We have a series of buildings that can be designed in three ways: None other are acceptable

Option A, when annualised over its lifespan, has a projected real lifespan of 60 years, a total embodied CO2 of 58 tonnes CO2 output from construction (together with renewals over its lifetime) and thus an annualised CO2 output of 12.9 kg/sqm/annum

Option B, when annualised over its lifespan, has a projected real lifespan of 100 years, a total embodied CO2 of 97.5 tonnes CO2 output from construction (together with renewals over its lifetime) and thus an annualised CO2 output of 13 kg/sqm/annum

Option C, when annualised over its lifespan, has a projected real lifespan of 150 years, a total embodied CO2 of 114 tonnes CO2 output from construction (together with renewals over its lifetime) and thus an annualised CO2 output of 13.1 kg/sqm/annum

All these options require heating & other energy at an annual cost of 25-40 kg CO2/sqm/annum (so pretty good perhaps 88kWh/sqm/annum). There is a good chance that this can be reduced with time as most of the CO2 supply is in appliances and relatively little in heat. (About 50-60% is heat & water)

Option A is 5% cheaper than B to build. Option C is 10% more expensive than B. We know that all of these options will become more expensive over the coming decades but that Option C will become more significantly expensive to construct than either of the other two.

Option A is least carbon now. Options B and C have the same annualised cost as A over their lifespans. However, there is an argument that B and C are more sustainable because they will provide homes for future generations at a lower financial cost (and, of course, they will still be standing in 70 years time whereas option A will not)

Edit: (thanks to Bowman) To the developer, all options are equal: If more money is spent on option C, the buyers are known not to fully appreciate whole life cost and will therefore only pay for a building roughly in proportion to the cost of the build.

Edit: (thanks to CWatters): The lifespans shown are based on average historical figures for the building types. However, an individual building could last significantly longer than shown.

Which should be built?

Why C Tony?
There are actually 6 options I considered for this one but the other 3 don't work out well or have associated planning difficulties.
They are all terraces as it happens. Let me know which bit you don't understand and I'll amend the question

Jon,

Option C = 2.5 x Option A

You need to keep the lifespan constant

Quite, think of it as car, both do roughly the same mpg, one cost £15K and will see you out maybe five years, the other costs £17.25K and will see you out twelve and half years. You'll be on your third £15K car by the time the £17.25K vehicle gives up.

Option A because Buildings last longer than expected and with luck in 80 years time the replacement building will need less CO2 to build.

CWatters, are "luck" and "expectation" mathematical/engineering terms such time, force etc? Either way you still have to apply the same variables to each option.

If the life span of a building is an exponential not a linear function of expectation, this would mean Option C should last 216 years. Pushing the balance even further in C's direction.

Surely luck or chance is a statistical term, i.e. a probability? This would need to be quantified in some way. But if CO2 build emissions are down by 30% in 60 years it would follow that they would be down by 75% in 150 years. If the buildings last longer than expected, Option C might even become a carbon sink in 216 years time.

> CWatters, are "luck" and "expectation" mathematical/engineering terms such time, force etc?
> Either way you still have to apply the same variables to each option.

Ah yes. Ok just testing. Wondered who would spot that one first said captain Manwaring.

Biff's point is very good. By 2050, 95% of the power we consume will have to be renewable, so these CO2 calcs you are doing now are meaningless.

There is a chance that we could be producing so much renewable power by then that we could afford to forget about low energy building altogether.

You last two posters have a very negative philosophy in my view.

Regardless of any other factors our homes should be green, this means well insulated so that we do not waste energy resources. In the long run they save money too especially as properly built quality homes would last a long time.

CO2 calculations have little real meaning in any case apart from them being linked to energy consumption.

Reduce waste, save energy, build well. This would be the basis of my philosophy.

You make me laugh! --- but I agree except for the bit about pigs sprouting wings.

Happy Christmas all.

I am away for a good few days so enjoy my absence!

Doesn't the question assume there is a trade off between longevity and low embodied energy? I don't accept that this is the case and I sight the example of cob (and other earth) buildings which have survived for 100s of years and have about about the lowest embodied energy of any permanent building I can think of. Would anyone seriously spend their own money on a building that was expected to pulled down in 60 years time? With a fair wind I might still be living in it!

Hi Chris

There's no bias: It's just the way the figures work out in this particular case. If we were to consider other examples where the embodied content was high and the longevity low, a non-linear result would occur: So you are right, it is not always linear, particularly if the designer chooses a mix of incompatible durabilities. However, the selection process has already ruled these out.

The current building regs require only 60- years. Option A is a very, very special case. Option B more realistically represents a building on the boundary of the current regulations.

You may be right about cob. I have looked made no evaluations at all looking at cob. I haven't evaluated it as an option because, for this building, it would not be possible to build in cob.

Since discovering recently that houses within the arctic circle are built so well-insulated that they very very rarely need heating (ie only if someone living there is ill or either a newborn or very old), I have to ask why is anyone considering building something that is going to need heating?

Also not sure if this question is a hypothetical problem or a real one? If its real I'd like more information - I can't picture it from numbers.

Why is cob not possible?

And how do I add smiley faces so this doesn't sound like an inquisition???

Thanks Richy & Pixie

Generally it seems the forum is 50/50 between A&C.
Option B is the one actually being built. Though the intention was for the question to be theoretical rather than real to investigate the forum's opinion on the background issue (cases where carbon can be reduced now at a cost of loss of longevity): The question is based on a real site with real building methods.
I can't release the details of the site. Cob could not be built: The building required is too tall, land in the area is scarce, the ground is very compressible clay and the site is in a potential floodplain. For the real site, options A&B would be structurally impaired by flooding with option A being almost completely destroyed by a serious flood. There were other options with much longer lifespans than C and similar annualised embodied contents but none of these would get through planning for environmental reasons (initial primary embodied content) and so were not listed.

Thanks everyone for the input!

jon