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Discussing a concrete-free, walk out basement for zone 4c (Vancouver, BC) – GreenBuildingAdvisor
I’m planning a concrete-free walk-out basement/garage/workshop using mechanically stabilized earth retaining walls and permanent wood foundation construction, up in the pacific northwest (Vancouver, BC). I’m hoping for some feedback from folks in the know about whether I’m making any predictable mistakes with the proposed detailing. This is a weird one, so please feel free to ask questions (or question my sanity)! I expect to have missed many obvious issues.
The self-imposed constraint driving this is to minimize the building’s embodied carbon. That in mind, I’m trying to avoid as much steel or concrete as possible, which has led down some unusual design paths.
I’ve attached a subset of a WIP foundation section [1] in order to help illustrate. Broadly, this is a trenched, PWF “slab on grade” with a mechanically stabilized earth retaining wall (Tensar’s SierraScape [2] or Temporary Wall [3], as specified by a geotechnical engineer) holding soil at a ~6″ air gap on all sides. The gap is capped off by a small deck around the building’s perimeter. Finally, the PWF wall transitions to interior untreated plywood sheathing when it’s 12″ above the interior ground level.
The interior floor will have two conditions: one will be for the workshop space (a concrete-free slab on grade: two layers of plywood, compacted 3/4 minus, generic foamed glass aggregate), and the other for a small parking area (a wood sleeper/interior “deck”, appropriately sized to handle car loads, and drained to the exterior). Zones are separated by a french drain.
I intend to handle the framing myself as a full-time job, so at least I won’t be roping anyone else into my terrible plans. There are some significant sequencing challenges, too: the retaining wall will need to be built first due to lot issues, so waterproofing + air sealing needs to be done with gaskets (or something similar). Lstiburek suggests against this [4], notably.
Looking forward to reading what people think!
[1]: https://imgur.com/a/jnTNlcO (I couldn’t upload an image because Cloudflare blocked my POST request … here’s the Ray ID, in case anyone is an admin: 9c84f39e98e92c71). For whatever reason, my ISP gets banned a lot)