Calculating in CO2 remains a challenge, especially when it comes to the remaining budget. There is in the Netherlands a small action group that is trying to define how we should deal with the remaining CO2 budget for the built environment. How much CO2 may be emitted (invested) per new house or per renovation. Earlier I argued that the remaining budget is actually entirely needed for renovation, and that new construction should be 0-CO2 from the start. That may be a bit much to ask for new construction, although the situation is serious enough for it, but then what? That raises a lot of questions. Such as how big is that budget then, what do you take into account or what are system boundaries, etc.

Mind that any CO2 budget calculation is not based on the building demand, it is based on the budget -limit: in other words, you can build 100 homes with x-CO2 or 1000 homes with budget 0.1 x CO2.

It is therefore not possible to set a budget per m2 floor or so, because then things can get out of hand. Below are some basic calculations , to get some feeling feeling for the actual numbers and their implications.

First, the budget: Actually, we should start with the last established budget by IPCC, per January 1, 2020, divide it proportionally to the population, and subtract what we have already emitted in 2020 and 2021, and possibly 2022.*

We read: The residual global carbon budget to remain within 1.5°C global warming with 66% probability is given as 400 billion tonnes CO2-eq from the start of 2020.

Dividing that by (nearly) 8 billion global population , times (nearly) 18 million inhabitants, gives 900 Mton for NL (per 1-1-2020).

Minus the emissions in NL over 2020, 2021 and 2022 : resp: 164.3, 167.8 and 164.3 Mton ( 2022 assumption as 2020)

This leaves 403.6 Mt CO2-eq for the Netherlands on 1-1-2023. If emissions continue at current levels, this will be exhausted in 2.5 years. (Budget thus reaching, at most, halfway through 2025).

Or we calculate per person: 22,4 ton per 1-1-2023 . That is, that person can use that in 1 year or in 10 years, but that is all. Nothing more will be added, not ever. (the budget is for everything, not just building)

Next, how to appoint that for houses?

There are several ways to approach this for homes. NIBE office, among others, has done extensive calculations and scenarios on this under a ‘ParisProof concept’. But to illustrate this, here are two simple straightforward approaches to get a feel for what is possible with that budget.

1 Suppose 40% of all CO2 is building related, for both energy and materials together[x] . then that remaining budget for construction is 160 Mt.

How much CO2 is the usual emission for the construction of a – common – house?

The construction of 1 house requires about 6 Gj/m2 of embodied energy ( average, ex infrastructure), at 100 m2 that is : 600 GJ, or 166000 kWh x 0.5 kg CO2= ca. 80,000 kg CO2 / house , for the construction : 80 tons CO2

A standard zero energy renovation of a house requires about 2 GJ/m2 of embodied energy: or ~ 25 tons CO2/house,

That gives a total for 7.5 million homes to renovate of 187 Mt , and for 1 million newly to built homes of 80 Mt, together 267 Mt.

So that’s already way over that budget of 160 tons. And that’s if everything were to be realized tomorrow, but not counting the emissions that will still be released into the air in the coming years for the houses have not yet been renovated… because we can’t tackle all houses in one day. We can only do a limited number each year. If that’s say 100,000 a year, then we’re busy for 70 years…. And all that time they’re still emitting operational CO2[x] .

So that’s not going to work, not if renovate whole houses, , and building another 1 million new ones, trying to make them completely energy neutral following current ideas about how and what. Then we’re going way over the CO2 budget and towards 3 or 4 degrees climate heating.

2

A completely different approach is to turn things around and divide that budget (160 Mt) in advance: we have 7.5 million existing homes to renovate and we want to add 1 million new ones, which adds up to 8.5 million. That would leave us with a maximum of 18 tons of CO2 per 100 m2 house (from the 40% divided by the 8.5 million) for materials and energy. To build one or to renovate one. And that’s the total available for any house, from now until forever, for operational, and embodied together…!….

In fact, this is the only correct approach, because the starting point is that we want to reach that 1.5 degree maximum warming, and that budget is therefore leading.

If then a new house requires 80 tons of CO2 (see 1, the common CO2 wasteful version) for 100 m2, then you can actually only build 23.5 m2. (Apart from scale losses) Everyone a studio, so to speak. Or start living together, sharing spaces, for instance.

Renovation: per dwelling of 100 m2 that costs 25 tons of CO2 ( in the usual way) , Then that is also already more than available per dwelling. It becomes a bit more favorable if we don’t retrofit the whole house, as if it were freezing 365 days a year, but make it a summer-winter house [x].( only insulate and heat the living room)

Or you could split a house, fit for two couples after renovation, and save a new constructed house, that gives an extra 18 tons. Which gives you a surplus of 11 tons… However: the average current energy consumption for a house in NL is (‘milieucentraal’) 3.7 tons CO2/year, so in 3 years that remaining budget will be used up anyway, in other words, that (shared/split) house will have to be renovated to 0-CO2 within 3 years….. Which counts for all homes actually.

This does not even include all other construction activities within that 40% budget, such as infrastructure, offices, etc. And for new houses, for example, this means that at most a gravel road/half-paved road can be built up to the front door.

This is also assuming a per capita distributed global budget. There is some consensus in science that this distribution is not entirely correct, that the poorer countries are entitled to a larger share than proportionally.

Another issue is that there is much discussion about CO2 storage via building materials, such as wood. That is possible, but only dynamically, as I argued earlier, i.e., the net contribution to storage only arises when the renewable materials used are regrowing. And you will have to reserve land use for that as well. [x] For wood, regrowth can take 40 years, and so does the build-up of the CO2 captured net extra . It can be faster, especially for materials like straw, which grows per year, obviously, but with a proportionate land use impact per year. ( the growth rate is not so much relevant, as the yield per hectare per year. (also applies to wood, by the way)

In short, from a CO2 budget point of view, the building task is not relevant, but the budget itself is normative. And that is rather limited, and time is running out….

* If we would take the current remaining budget, that would imply that we, with our high above average emissions, would have profited from countries with low emissions, since averaged over a global budget for two years!