Record ADR-0050 (underfloor override companions) and ADR-0051 (wall U-value gate) 🟪

WS2 of PRD #1435, diagnosed on property 711795: an electric-underfloor
override inherits the replaced community-heating category (no pathway,
baseline mis-rated 51 vs 67), and wall generators offer £4k insulation
that cannot change an age-band-L wall's derived U (0.280 → 0.280).
Glossary: Wall U-Value Gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CONTEXT.md

@@ -317,7 +317,11 @@ The two competing **Measure Options** for insulating a *solid* (non-cavity) main
 _Avoid_: solid wall insulation (that names the pair, not one Option), cladding, drylining
 
 **Wall Insulation Eligibility**:
-The rule fixing which wall Option(s) the main-wall **Recommendation** offers, by wall construction then planning status. By construction: **cavity** → cavity fill only (never solid insulation); **solid brick** / **system-built** → IWI + EWI; **timber-frame** → IWI only (EWI is not constructable); **cob** / **granite or whinstone** / **sandstone or limestone** → none (breathable fabric — standard insulation risks trapping moisture). Then, as planning gates: a **conservation area** or **flat** removes EWI (external-appearance / whole-block constraints), and a **listed** or **heritage** building removes **both** EWI and IWI (protected fabric).
+The rule fixing which wall Option(s) the main-wall **Recommendation** offers, by wall construction then planning status. By construction: **cavity** → cavity fill only (never solid insulation); **solid brick** / **system-built** → IWI + EWI; **timber-frame** → IWI only (EWI is not constructable); **cob** / **granite or whinstone** / **sandstone or limestone** → none (breathable fabric — standard insulation risks trapping moisture). Then, as planning gates: a **conservation area** or **flat** removes EWI (external-appearance / whole-block constraints), and a **listed** or **heritage** building removes **both** EWI and IWI (protected fabric). Finally the **Wall U-Value Gate** applies.
+
+**Wall U-Value Gate**:
+The eligibility rule that a wall insulation Option is offered **only when it would lower the wall's derived U-value** (by more than a 0.01 W/m²K float-noise guard) — comparing `u_wall` on the effective wall's current state vs its post-measure state (ADR-0051). Kills the **no-op fabric offer**: on a modern construction age band (e.g. band L, 2012–2022) RdSAP's cascade assigns the as-built wall its at-regs U-value and added insulation resolves to the *same* value, so the measure costs thousands for +0.00 SAP. The gate is physical possibility only — how *much* improvement is worth the cost stays with the Optimiser (ADR-0016/0024).
+_Avoid_: no-op filter (it gates generation, not scored output), U-value threshold (there is no absolute target-U; the comparison is before-vs-after)
 _Avoid_: restricted measures (legacy collapsed conservation/listed/heritage into one boolean — they now gate different Options, so keep them distinct)
 
 **Roof Insulation Eligibility**:

docs/adr/0050-electric-underfloor-override-drags-category-8-and-conservative-group-7-control.md

@@ -0,0 +1,84 @@
+# An electric-underfloor override drags category 8 and the conservative Group 7 control
+
+## Status
+
+accepted
+
+## Context
+
+[ADR-0048](0048-heating-override-forces-category-and-control-defers-fuel-and-meter.md)
+established that a system-replacing heating override must emit a **complete,
+coherent companion set** — category and control are archetype-forced, never
+inherited — and explicitly deferred the category expansion for the still-`None`
+archetypes ("electric boiler/CPSU→2, underfloor→8 … flows into the
+category-keyed Table 12a resolver and needs a corpus check") to its own change.
+
+The portfolio-796 audit (PRD #1435, WS2) found the underfloor half of that gap
+end-to-end on property **711795** (uprn 10090343115): a lodged **community
+heating** cert (SAP 301, category 6, control 2311) overridden to *"Electric
+underfloor, in screed above insulation"* (SAP 424,
+[ADR-0046](0046-electric-underfloor-scored-by-its-own-codes-meter-deferred-when-tariff-ambiguous.md)).
+`_category_for(424)` and `_control_for(424)` both returned `None`, so the
+effective cert kept the replaced heat network's **category 6** and community
+control. Two independent mis-ratings followed:
+
+1. **No heating pathway.** `is_heat_network_main` keys on category 6, so the
+   heating Recommendation Generator excluded both HHRSH and ASHP by the
+   heat-network topology rule — `recommend_heating` returned `None` and the
+   flat's whole candidate menu was a no-op wall measure plus lighting. The
+   dwelling was unliftable from band E (the `plan-stuck-in-low-band` audit
+   group counts ~7 electric-underfloor dwellings).
+2. **Baseline mis-rated.** The calculator scored the electric underfloor as
+   community heating: baseline SAP **51 (E)** vs **67 (D)** with coherent
+   companions. With the pathway restored, HHRSH scores **80 (C)** for ~£1k —
+   the dwelling reaches the scenario goal.
+
+## Decision
+
+The three electric-underfloor archetypes (SAP Table 4a **421 / 422 / 424**,
+ADR-0046) join the forced-companion branches of `main_heating_system_overlay`:
+
+1. **Category 8** ("Electric underfloor heating") in `_category_for` —
+   deterministic from SAP Table 4a, per ADR-0048's category rule.
+2. **Control 2701** (SAP Table 4e **Group 7**, "no time or thermostatic control
+   of room temperature", **+0.3 °C**) in `_control_for` — the conservative
+   default within the group the category fixes: the largest mean-internal-
+   temperature adjustment in Group 7, so an unobserved control is never
+   over-credited. Mirrors the storage manual-charge 2401
+   ([ADR-0035](0035-coherent-heating-system-synthesis.md)) and room-heater 2601
+   (ADR-0048) precedents.
+
+Fuel (electricity 29) and meter (Dual for 421/422; **deferred to the cert** for
+tariff-ambiguous 424) are already correct per ADR-0046 and are unchanged.
+
+**Scope**: underfloor only. Electric boiler / CPSU (Group 1) remain
+log-and-continue gaps tracked by #1444 — their category (2) feeds the wet-boiler
+fallback classification in the generators and needs its own check.
+
+## Consequences
+
+- An underfloor override on any lodged system now yields a coherent cert: the
+  Table 12a resolver and the heat-network gates read the dwelling as electric
+  underfloor, the baseline re-rates correctly, and the HHRSH bundle is offered
+  where eligible (a flat's only heating pathway — ASHP stays house/bungalow-only
+  per ADR-0024).
+- The "incomplete companion set" error log no longer fires for 421/422/424.
+- **Known deferred gap (pre-existing, unchanged by this ADR):** the Table 12a
+  space-heating resolver's underfloor rows are wired in the fraction table but
+  `_table_12a_system_for_main` never returns `UNDERFLOOR_HEATING` ("Underfloor
+  heating (421-422) — TODO"), so an off-peak-metered underfloor dwelling falls
+  to the all-low-rate fallback. That affects genuinely-lodged underfloor certs
+  identically and needs spec sight (SAP 10.2 Table 12a Grid 1, PDF p.191) to
+  split slab (storage-like) from screed (direct-acting-like); tracked as a
+  follow-up, not ridden on this override fix.
+
+### Alternatives rejected
+
+- **A zero-adjustment Group 7 control (2703/2704).** Rejected: credits a room
+  thermostat the landlord never reported; the conservative default is the
+  ADR-0035/0048 pattern.
+- **Completing electric boiler / CPSU (category 2) in the same change.**
+  Rejected: no evidence property in this workstream, and category 2 flows into
+  the generators' wet-boiler fallback (`_WET_BOILER_CATEGORIES`), which would
+  make an electric-boiler override newly eligible for tune-up options — a
+  behaviour change needing its own corpus check (#1444).

docs/adr/0051-wall-insulation-offered-only-when-it-lowers-the-derived-wall-u-value.md

@@ -0,0 +1,71 @@
+# Wall insulation is offered only when it lowers the derived wall U-value
+
+## Status
+
+accepted
+
+## Context
+
+The wall Recommendation Generators trigger on the wall's **insulation state**
+alone: `recommend_cavity_wall` offers cavity fill for any cavity wall whose
+`wall_insulation_type` is as-built (4), and `recommend_solid_wall` offers
+EWI/IWI for any as-built suitable construction
+([ADR-0019](0019-solid-wall-insulation-options.md)). Neither asks whether
+adding insulation can actually **change the modelled wall**.
+
+For modern construction age bands it cannot: RdSAP's Table 6-10 cascade
+(`u_wall` in `domain/sap10_ml/rdsap_uvalues.py`) assigns an as-built wall its
+age band's at-regs U-value, and the post-measure state resolves to the **same
+value**. Portfolio-796 evidence (PRD #1435, WS2): property **711795**, timber
+frame at age band **L** (2012–2022) — `u_wall` returns **0.280 W/m²K both
+as-built and with 100 mm IWI**, so the persisted plan offered
+`internal_wall_insulation` at **£4,027 for +0.00 SAP**. Any modern-age-band
+dwelling gets the same no-op fabric offer; on an unliftable dwelling it is the
+*entire* remedial menu, which reads as "we recommend spending £4k for nothing".
+
+Scoring already prices the option at +0.00, but the Optimiser keeps
+zero-uplift candidates in the persisted menu (they are legitimate for some
+measure types, e.g. secondary-heating removal saves bills without SAP), so the
+fix belongs at **generation eligibility**, not a blanket score filter.
+
+## Decision
+
+Both wall generators consult the **derived wall U-value** before offering: the
+measure is offered only when the post-measure state's `u_wall` is **lower than
+the current state's by more than 0.01 W/m²K** (a float-noise guard, not a
+materiality threshold — economics stay with the Optimiser, ADR-0016/0024).
+
+- **Current state**: `u_wall` with the effective main wall's construction, age
+  band, lodged insulation type/thickness — a thin parameter mapping in the
+  `envelope.py` style, deliberately omitting the calculator's rich extras
+  (surveyor description, dry-lining, documentary-thickness routes). Omissions
+  bias the gate toward **offering** (both sides are computed identically, so a
+  no-op still compares equal), never toward suppressing a real measure.
+- **Post-measure state**: the same call with the Option's overlay applied —
+  filled cavity (`wall_insulation_type=2`) for cavity fill; 100 mm
+  internal/external insulation for IWI/EWI.
+
+## Consequences
+
+- Age-band-L (and any at-regs age band) walls stop polluting candidate menus
+  with £4k no-ops; genuinely treatable walls (e.g. age-band-D solid brick,
+  U 2.1 → ~0.3) are offered exactly as before.
+- The generators gain a read on `domain/sap10_ml/rdsap_uvalues.u_wall` — same
+  precedent as the heating generator reading the calculator's PCDB/Table 4b.
+- **Deferred:** (a) a cert that lodges an explicit `wall_u_value` overrides the
+  cascade in the calculator, making fabric measures score +0.00 regardless of
+  this gate — the measure overlay should adjust the lodged U; (b) roof/floor
+  generators have the same anti-pattern in principle (roof age-band eligibility
+  is partly covered by ADR-0047); both tracked as follow-ups.
+
+### Alternatives rejected
+
+- **Filter zero-SAP candidates after scoring.** Rejected: zero-uplift options
+  are legitimate for some measure types (bill-driven), and the pollution would
+  survive in any consumer that reads Generator output before scoring.
+- **Hard-code "age band ≥ K ⇒ no wall offer".** Rejected: duplicates the U
+  tables coarsely; the tables already encode exactly which
+  construction × age-band × insulation combinations move.
+- **Full-fidelity U parity with the calculator (description, dry-lining,
+  lodged U).** Rejected for now: heavier coupling for no observed benefit —
+  the omissions only ever bias toward offering, which is the status quo.