Probably the best place to start on this one is to clarify some of the terminologies we use:

• Embodied carbon: The carbon that is "embedded" into the construction of the building and includes all the manufacturing and transportation of the materials as well as the carbon "locked-in" to the materials themselves
• Operational carbon: The carbon associated with running the building. So the heating fuel, be that gas oil or electricity and the electricity for small power.
• Lifecycle carbon: Adds these two together and methodologies generally assume 60-years of operation

I think we can all agree that saving carbon is good (hopefully!!) but the dichotomy is that generally speaking, the best insulations, by which I mean those with the best thermal efficiencies at the slimmest thicknesses, have the highest embodied carbon emissions. A standard PIR for example has about 170kg CO₂/m³ vs 70kg CO₂/m³ for a wood fibre. 

You would however need roughly double the thickness to get the same level of thermal efficiency or heat retention from the wood fibre and that would still be a slight win from a carbon point of view (because now we're comparing 170kg for the PIR against 140kg for the wood fibre). While on paper this seems like a no brainer, practically the difference between 100mm and 200mm can be significant; particularly when considering external walls.

A homeowner can decide to insulate internally or externally:

• Internally and that's a fairly intrusive loss of floor space
• Externally the Architect will need to think carefully about window reveals, roof overhangs and gable ends & there will be planning implications of increasing the property size
• It may be that it's simply not possible to reach that thickness
• They may ultimately be tempted to just do the best possible thickness with the more sustainable material

In my opinion that would be a mistake and here's why…

If you take a typical 100m² three bed 1900s detached property and insulate it to current Building Regulations (SAP notional dwelling) for new builds you'll reduce the space heating requirement by around 85%. Which with a gas boiler is about 6 tonnes of CO₂ annually.

You could reach that saving using either 100mm PIR or 200mm wood fibre, to all thermal elements (I am using a pinch of poetic license here these are a wide variety of practical considerations that this post glosses to prevent it from becoming a monster!) the difference in Embodied Carbon terms is about half a tonne.

If you only have space for 100mm you could reach roughly 1990s standards with 100mm wood fibre, or use only approx. 50mm PIR

Here is the operational and embodied carbon compared…

But when we consider the Lifetime and the 60 year impact of that operational carbon the embodied carbon pails into insignificance...