[Draft written for review, edit before publishing]
Almost every sustainable building I was shown in my first year was new. Timber frame, triple glazing, a green roof, a diagram of air moving through a section. All of it was good work, and none of it addressed the question that now seems to me the most important one: what happens to the building that was already standing on that site?
The arithmetic of starting over
A building’s emissions come in two forms. Operational carbon is what it takes to run — heating, cooling, lighting, hot water — and it accumulates year after year. Embodied carbon is what was spent making it: quarrying, firing, smelting, milling, transporting and assembling the material, plus whatever will eventually be spent demolishing and disposing of it. Operational carbon is the one we have spent forty years learning to reduce, and we have got genuinely good at it. Embodied carbon is the one that is already spent and cannot be recovered.
Which means that demolishing a serviceable building has a cost that does not appear in the new building’s energy model. You write off everything embodied in the old building, you spend a new round of embodied carbon on the replacement, and only then do you begin to collect the operational savings that justified the exercise. The 2011 Preservation Green Lab study found that recovering the carbon spent on a new, more efficient replacement takes between ten and eighty years, depending on the building type and how it is built. Eighty years is beyond most planning horizons.
There is a waste argument alongside the carbon one. The EPA has put construction and demolition debris in the United States at around 600 million tons in a single year, roughly double the municipal solid waste stream, and most of it comes from demolition rather than construction.
An energy model for a new building starts its count on the day the old one comes down. The atmosphere does not.
Most of 2050 is already built
The figure usually quoted is that something like four fifths of the buildings standing in North America and Europe in 2050 exist today. Treat the exact number carefully — it depends on the region and the assumptions — but the direction is not in doubt, particularly in older cities where the stock turns over slowly.
If that is true, the performance of the existing stock is the climate question for buildings, and a perfect new building does not fix a city of leaky old ones. It is also a different kind of design problem: less about inventing a form, more about diagnosing a building and intervening precisely, which is closer to medicine than to sculpture.
Retrofit in the right order
What struck me, reading retrofit case studies, is how unglamorous the effective measures are and how reliably they come in the same order.
- Fix the water first. Gutters, downpipes, flashings, grading away from the wall. A wet wall conducts heat far better than a dry one, and every other measure fails in a wet building.
- Air sealing. Uncontrolled air movement is usually the largest single loss in an old building and the cheapest to reduce. It is also almost entirely invisible, which is why it is skipped.
- Insulate the roof. Most heat leaves upward, the attic is usually accessible, and nothing historic is normally at risk.
- Then the walls, carefully. Insulating a solid masonry wall on its inside face changes where it gets cold and how it dries, and done without understanding the wall’s moisture behaviour it can cause frost damage and rot the uninsulated wall never had.
- Then the systems. A heat pump sized for a sealed and insulated building is much smaller, and cheaper, than one sized for the building as found.
The order matters as much as the measures. Install the heat pump first and you buy a bigger machine than you need, forever.
Windows, the argument that will not die
Replacement windows are the most commonly sold energy retrofit and one of the least efficient uses of money in an old house. The glazing is a small fraction of the envelope area, much of the loss through an old window is air leakage at the sash rather than conduction through the glass, and repairing and weatherstripping an existing sash and adding a storm window gets you close to a replacement unit’s performance at a fraction of the cost.
It also keeps a repairable assembly. An old sash is wood, glass and putty, and any of those can be replaced individually with hand tools. A sealed double-glazed unit is a manufactured product with a service life, and when the seal fails the whole unit is replaced. That is the embodied carbon argument again at the scale of a component: the repairable thing is usually the sustainable thing, even when its instantaneous performance is worse.
What makes reuse hard is rarely the building
The obstacles are mostly procedural. Energy codes written around new construction can be difficult to satisfy with a solid wall, even where the building performs acceptably in use. Appraisals and lending favour new construction because the comparables are easier. Demolition is fast and its costs are immediate, while a retrofit is slow and its costs are discovered as you open things up. And the carbon already in a building has no value on anybody’s balance sheet, so nothing is lost on paper when it is destroyed.
Some counterweights are real, though. The federal historic rehabilitation tax credit — twenty per cent of qualifying costs for a certified rehabilitation of a listed income-producing building — exists precisely to close part of that gap, and it has been the deciding factor in a very large number of projects. It is also, notably, a preservation instrument doing climate work it was not written for.
The honest counterargument
“The greenest building is the one that is already built” is a good slogan and not a universal truth. A building kept in use but never improved can burn fuel for decades and overtake the embodied cost of replacing it. A single-storey building on a transit-served site may be the wrong use of land, and density has a carbon value of its own. Some structures are genuinely at the end of their lives, and some are contaminated, unsafe, or so specific that no second use exists.
So the honest position is not “never demolish”. It is that demolition should have to make its case, with the embodied carbon counted on the same page as the operational savings, instead of being the default that happens when a building has no obvious advocate.
What I would want to design
Working on Reuse of the Ropewalk changed how I read a brief. The building was difficult, enormously long, and committed to a process that no longer exists, and the only way through was to accept its geometry and bend the programme. The result is less free than a new building on an empty site and, I think, better for it — the constraints did the work that an invented concept would otherwise have had to do.
Light Register came from the same instinct at a smaller scale: measuring how much daylight an existing section already delivers before deciding to cut anything new into it. And doing the condition survey for Conserving the Cast Collection taught me the habit underneath all of this, which is to find out what something is made of and how it fails before deciding what to do with it.
Sustainability is a question about time: how long will this last, and how much can be repaired rather than replaced? Asked that way, the buildings we already have are not a constraint on sustainable design. They are the material we are designing with.
