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Materials

Steel and framing for hempcrete

Hempcrete needs a structural frame. MicroHom compared timber, US light-gauge steel and imported aluminum for a 400 sq ft one-storey home.

Wall-frame options with hempcrete infill, 400 sq ft home
OptionWall-frame material (estimate)California code pathHeat flow through the frame
Timber, 2×6About $340–1,000Simplest: Appendix BL figures are drawn for wood studs; untreated wood may touch hempcrete (BL103.3.6). Engineer still required in SDC D0–D2.Low: softwood about 0.10–0.13 W/m·K
Light-gauge steel, 600S162About $1,000 (18 ga) to $1,240 (16 ga), plus required coating (cost unknown)CRC R603 with an engineer, under Appendix BL's "approved alternative design."Steel about 48–58 W/m·K; bury it in at least 3 in of hempcrete and add breaks at the tracks
Light-gauge steel + engineered bambooSteel as above, plus bamboo floor about $1,300–1,800 at $3.29–4.55/sq ft retail; beams price unknownAs steel, plus structural bamboo only with an evaluation report and an engineerAs steel
Aluminum extrusions (imported)About $3,100–4,350 landed before freight; dies $300–3,000 per profileHardest: CBC Chapter 20 requires the Aluminum Design Manual; full engineering plus an R104.11 requestHighest: 6063 aluminum about 201 W/m·K; thermal-break profiles needed

MicroHom arithmetic on sourced unit prices: timber at $480–1,004 per thousand board feet; steel studs at $1.56–1.94 per linear foot (May 2025 list); aluminum at $3.01–4.20/kg × 1.55 for 5% MFN plus 50% Section 232 duty. Wall framing only.

Metal and lime

  • Any metal touching hemp-lime must be stainless or coated with epoxy, oil, bituminous paint or another approved coating; galvanizing alone does not meet BL103.4, except for the heads of hot-dip galvanized nails.
  • Wet alkaline material attacks aluminum surfaces; coatings and isolation from steel are recommended (NBS Report 9521). Anodizing is not a listed coating under BL103.4.
  • Aluminum and steel used together need a non-conductive barrier between them.

Seismic design

Most of California is in IRC Seismic Design Category D0, D1 or D2, and Appendix BL requires an engineered design there. In practice every frame option needs a California engineer. Hempcrete can weigh up to 65 psf of wall, which adds seismic mass.

What MicroHom's research recommends for a first build

A US-made light-gauge steel frame, coated per BL103.4, with precast hemp blocks and at least 3 inches of hempcrete cover, on a permanent foundation, with timber as the permit fallback. Aluminum is set aside until a steel or timber build is permitted, because of its cost, the 50% duty, full engineering and heat loss. This is a research recommendation, not an engineering design.

Steel framing suppliersAluminum suppliers

Sources

  1. 2025 California Residential Code, Appendix BL, Section BL101 General (gocodebook). Checked Oct 17, 2026.
  2. 2025 CRC Appendix BL, Section BL103 Hemp-Lime Construction (gocodebook; effective Jan 1, 2026). Checked Oct 17, 2026.
  3. 2025 CRC Section R104, duties and powers of the building official (alternative materials, R104.11). Checked Oct 17, 2026.
  4. 2025 California Building Code, Chapter 20 Aluminum, Section 2002. Checked Oct 17, 2026.
  5. FEMA P-2192 Seismic Design Category maps for the 2024 IRC (July 2024). Checked Oct 17, 2026.
  6. US Frame Factory, metal stud price list (updated May 1, 2025). Checked Oct 17, 2026.
  7. Natural Resources Canada, current lumber prices (Random Lengths composite). Checked Oct 17, 2026.
  8. Gordian, lumber price updates (Q4 2026). Checked Oct 17, 2026.
  9. Westmetall, LME aluminium cash price. Checked Oct 17, 2026.
  10. USITC Harmonized Tariff Schedule search, heading 7604 (2026). Checked Oct 17, 2026.
  11. U.S. Customs and Border Protection, Tariff Overview (Jan 14, 2026). Checked Oct 17, 2026.
  12. Congressional Research Service IN12519, Section 232 tariffs (Sep 17, 2026). Checked Oct 17, 2026.
  13. American Galvanizers Association, galvanized rebar reactions in concrete. Checked Oct 17, 2026.
  14. NBS Report 9521, Corrosion of aluminum embedded in concrete. Checked Oct 17, 2026.
  15. Thermal conductivity chart (calibr.pro). Checked Oct 17, 2026.
  16. Energy Code Ace, 2025 Reference Appendix JA4 (U-factors, framing factors). Checked Oct 17, 2026.

Last updated Oct 18, 2026. Each source above was checked on the date shown.