The Co2 emissions of our housing stock needs to brought to 0. And in the Netherlands we have the target to become natural gas free as well , and even sooner. Plans how to act are launched one after the other. But they are all object-oriented, the house, or technology-oriented, or at best “neighborhood-oriented”. I have been contributed to that approach as well, but gradually over the years came to other insights: It won’t work that way. In two episodes the approach detailed and explained.
About 10 years ago, the first major project for the sustainable retrofit of houses while inhabited arose in Kerkrade: the houses were given new, heavily insulated facades and roofs and a new heating and ventilation system. All in 10 days. The renovation team moved like a retrofit-train from house to house:, 1 day preparation, 1 day removing old facade parts, 1 day fitting new facade parts, and so on. One home per day was delivered once the train was on speed, ultimately 150 in total. A great project for which the Housing corporation Heemwonen took the lead and should get the honors. It was an important example for later projects , (even retrofitted within 1 day) and for the elaboration of plans for making the entire housing stock sustainable with prefab elements for roofs and facades, the basis for current retrofit plans. At the time, I was enthusiastic about that, we were completely satisfied, we would let the train roll throughout the Netherlands.  Until the first crack in that thinking arose. Our research showed that in some cases, such as for a row-house, , Taking into account the material impacts, such a complete prefab make over was not the optimum choice: less insulation and an additional solar panel turned out to score better overall, still being ‘0-energy’.  (1)
Then came Paris 2015, and the IPCC published the “Carbon budget” for the first time: the maximum amount of CO2 (equivalents) that could still be emitted to stay below two degrees or close to 1.5 degrees. Which is a absolute ceiling.
Its as if you have 1000 Euro’s in your wallet, and that’s all you have to spend for the rest of your life . You can spent it in 1 year or in 30 years, but once spent that’s it. The fact that we have to achieve 0-CO2 emissions by 2050 is only a political choice: you will have 30 years of budget, but less per year. But if we continue on the current CO2 spending track , a fair share of the Netherlands will be exhausted in 2027. (fair share: the remaining global emission budget is evenly distributed among all earthlings). Either way, exceeding the 2-degree standard is virtually irreversible. So it is important to reduce those emissions as quickly as possible. The longer that takes, the more of the remaining budget is already is spent before you start taking any measures.
Taking the housing stock as an example: every house that is retrofitted is then energy neutral and causes no more CO2 emissions over time (in theory). But only a limited number of houses can be renovated per year, so that the non-renovated homes continue to contribute 100% to CO2 emissions. If 100,000 houses are retrofitted each year, there will be 1 million done after 10 years, or only 13% of the stock ( in NL). 87 % still contributes fully to the CO2 emissions, as before (in 2030, if we already have to be at 50% reduction). That is far too slow anyway, and therefore does not offer the solution to stay within that budget . (2)
But a new insight emerged, in collaboration with European colleagues, in which we made exploratory calculations with regard to that budget: If we distribute the Dutch emission budget pro rata across the sectors, say 20% for the housing stock, then all materials for retrofitting the entire stock together already produce more CO2 emissions, as s available as proportional share for construction. Although it seems to be a good thing for each home separately, the sum total for the entire stocky exceeded the budget for 1.5 degrees  (3). ( Still not counting the the time delay in implementation -as under 2)
Incidentally, assuming that new construction causes 0-CO2 emissions from the beginning, otherwise each new building will reduce that remaining budget for retrofits. New constructions must , so to say, already comply with the 2050 building regulations by now, in order to keep that emission budget available for adjustments to the existing stock.  (4).
This calculation of the cumulative emissions of all homes over time is rarely made, usually one technique or product is targeted and optimized. A similar effect occurs with, for example, solar panels: A panel produces renewable energy. Yes. But Solar panels cause CO2 emissions in their production (directly). And that can take 3 years before they are compensated by the CO2-free production of electricity from the panel itself. If more panels are installed (and produced) every year than the year before, CO2 emissions continue to rise every year. There is only net profit 3 years after stopping production and installing. That total effect is usually overlooked. This effect can only be seen if you look at the higher scale of the country, and over time. After a few years there seems to be a profit per panel or project. But not at national or collective level. This also applies to housing, which must be considered as a whole stock.
How then to reduce that cumulative CO2 curve? Not home for home or neighborhood for neighborhood, and taking 30 years for the job, the solution must be found in limited measures that can be applied in all houses in a very short time. This means that the cumulative CO2 emission curve is immediately deflected and less steep, which means that on the one hand we already strongly reduce , with limited rebound effects in materials, creating more time to bring the remainder to 0. One option is, In the Netherlands that is, to fit all homes with a small heat pump in hybrid mode next to the (natural) gas boiler. It can take over 40-70 pct of the heating load, and therefore immediately reduces (natural) gas demand. If we do this at the same time as equipping all homes with solar panels on the roof, south east or west, it doesn’t matter, we immediately remove a large part of the new electricity demand, and then immediately have deflected the CO2 curve . (I propose: nationalize all roofs and put solar panels on It ) Within a few years, and the curve of both gas and CO2 flattens sharply, for years, for the entire supply. It is not a divestment, since these investments have already been paid back before entering a next retrofit phase . Which might look quit different, if despite our efforts the climate has nevertheless settled at a higher average temperature level, and the gas boiler rarely switches on again.
In short, in order not to have to conclude after 20 years, that we have failed in the individual full retrofit approach, it is necessary to base our strategy on the cumulative CO2 emission curve, and not focus at each product or house. We have to calculate for the entire stock, and the curve over time. And adjust the plans accordingly, of course.
(more in part 2)
 New energy housing retrofit concept: “renovation trains for mass housing, Building Research & Information Volume 42 issue 6 2014 , https://www.tandfonline.com/doi/abs/10.1080/09613218.2014.926764?journalCode=rbri20#.VKquEI-RA5Y%20
 Environmental impact evaluation of energy saving and energy generation: Case study for two Dutch dwelling types, Michiel Ritzen, T.Haagen, Ronald Rovers, Chris Geurts. in: Building and Environment 108 ·July 2016, https://www.sciencedirect.com/science/article/abs/pii/S036013231630275X
 buildings carbon budget workgroup http://www.buildingscarbonbudget.org/ see chapter 3.1
 new buildings should comply with 2050 regulations http://www.ronaldrovers.com/new-construction-must-comply-with-regulations-2050/