Locate ducts within the conditioned envelope — eliminate the ~20% energy penalty of unconditioned-attic ducts
BASC Guide — guides/ducts-located-conditioned-spaceDescription
Ducts located in unconditioned attics or crawl spaces lose 20-30% of conditioned air to the outside via duct leakage + conductive losses. Best practice: design the structure so ALL ducts fit within the conditioned envelope. Strategies: open-web floor trusses with ducts in the floor cavity; soffit chases inside conditioned space; unvented (sealed + conditioned) attics; conditioned crawl spaces; mechanical closets located centrally.
Why this exists
This is the single largest HVAC efficiency lever — and it's a SCHEMATIC-design decision, not a mechanical-design tweak. The architect chooses the strategy when the floor system is selected. Adding an unvented attic detail or a 12-in dropped soffit chase to the floor plan is far cheaper than retrofitting an air-sealed/insulated attic later.
Categories
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Continuous load path from roof to foundation · HUD RSDG §2.4
- Residential structural reliability targets 1-in-100 to 1-in-1000 annual probability of failure · HUD RSDG §2.5
- Residential floor live load: 40 psf minimum (30 psf sleeping rooms) · HUD RSDG §3.4
- Wind load design uses ASCE 7 basic wind speed for the locality · HUD RSDG §3.6
- Ground snow load for Virginia: 25 psf eastern, up to 40 psf western mountains · HUD RSDG §3.7
Last reviewed 2026-05-15.