Hello, welcome + lifespan assumptions

This focus group is a forum for those who use the Revit plug-in Tally to account for the embodied carbon impacts of their designs. This is the place to ask detailed questions related to the tool, post lessons learned & share best practices with fellow Tally users.

To kick things off, I’d like to share a Tally oversight that we spotted recently when running Tally studies - for the domestic softwood material definition, the default lifespan assumption is 30 years. This may be appropriate for wood products that are exposed to the elements, but is not representative of wood framing members on the interior of wall assemblies. This simple lifespan assumption was very consequential in our Tally results, and can cause your LCA study to inadvertently double-count the impact of wood members if the building lifespan is set to default of 60 years.

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This is a very interesting assumption that Tally makes. There is lots of good wood durability work out there, and useful lifetimes of 100+ years are achievable, even when exposed to the elements, whether through natural durability, treatment, or coating. The Horyuji Temple in Japan is the oldest wooden structure and has wooden members that are ~1,300 years old. In reality durability is a probabilistic phenomenon, and it would make more sense to assume that some portion of timber (say 5%) requires replacement at 30 years, with other maintenance activities also taken into account (recoating for example).

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Hello! Thrilled to have you all starting a Tally forum :smiley:

In terms of the wood assumptions for Softwood Lumber, at Tally we’ve taken the approach of trying to match EPD data or PCRs (where possible), and in absence of that, using sources that we can cite. Obviously, the use of the material influences the service life significantly, so where it seems like there may be a lot of variety in application (such as generic softwood lumber), we typically chose a more conservative estimate. That being said, we absolutely expect people to update those service life numbers based on the use case - that’s why we left the field so easy to update!

Two of us on the Tally team actually published a paper in IJLCA a few years back on the differences that service lives make in LCA results and the variety of service lives that can be assumed, based on different maintenance regimes: https://link.springer.com/article/10.1007/s11367-016-1093-x

For specifically where the 30-year assumption came from, it was from a study published by the National Homebuilders and Remodeler’s association looking at average lifespans of typical materials and assemblies in residential applications.

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Thanks very much for the paper, I think the maintenance scenarios are important to understand. I was also surpised at the number of other materials that go into the casement assemblies, is this why wood fared poorly compared to aluminium? In the paper you used a 2% hardware replacement per annum, can this approach be intergrated into Tally?

In Australia, AS 5604 provides advice on timber durability, and is based on extensive research by CSIRO. Of course, this doesn’t reflect the actual use lifespan, but I guess potential lifespan. Do you have a link to the National Homebuilders and Remodelers data, and are there any other datasets available?

Thanks Alex! I hadn’t caught this yet - a heads up…I’m seeing the same thing for Gypsum Wall Board! Tally defaults to 30 years, but many manufacturers (e.g. CertainTeed) state reference service life is 60 years or more. Good reminder for due diligence in cross referencing specs and Tally assumptions.

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Alex, thanks for bringing this up. I’m also seeing interesting replacement rates within the material assemblies themselves. For instance, the GWB has a default service life of 30 years, but if you add foil facing to this entry, the foil facing is given a default service life of 75 years. This can be adjusted manually, but if you quickly use defaults, it results in an impossible condition where the underlying GWB is replaced and the foil facing is not, undercounting the impact of the foil facing. I’ve seen similar issues with the aluminum mullions and the fluoropolymer coatings, as well as safety glass used within a custom IGU (30 years instead of 40 years for all other items). This doesn’t appear to have a major impact on the the overall numbers, but it’s another item that easily slips by and needs to be checked if you want to dial in your studies.