Thursday, June 5, 2014

Quick Look at Embodied Energy of Two Roof Solutions

A friend who is proposing a vaulted roof made from compressed earth blocks / tiles asked me to take a look at embodied energy between his proposed solution and a typical concrete roof. My response is below. Note the initial roof area, thickness, and materials for both the proposed and typical roof were provided by the friend. It appears that the compressed earthen blocks have significantly less embodied energy compared to a concrete dome solution. Another comparison might be the domed earthen blocks to a flat (less area) concrete roof, but this is not represented below. Let me know if you have any questions. Nathan Gauthier

Here is a summary of your emissions and embodied energy comparisons for the two roof options. The earthen dome option has significantly less embodied CO2 emissions (81% less). See below. All conversion factors come form the Inventory of Carbon and Energy (ICE v2.0) put out by Bath University.

Most of your savings comes from the tile roof not having steel reinforcement (very high embodied energy compared to other materials) and the earthen tile roof thickness (150 mm) being much less than the concrete (250 mm). Sand, aggregate and cement have the same CO2 per unit for each roof, but there is more volume / mass in the thicker concrete roof. I had to make a number of assumptions (type of lime, strength of concrete, percent recycled content for steel, etc.) but they don’t have a significant impact. Detailed breakdowns follow:

All volume to mass conversions from:
I didn't have Rwanda-specific emissions data, but sand, water, aggregate will all be collected locally with hand labor (aggregate will be crushed by hand), the earthen blocks are hand pressed (machine originally from South Africa), and concrete, steel, and anything else that is imported will come by truck into Rwanda and probably by boat to Mombasa, Kenya. 

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