CLT (kgCO2/m3) similar to CONCRETE (kgCO2/m3) ¿why?


We are developing an Office Building in Spain. The structure above grade is made with wood (CLT).

We are studying the differences between the GWP impact of that proposed building (CLT structure) with a baseline (reinforced concrete structure). Both, proposed and baseline have similar below grade structure made with reinforced concrete.

We have found that, talking in m3, the amount of CLT and reinforced concrete above grade is similar, in fact we are using more m3 of CLT structure than concrete. We are aware that the kg are higher in the case of reinforced concrete structure.

What surprises us is that 1m3 of CLT can have similar kgCO2/m3 than 1m3 of concrete. ¿why is this possible? We expected to find big differences between CLT and Concrete, no matter the type of CLT or Concrete, but it is not the case.

The program is taking into account same location and same transport for each CLT option. The different CLT options included are from 450 to 550 kg/m3

To sum up: we were expecting that the results of A1-A3 of CLT structure (without biogenic carbon) would be much lower than the results of A1-A3 of Reinforced concrete strcuture, but if we compare both structures with similar m3 of material, results are not reflecting that difference.

One of the reasons we are considering is the election of CLT.

We have found big differences between the CLT options. ¿why is that?

We will start conversations with the 3th party behind the EPDs of CLT to see how the impacts of A1-A3 were calculated.

We don’t think the inclusion of biogenic carbon or not could be the issue.
The transport it is not the issue, neither the location.

Can be the FSC certificate the main reason of the different results?

If it is the main reason, a final conclusion I see is that, not always a CLT structure will have less GWP A1-A3 than a Reinforced Concrete structure. We will have to be sure that the CLT is FSC, if its not, the final GWP could be similar to the reinforced concrete option.

Does anyone have any comments that can bring some light?

thanks a lot!

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Hi Xavi,

Getting exact numbers for CLT and all mass timber is difficult given our current data. I would be lying to say that I have a perfect understanding of the entire issue, but maybe I can start to help and someone else can jump in to fill in the gaps. CLT has a wide range because the carbon accounting for wood can be quite complex - to illustrate this point, the CLF has a seven part series discussing carbon in wood, linked here:

There’s a lot of questions about biogenic carbon, forestry practices, the health of forests after logging, the regrowth of forests after harvesting, and the amount of carbon that remains in the soil. I see that you removed biogenic carbon from the numbers. This helps clear up some of carbon questions, but it means that the wood isn’t receiving any credit for sequestered carbon, so you’re left with the carbon intensive process of harvesting, milling, and producing the CLT panels.

You’re not alone in noticing that CLT can perform as poorly as concrete. I’ve seen reports from Miller Hull and MKA both looking at the range of carbon in mass timber, and they’ve also identified that CLT can range from being much better than concrete to being equal. My understanding of these studies is that the general belief is that CLT will perform better than concrete, but sourcing the wood from well managed forests is key. Maybe @iancho or @ddavies can weigh in on their studies.

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Hi Xavi, I haven’t looked into this exact problem too much but when I don’t confidently know the manufacture then I’m likely to use the Conservative values from EC3 or the 80th percentile values from One Click (if the material isn’t in EC3). The following compare CLT (25 EPDs) and 35MPa concrete (7861 EPDs!) in EC3. You can see that EC3 is suggesting that most CLT products have a lower EC than concrete, even though you can get intensive CLT products and less intensive concrete products.

This is an aside from your question but CLT and concrete structures won’t be designed to have the same spans so this should be taken into account, just so you’re not considering 7-ply CLT if that isn’t applicable. Similarly, the weight of a CLT building will reduce foundation loads, which you may consider depending on how in-depth this comparison gets, but it is worth acknowledging.

Another study that may be useful is this … SE2050 EC Intensity Diagrams
It shows that CLT floor plates are typically lower than PT concrete alternatives but there can be some cross over, which is what Justin was saying too.


Hi Xramon,

I believe there are a few reasons for these variations:

1- What type of CLT is chosen for the project, as the carbon intensity can be ranged from 0.15 kgCO2-e/kg to 0.4kgCO2-e/kg. Please see the following report by IStrucrE. This also applies to concrete, what type of concrete mix designs are compared with CLT.

2, it is reasonable to say that CLT slab requires a higher volume of products in comparison with concrete, as a result, the kgCO2-e of the project would highly likely be equal to or higher than concrete if we normalise per volume of products (assuming higher range carbon CLT with normal of low range concrete). PT-Slab for the concrete building may be between 180 and 250 mm thick, while the CLT equivalent would most likely be over 380 mm thick. These numbers are based on high-level assumptions and they can be changed across projects.

I think it would be also good to normalise the overall carbon of projects per GFA (m2) or GBA (m2) area instead of the volume or mass of products. The main point of saying GBA is sometimes basement areas are not included in the GFA. It is good to be double-checked with your cost planner.

All the best,

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Thanks @jschwartzhoff, I would like to remark what you said “the general belief is that CLT will perform better than concrete, but sourcing the wood from well managed forests is key”.

Thanks for the video! It refreshed me the difference between storage and emissions of biogenic carbon. We didn’t included the “biogenic carbon stored” sure, as the final result would be then negative. I like a lot this image as it illustrates the issue:

The “biogenic carbon emitted” is something that I don’t think is either included, see attached the detail of the Egoin EPD:

I have found interesting also this:

@jkitchin thanks! after seeing your numbers in EC3 I have done the same but applying the Spanish numbers. It is great to play with the numbers, and it shows how different can be the results looking at specific EPD’s, or using the 80th percentile values (even global or by countries). The conclusion I take from this is that sometimes using the specific EPD of the material used in the building can be worst that using the 80th percentile. ¿why? because there are lots of assumptions behind the different EPDs. We may think that the more accurate we are (by using the EPD of the material) the most accurate results we will get. But in this case, it doesn’t seem the case.

Another interesting issue I have found comparing the results of the same EGOIN CLT EPD in EC3 to the same EPD in OneClick is that from the same impact category A1-A3, the results of GWP are different, whereas the results of the other environmental impacts (acidification, eutrophication, etc) are the same! ¿can there be a problem of transcription? I have already asked to OneClick to double check.

Regarding your last comment, we have worked with the structural engineer and they have said that the difference between the mass of concrete and CLT structures might reduce the foundation loads, but not that much. It is true that we could work on that to compare, but for now we will not take it into account. I prefer to keep as few variables as possible unless they change the results a lot. Regarding the structure of the basement, another conclusion that we have noticed is that in addition to the optimisation that the engineer could do (which could be a lot), in Spain (for the building type we are working with) the law obliges us to include parking for cars in most cases. This forces the project to build from 1 to 3 floors below grade, with the associated retaining wall and load-bearing mass. We will use reinforced concrete for those bellow grade structure, and this amount of reinforced concrete could be a lot if compared with the concrete above grade structure, which makes the reduction of concrete less important in %. We have found here that will be necessary to start working with low embedded concrete carbon. The architecture team also requested the city council for an exception of the norm, and they allowed us to construct less parking cars than usual, allowing the project to include just one floor bellow grade instead of two. This reduced a lot the amount of concrete, but we won’t include this as an strategy of reduction because the GFA won’t be comparable then.

Anyway… just to share! thanks for your feedback!

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Thanks also @mr329 ! the report you shared has been quite interesting! Stora Henso “wins” again if we compare the results, one main reason I see could be because of the mixture of renewable and non-renewable primary energy quoted by each manufacturer (including renewable energy from burning waste wood on-site)… it’s a pity we cannot see the energy/m3 of CLT panels which could have been useful for comparison!

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