Tim Martel explains how to model the embodied carbon of your build using PHribbon, a tool that plugs in to PHPP (Passive House Planning Package).

Interview with Tim Martel

Tim is a qualified architectural technologist and Passivhaus designer. He also works for the AECB and is a course tutor for the CarbonLite retrofit course. Helped by a background in programming he has used his research and experience of working on new builds and retrofits to create PHribbon – a tool which enables people to model the embodied carbon on their build.

PHribbon is designed to specifically work with the PHPP software

While there are other specialists who do work on embodied carbon, PHribbon is unique in that it has been designed to work specifically with the Passivhaus Planning Package (PHPP). This is a benefit because around 80% of the information needed is already in PHPP. The additional information needed will be the things that aren't covered by PHPP, such as internal walls, floors and roof coverings.

The calculation is built around a RICS document

The Royal Institute of Chartered Surveyors (RICS) wrote a document in 2017 called ‘Whole Life Carbon Assessment for the Built Environment.' It's a statement of what members have to adhere to and sets out in detail what is expected of the calculation. While the RICS calculation can only be done once a building is complete, Tim is able to use quantities that are in PHPP to give a good estimate of what the embodied CO₂ of the building is likely to be.

Information is gathered from Environmental Product Declarations

The RICS calculation uses information from Environmental Product Declarations (EPDs), which are documents that work to European EN standards and which quantifiably demonstrate the environmental performance of a product. RICS also go through the whole lifetime calculation with detail on what should be done at each stage and what assumptions should be used. Assumptions being necessary sometimes for standardisation, because of the inconsistencies of different people coming up with different answers even for the same building. With lots of companies producing these declarations, (Tim now has around 200 in PHPP covering many different materials) the challenge might be in convincing manufacturers of products with lower environmental credentials to share their information.

CO₂ results can be shown in different ways

As long as you are able to provide the net internal area (NIA), results can be shown as kilograms of CO₂ per metre squared of net internal area. In addition the graph can be customised to show total CO₂ for the whole building, kilograms of CO₂ per metre squared of TFA (that's usable space in Passivhaus language), and even CO₂ per occupant.

Awareness is drawn to potential inaccuracies

Tim stresses this isn't an official calculation, but it does closely follow the RICS standard. There will however be some inaccuracies, and these are highlighted by a colour coded system in PHribbon to draw attention. Some EPDs they can be fairly certain about so will be coloured green, but others that they're less confident of will be orange or red. They might be ones that are based on RICS assumptions like 300km being the assumed transport distance from various manufacturers to the site if its a national product, whereas in reality it's likely to be more or less than that.

Generally speaking, the information is likely to get more accurate as more EPDs are produced, and with more available it's highlighting just where the missing information is. In particular, it would be useful to show information for the end of life stage, and include reuse and recycling benefits.

Results can be tailored to show alternatives

With several examples already completed, Tim says his findings show a typical semi-detached house as having around 14 tonnes of CO₂ for the whole life of the building, with about 10 tonnes of that just for the materials themselves. The benefit of using PHribbon is that comparisons can be made in a graph by using different options, such as what the figure might be if the house was made out of brick, as opposed to timber.

More information is needed about the end of life stage

With some of the more typical timber constructions where I-beams and cellulose are used, both of those have quite a large storage component so a large bar represents that storage which offsets all of the emissions in the RICS calculation.

Tim says there's a catch with this though, as one of the assumptions is that in order to include storage you have to do a full life calculation. That then uses the RICS assumption that 75% of the timber is incinerated at the end of life stage, with the other 25% going to landfill. Unfortunately there isn't currently enough information about recycling and reuse to produce those in the EPDs as an alternative.

“If you could say, ‘this building has been built so that it can be taken apart and recycled. This material is ideal for being recycled so it can be made into OSB, MDF or anything’, if that can be done and we can prove the figures for it with an EPD for that process then you could legitimately add that as another option.”

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