OPTION_A
Enhanced envelope
Credit
0.5 – 1.0 credit
Cost add
+$1.50 – $3/sf
Bump wall to R-25 c.i. equivalent or add R-10 continuous above code minimum.
Best fit: Owner-occupied AADUs where comfort matters and HVAC sizing benefits.
GUIDE · WSEC R406 · 2026
Washington energy code.
The R406 credits ADUs actually use.
The 2021 WSEC (in force statewide) requires every Washington ADU to hit a prescriptive envelope plus collect R406 credits. This guide breaks down envelope minimums, every credit path, the blower-door and duct tests required to prove compliance, and how Puget Sound jurisdictions actually enforce it.
SPEC_01 / ENVELOPE
Prescriptive envelope minimums
Climate zone 5 values (all of Puget Sound). Hitting these is the floor — R406 credits stack on top.
| Assembly | Code minimum | Practical build-out |
|---|---|---|
| Ceiling (vented) | R-60 (climate zone 5) | R-49 blown-in + R-11 batt at energy heel, or R-60 blown loose-fill cellulose. |
| Ceiling (unvented / cathedral) | R-49 c.i. equivalent | R-21 batt + 4" rigid above sheathing, or all-foam closed-cell to R-49. |
| Walls (above grade) | R-21 cavity + R-5 c.i. | 2x6 + R-21 mineral wool + 1" R-5 c.i. (rigid foam or Rockwool ComfortBoard). |
| Walls (basement) | R-21 cavity OR R-15 c.i. | 2" XPS direct-to-wall + 2x4 + R-15 batt is the standard Puget Sound stack. |
| Floor (over crawl) | R-30 | R-30 batt + air-sealed rim joist + vapor barrier on warm side. |
| Slab edge | R-10 to 24" below grade | 2" XPS at slab perimeter; insulate under slab where heating system runs in-floor. |
| Fenestration U-factor | U-0.30 max | Most code-compliant vinyl windows; aluminum-clad wood typically U-0.28 standard. |
| Skylight U-factor | U-0.50 max | Velux + similar code-stock units pass. Avoid acrylic bubbles. |
SPEC_02 / R406_CREDITS
Six R406 credit paths
ADUs under 1,500 sf need 3.0 credits; larger units need more. Stack any combination — heat pump (1.0 – 2.0) plus reduced air leakage (1.0 – 1.5) clears most projects.
OPTION_B
Reduced air leakage
Credit
0.5 – 1.5 credits
Cost add
+$1 – $2/sf
Blower-door test below 3.0 ACH50 (0.5cr), 2.0 (1.0cr), or 1.5 (1.5cr).
Best fit: Highest dollar-per-credit in our experience — the trade is taping & gasket discipline.
OPTION_C
High-efficiency HVAC
Credit
1.0 – 2.0 credits
Cost add
+$1k – $4k all-in
Cold-climate heat pump (HSPF ≥ 9.5) or VRF mini-split is now the default code-compliant path.
Best fit: Every ADU we build — gas-only systems no longer pencil here.
OPTION_D
Heat-pump water heater
Credit
0.5 credit
Cost add
+$1.2k – $2k vs gas
UEF ≥ 2.0 HPWH (e.g., Rheem ProTerra, AO Smith Voltex).
Best fit: AADUs with conditioned utility room (HPWH needs ~700 cf to operate well).
OPTION_E
Drain-water heat recovery
Credit
0.5 credit
Cost add
+$900 – $1.6k
Vertical copper coil on shower drain (PowerPipe / EcoDrain).
Best fit: Plumbing-accessible basement AADUs; cheap when planned at rough-in.
OPTION_F
On-site renewables
Credit
0.5 – 1.5 credits
Cost add
+$8k – $22k
PV system sized to ≥0.5 W per conditioned sqft for full credit.
Best fit: Owner-financed long-hold projects; capture WA sales-tax exemption (RCW 82.08.962).
SPEC_03 / TESTING
Required testing & commissioning
| Test | Target | Performed by | Cost add |
|---|---|---|---|
| Blower-door (whole-house) | ≤ 5.0 ACH50 (code) · ≤ 1.5 for max credit | 3rd-party rater (BPI / RESNET) | $350 – $650 |
| Duct leakage (if ducted) | ≤ 4.0 CFM25 / 100 sf conditioned area | Same rater, usually same visit | $150 – $250 (add-on) |
| Mechanical ventilation balance | Within ±10% of design CFM (IRC M1505 / WSEC C403) | HVAC sub or rater | Often bundled with HVAC commissioning |
| Whole-house ventilation fan | Sized per ASHRAE 62.2 (typically 50 – 75 cfm continuous) | Code requirement, not a credit | $400 – $1,200 (Panasonic WhisperGreen tier) |
SPEC_04 / CITY_AMENDMENTS
How Puget Sound cities enforce WSEC
Seattle
Amendments · SDCI requires energy form at intake; 1.5 R406 credits min for ADUs over 1,000 sf.
Notes · Most AADUs hit credits via heat pump + 1.0 ACH50 reduction.
Bellevue
Amendments · Standard WSEC; no city-specific amendments beyond plan-review checklist.
Notes · Reviewers strict on penetration sealing detail at intake.
Kirkland / Redmond
Amendments · Standard WSEC; online portal accepts REScheck or REM/Rate.
Notes · Faster turn when energy package submitted as a separate sheet.
Tacoma
Amendments · Standard WSEC; sometimes accepts prescriptive path without R406 credits for sub-700sf ADUs.
Notes · Confirm with TMC reviewer — varies by examiner.
Snohomish County
Amendments · Standard WSEC; PUD has incentive program for heat pumps (~$1.5k rebate).
Notes · Stack the PUD rebate with the R406 credit for net-cost wins.
AUTHORITY / Verify the spec
Permit & code references
NAV_GRAPH / Local data
Use this next
Related guides
Permit & cost
Spec your energy package with us.
We'll run the R406 credit math against your jurisdiction, pick the cheapest path to compliance, and bake the rater test fee into your quote — no surprise line items at inspection.