Decarbonising Buildings Part 1: What your choice of materials actually means for carbon emissions
Everyone's talking about decarbonising buildings. And that's great — we should be. But a lot of the conversation gets stuck on a single number: upfront carbon. The emissions baked into a material before it even reaches your site.
Upfront carbon matters. But it's only part of the story. A material with a high upfront carbon cost can still be the right call if it dramatically cuts the energy a building uses over its lifetime. And a "low carbon" material used badly can actually make things worse.
So instead of giving you a league table of good and bad materials, let's talk about how to think about them — and where the real opportunities to reduce emissions actually sit.
Concrete
Concrete gets a hard time in sustainability circles, and not entirely unfairly — it's one of the most carbon-intensive materials in construction. But used well, it's also one of the most useful. Concrete has excellent thermal mass: it absorbs heat during the day and releases it slowly at night, which helps keep indoor temperatures stable and reduces how hard your heating and cooling systems have to work.
The catch is that thermal mass only works if you insulate on the outside. Put the insulation on the inside and you've essentially cut the concrete off from the room — it can't do its job. External insulation keeps the concrete in play thermally, reduces heat loss, and eliminates most thermal bridging issues. It also tends to handle moisture better, often removing the need for a separate vapour barrier.
This holds true whether you're in a cold, temperate, or hot climate. The physics doesn't change — only the specifics of what you're trying to achieve.
Where concrete earns its keep most efficiently is in the structural core of a building — think columns, cores, and floor plates — rather than spread through the external envelope. Concentrate it where strength and thermal mass are genuinely needed, keep the quantity down, and you reduce the carbon hit without giving up the benefits.
Steel
Steel doesn't have thermal mass to speak of, but it more than makes up for that with structural performance. High strength-to-weight ratio, design flexibility, and it goes up fast. For tall buildings or long spans, there's often no better option.
The problem is that steel is highly conductive. Leave it exposed to the elements — or connect it directly between inside and outside — and you've created a thermal bridge: a fast lane for heat to escape your building. In cold climates that means energy haemorrhaging out. In hot climates it means heat pouring in.
The fix is external insulation, same as concrete. Wrap the structure, break the thermal bridge, and steel becomes a much more manageable material from an energy perspective.
The sustainability play with steel isn't really about thermal mass — it's about using less of it. Design the structure to use steel only where its strength is genuinely needed, combine it with lower-carbon materials where possible, and you can keep the carbon cost reasonable. Steel is also highly recyclable, which helps offset some of its upfront impact over the long run.
Timber
If concrete and steel are the workhorses, timber is the overachiever. It's structurally capable, naturally insulating, carbon-sequestering, and — when it comes from responsibly managed forests — genuinely renewable. It's also the material that tends to make buildings feel like actual places to live in, which counts for something.
Unlike concrete or steel, timber provides inherent insulation. In a timber-framed building, insulation sits between the studs. In mass timber construction, it goes on the outside — same principle as concrete. Either way, the building envelope performs well without heroic engineering effort.
Timber and wood fibre insulation also have a useful trick: they store heat and release it slowly, smoothing out temperature swings through the day. In summer this delays peak heat transfer into the building. In winter it helps retain daytime warmth into the evening. It also manages moisture well — absorbing and releasing humidity rather than letting it accumulate — which contributes to comfortable, healthy indoor air.
Used cleverly, timber can do double duty: structure and finish in one, reducing the need for additional lining materials. And because it sequesters carbon throughout its life, it can actively offset some of the emissions associated with the rest of the build.
Windows
Windows are where a lot of buildings quietly haemorrhage energy. Get the specification wrong and all the effort you've put into insulating the walls becomes somewhat academic — heat will find the weakest link.
Modern glazing has come a long way. Double and triple glazing with inert gas fills and low-emissivity coatings can turn glass from a thermal liability into a genuine asset — capturing passive solar heat in cold climates, blocking unwanted heat gain in warm ones. But the frame matters just as much as the glass.
| Aluminium (standard) Avoid | Highly conductive. Creates thermal bridging and surface condensation. Needs a thermal break to perform acceptably. |
| Thermally broken aluminium Acceptable | A non-conductive barrier interrupts heat transfer. Significantly better than standard aluminium — a reasonable choice when properly specified. |
| Steel Context-dependent | Strong and slim sightlines, great for large glazed areas. But like aluminium, needs thermal breaks and careful glazing specification. |
| Timber Strong choice | Naturally insulating, lower embodied carbon, less prone to condensation. Performs well thermally and aesthetically. |
| uPVC Strong choice | Excellent insulation, minimal maintenance, non-conductive. Not the most glamorous option but consistently performs well. |
The right choice depends on your building design, budget, and climate. When in doubt, model it — the difference between frame types can be significant enough to swing your overall energy performance.
Insulation
Here's a take that might surprise you: almost any insulation is better than not enough insulation. The obsession with finding the "greenest" insulation product can sometimes distract from the more important question — are you using enough of it, and is it in the right place?
That said, different products have different strengths, and using the wrong one in the wrong location can cause real problems — particularly around moisture.
PIR and PU foams
High insulation value per millimetre — useful where space is tight. But they're petroleum-based, so the upfront carbon cost is higher. They're also not great with moisture, particularly when foil-faced and used between timber framing. Best used in SIPs panels (structurally insulated panels) with an OSB or metal skin, where moisture behaviour is more predictable.
Mineral wool and glass wool
Workhorses of the insulation world. Good thermal performance, decent sound absorption, made from abundant or recycled materials. Relatively low embodied carbon. Widely available and easy to install correctly.
Wood fibre
Renewable, carbon-sequestering, and excellent at managing moisture within the building envelope. Thermally it's not quite as efficient per millimetre as foam products, but the hygroscopic properties make it a standout choice for timber-frame buildings where moisture management is critical.
Sheep wool
Low embodied carbon, naturally moisture-managing, and very forgiving in timber construction. Not the cheapest option, but a genuinely sustainable one for those who want to go the extra mile.
Polyester
Often made from recycled PET plastic, which gives it a better environmental story than virgin foam. Moderate thermal performance. A reasonable mid-ground option.
Stone wool
High thermal resistance and fire retardancy make this a popular choice for external insulation on multi-storey buildings, where fire performance is non-negotiable. Energy-intensive to produce, but its durability and performance tend to offset the upfront cost over time.
Part 2 covers building systems, mechanical services, and where the rest of your operational carbon comes from.
