Europe, recently among others within the EU H2020 project More-Connect, in which 5 climate zones develop concepts and packages for pilots. [1] . And in principle it can be done, retrofit for 0-energy, in several ways. (And just this week a new Dutch government is launched in the Netherlands, and planning a policy for 50.000 retrofits a year).

But there is a problem.

Suppose we would succeed in doing even 100.000 retrofits for 0-energy performance a year, that would imply that, with 7,2 million houses in NL, it would take 72 years to make the total stock 0-energy.

Now every house retrofitted for 0-energy has reduced operational related energy emissions drastically: The demand is reduced, and the remaining need is supplied by renewable energy ( Solar panels on the roof) As a result, fossil fuel, and in the Netherlands 90% of houses has a gas grid connection for this, is abandoned . Nothing wrong with that. There is a rebound effect in Embodied energy: large amounts of materials are invested that come with (fossil) energy used for processing and transport. But even this can have a (energy-) pay back of say 10 or maybe even 7 years. So after that period the house functions more or less emission free. Now whats wrong with that?

The problem is, that the so far non-renovated houses ( millions that have to wait for years for their turn), will still cause CO2 emissions. And if you look at the housing stock in stead of a individual house, it does not make sense anymore !

With 100.000 houses a year the progress is that slow, that the Embodied energy even raises the CO2 emissions the first years. As academic researchers within the NGO iiSBE, we did a study on the CO2 budget remaining tot stay below 1,5/2 degrees, and found that the rebound effect in embodied energy from the materials invested, is nearly half the remaining budget under the 2 degree scenario ( or the total under the 1,5 scenario) . That wont work, while the non-renovated houses, would still have years to wait for their retrofit moment and emit a lot of operational CO2 in the mean time. [2]


I have had some discussions about this in the Netherlands.* The problem is, the focus on individual house retrofits, whatever profitable they are individually, troubles the vision at the total stock effect: And what we need is a fast reduction in CO2 levels. Not in 72 years.

In the Netherlands we have a second problem: nearly all houses work on (natural) gasboiler systems with hot-water fed radiators . And we have a problem with gas extraction in the North of the country, causing earthquakes and damage to houses in that region., So we want to shut down the gas fields.

So we have two targets: cut down on gas and cut down on CO2. Only gas , and shifting to coal power plant electricity , would solve the gas issue, but not the CO2 issue.

All concepts I have seen so far, focus at individual house retrofits, or small groups. No single concept or strategy looks at large scale solutions that can be implemented in short time, with large gas and CO2 reductions.

The only option I can think of , to reduce the gas extraction and CO2 emissions fast, ( since that is the reason after all for our energy-retrofit focus) is a complete different approach: the hybrid concept. We wont start with doing complete house make-overs’s , and change our housing stock completely, but start with a simple adjustment with large effects to the installations: we add a small heat pump (air to water) to the gas boiler system. Its an option available in the market, though not well known at the moment, and that can be enrolled fast: in 2 or say 3 years all houses in the Netherlands could have been supplied with this option. Of course, this is for he Netherlands, in other countries it only works if the heating system is water driven, or maybe there is another similar solution. But here in the Netherlands it can be done. We have done it before, back in the sixties when we found the gas, in a few years the whole country was retrofitted with a gas driven boiler system.

The hybrid heat pump, takes over the heating of the house, and only when temperature levels are to low for the heat pump to function with a reasonable performance, the gasboiler comes in and takes over. The precise moment can differ by the house, but sources show that the reduction in gas can be somewhere between 40 and 75 %. Now lets not be to optimistic and take 50% as the potential: In that case we have reduced gas extraction for housing heating with 50% in just a few years. For the whole housing stock. There is no other option with such a potential. The inhabitants nearly will mention, the installation nor the difference.

Another development, how cynical, may even contribute to a larger profit: as a results of climate change, cold days in the Netherlands are reduced, and the moments the gasboiler has to take over will be less and less during the years…

Advantages are big: 1) fast gas reduction overall, 2) little rebound effect in embodied energy, 3) hardly any trouble for inhabitants 4) fast pay back period 5) no waste of demolished gas installations 6) no sub-optimisation of housing retrofits, and further steps can still be taken later.

Of course, some basic additional measures can still be applied now: double glazing, cavity wall insulation, ( more is not always needed [3]), but these are less disruptive and impacted.

But then we have tackled the gas problem, but not yet the CO2 emissions. For this, we will have to produce the electricity with solar panels: all roofs at the same time are covered with PV. Two problems solved in one action: within a few years gas reduced by half, and CO2 emissions reduced by half , for more then 7 million houses. Which has another advantage: 7) it provides us time to consider the next steps, since the cumulative growth in emissions has already halted largely .

Think of this: with the hybrid approach we have reduction in three years that would take 10 times longer with the 0-energy house make over approach. The time can be used for instance to first tackle the building and installations industry: if they produce by renewable energy, there will be no rebound effect anymore from embodied energy in later measures for housing retrofits: since produced by renewable energy. And the industry has to make that transition anyway as well: better do it first, and profit from that in the end product phase.

Now how could the measures in phase 2 to look like, to tackle the remaining 50% ? A natural moment would be fro instance when the old gasboiler breaks down. There are several options, one looking interesting is to continue with the heatpump, and have additional infrared heating panels installed, to cover the deficit from the heatpump in coldest periods. Or even better, at the same moment by reducing the heated space , which also reduces the need for insulated surface. Since we hardly can expect that we will continue to heat the whole house for 24 hours, when there is just a small period in the year left that requires gas heating. Its was the luxury obtained in the Netherlands when finding the large gas fields. But now we have wasted those, and causing earthquakes , it would be another waste of resources to retrofit a whole house for such a short period in the year.

I call this the Summer-Winter house option: Its makes more logic to reduce the living area in the short cold winter periods and just heat the most important rooms.

But thats for later, lets us start with the first 50% profit, with the hybrid option. Its the no regret option that makes us profit fast.

To explore this option, is exactly the difference between the building and housing sector focusing at individual cases, and analyzing on stock level with a focus on gas and CO2 reduction. Which leads to a different approach. And although I did not make the calculation, but it seems to me a solid (financial) business case as well. And if not: government and politicians: make it a business case !! Thats our main focus now: to reduce gas and CO2 fast, not to have 72 years of work for the building sector.




* Just before summer, Jan Willem van de Groep, a well known and respected innovator in the Netherlands , introducer of the NOM concept ( nillonthe meter retrofits/ NOM, ‘energiesprong’ ) and very active on social media, wrote a column, under the title sustainable nonsense , stating that CO2 is our most important problem, and that materials invested in the transition have a marginal effect (my summary words)

This was far too simple arguing, not to react, so I did, the more since we had a two hour discussion a few months ago about similar issues. I published it as my ( dutch) blog and via twitter announced. Stating, that the transition to renewable energy is a materials problem, and his column is dangerous since it suggests that we can solve everything, just by (energy) innovation.

This caused a strong debate via twitter. A week later Jan Willem vd Groep wrote a column, this time more thoughtful, (in which he named me as the one that made him aware of 0-energy retrofit possibilities), and yes, he was right this time in his column: A 0-energy retrofit saves a lot of energy/CO2 over time, and yes after x years the (embodied energy ) investment had been paid back . Whether thats 3 or 6 or 10 years is not so relevant in this discussion, it has been paid back in relative short time. The NillOntheMeter concept is very interesting, and we should continue of course to aim for 0-energy houses. Only ,due to my experiences since the first projects, maybe not in this form exactly…!

Still,  my major point was not addressed ( or I did fail to make it clear enough). This article ( and a dutch column in the Dutch Construction newspaper) is/was another attempt, combined with a different strategy proposal. The debate is continuing…


[2] see the report at

[3] Ritzen, M. et all (2016) Environmental impact evaluation of energy saving and energy

generation: Case study for two Dutch dwelling types, in: Building and Environment 108

·July 2016