The new UK benchmarks being proposed by the RIBA 2030 Climate Challenge arise out of reports and research from the Committee on Climate Change and the Green Construction Board. The 2030 target is <300kgCO2e/m². This is based on Net Internal Area, NIA (RICS doc 3.2.7) which is very close to the TFA (Treated Floor Area) figure used by Passivhaus Designers and covers the RICS Whole Life Carbon model which takes you right from material sourcing through to 'end of life'. This links back to a current benchmark (M4i) of <1000kgCO2e/m².
What does 300kgCO2e/m² look like?
A published footprint study by XCO2 Ltd. looks at a notional two-storey house of 160sqm house for three different construction types and the embodied carbon from cradle to gate – masonry, SIPS and a light-weight timber frame. The results (once adjusted to ensure like with like comparisons) are challenging. The results for above ground between Masonry/SIPS/Timber are 159/120/40 kgCO2e/m² – that’s a fourfold difference between masonry and timber.
The largest impact by far is the embodied carbon within the groundworks (in this example a typical trench fill footing with a 150mm concrete slab). This standard UK construction solution for this detached house emits circa 30 tonnes of carbon or around 188kgCO2e/m². Once this is added into the mix the carbon cost of both the masonry solution (348kgCO2e/m²) and SIPS solution (308 kgCO2e/m²) fail the 2030 challenge. Note that the groundworks account for over 50% of the embodied carbon of the masonry building.
The embodied carbon within the SIPS solution is also higher than one might expect due to the use of oil-based insulations. The light-weight timber construction has been calculated alongside the use of cellulose insulation (oil-based insulations can be up to 30X more carbon intense than cellulose). Given the volumes of insulation required for a low energy construction, the choice of insulation type will have a significant impact on overall embodied carbon.
What happens if you compare these embodied carbon figures with the operational carbon if the house was built to meet the Climate Challenge 2030 target. The 2030 Climate Challenge for operational energy is comparable to a Passive House Classic.
Operational carbon will reflect the energy supply solution as well as the efficiency of the house itself. For example compare a gas boiler for space heating with a 4kw PV (photovoltaic) array (a common Passivhaus choice) against an all-electric solution with an ASHP (air source heat pump) plus a 4kW PV array, both options modelled in PHPP software.
Taking the standard assumption of a 60-year life the gas solution emits 65 tonnes of carbon while the ASHP solution emits a much smaller 12 tonnes of carbon. The all-electric solution is much less carbon-intensive because of the decarbonisation of the grid over the last seven years – electric energy is currently 134 gCO2e/kWh lighter than gas and the gap will widen even further over the next ten years. This is quite a game-changer, mainly the outcome of closing coal-fired power stations and new large offshore wind farms.
So, the current direct comparison between the embodied energy of a Passive House standard light-weight timber frame house (tonnes carbon) and its operational energy (tonnes carbon over 60 years) might be anything between 37:65 (50%) or 37:12 (300%).
If this embodied energy target is robustly conceived then it looks like we are set to build our new housing mainly from timber. If timber is set to become a much more common building material in the UK, then we also should use it carefully – minimal use like small section I-joists are a good solution alongside incorporating supply methods which avoid waste on-site e.g. offsite cutting at a factory or pre-insulated panels plus the much greater use of timber insulation products such as flexible batts, rigid boards or floc.
Thoughtful management of a sustainable supply chain to meet the increased demand will be a critical component for success. According to Jae Cottrell of PH Homes using timber solutions like CLT (which is timber heavy) should only be used where the strength of CLT is needed and not elsewhere. CLT that replaces lots of steel or concrete is great, but not where lightweight, small section timber will do.
Carbon storage within the timber structure is accounted for only at end-of-life and this avoids the danger of thinking that using timber is carbon negative, that maximising timber in constructions will be best, and thereby encourage profligate use.
The challenge of embodied energy is that the emissions related happen NOW and carbon is sent out into the atmosphere – this is unlike operational carbon where the impacts of our choices today might be mitigated to some degree over time (e.g. swapping out a gas boiler for an ASHP after 5-10 years, or further decarbonising of the grid).
There is a lot of good stuff happening in this area. There is the latest version of the ICE published in 2019 (Inventory of Carbon and Energy) and launched at the RICS. There are more materials listed and greater detail incorporated. Products are also starting to come with Environmental Product Declarations (EPD’s) which cover life-cycle environmental impacts large manufacturers of timber products such as Steico have EPD’s for all their products.
The AECB recently brought out version 2 of PHribbon, a plug-in software for PHPP to measure embodied energy (it gathers lots of the ICE and EPD data for you) – it is not expensive and is a valuable addition.
Tim Martel, the author of the PHribbon software has done similar calculations to the above with the new version of PHribbon, from the AECB. This now includes the full life cradle to gate calculation required for the RIBA 2030 challenge. So in addition to stage A covered above it also includes the replacement of products with a short lifespan (stage B) and the end of life (stage C). Together, stages B and C can double the footprint over stage A (cradle to gate) and there is a further storage effect of timber and timber-based materials that can be legitimately included in a full life LCA if the timber is from FSC or PEFC approved sources.
It works off quantities provided in the PHPP energy calculation that can be produced quite quickly at the design stage and follows the RICS professional statement that guides the RIBA 2030 challenge. (though not an official RICS calculation, since that can only be done when the building has been built). He compared a new semidetached house of 78m2 constructed using Passivhaus principles in masonry, SIPS and I-beam/cellulose. The concrete raft foundation was the same in each case. The extra stages substantially change the result and the largest effect is from stage C, the end of life or disposal stage.
Masonry and SIPs have emissions from all stages of 201 and 214 kgCO2/m2 respectively and would pass the 300 kgCO2/m2 NIA limit. However, the timber frame/cellulose option is over 30% lower at 135 kgCO2/m2. Carbon storage of the timber products is included in this figure, assuming they are FSC/PEFC grade according to the RICS methodology. Amazingly, that already low result can be improved further by refinement of stage C scenarios.
In stage C, the standard RICS scenario assumes that 75% of timber is incinerated and 25% goes to landfill, this is based on research into current practice. Half the emissions from the timber option are from this incineration assumption. However, we'd want to be using our resources much smarter than that in the future, even if the incineration has heat recovery. Timber captures CO2 and stores it, but it takes 40 years or more, so that is also a good reason that the life of the products should be as long as possible and it should be reused and recycled rather than burnt. If reuse and recycling were chosen instead of incineration (in stage D) then there is great scope for further reducing the timber frame footprint, and this is still with a concrete raft foundation, there are ways of removing that too with timber.
Tim says we should also be aware of recycling possibilities for oil-based insulation. It makes no sense to cut down trees and throw away plastic. Plastic has a long lifespan and foam insulation of various types could, he suggests, be shredded and used as lose fill locally without the need to take them to specialist facilities to melt them down and reform them. They would be a very similar product to foam bead approaches in new build and of course need the appropriate membranes. Perhaps they might even work with the timber frame options that have already been shown to work so well.
Author’s note: This article is based on an original piece by Jae Cotterell and Anna Carton with additional material from Tim Martell