feat(heat-transmission): door to unheated corridor uses Table 26 U=1.4 on the sheltered wall

A door opening to an unheated corridor/stairwell takes U=1.4 W/m²K (RdSAP 10
Table 26, p.51 — any age band) instead of the 3.0 external-door default, and
its area deducts from the SHELTERED wall, not the main wall (RdSAP §3.7,
p.18: "the door of a flat/maisonette to an unheated stairwell or corridor
... is deducted from the sheltered wall area"). The cascade previously
billed every door at the external U on the main wall.

Signal: a SHELTERED alternative wall (`is_sheltered`, the RdSAP §5.9
wall-to-unheated-corridor surface, already modelled) is the evidence that
the dwelling is accessed via an unheated corridor, so one lodged door opens
to it. `_corridor_door_count` returns 1 when a sheltered alt wall is present
and >=1 door is lodged, else 0 — so the door channel is unchanged for every
non-corridor dwelling (houses, exposed-gable flats). `heat_transmission_
from_cert` gains a `corridor_door_count` param (default 0): it splits the
door area into external (main wall, age-default U) + corridor (sheltered
alt wall, U=1.4), threading the corridor door's area into that wall's
opening deduction and billing it at 1.4.

Validated on TWO faithful worksheets: simulated case 34 (cert 001431
storage flat — doors 8.14 exact, fabric 207.47 ≈ ws 207.48) and the
long-standing worksheet-harness diverger cert 2474 (−0.87 → −0.32, the
"space-demand thread" was the dropped corridor door). The worksheet harness
is now 47/47 with ZERO divergers.

API SAP gauge: 57.6% → 60.0% within 0.5; mean|err| 1.185 → 1.167; signed
−0.165 → −0.115 — ~22 sheltered-corridor flats were a systematic gap.
Regression gate green (3 pre-existing fails unrelated); pyright net-zero.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -4011,9 +4011,31 @@ def heat_transmission_section_from_cert(epc: EpcPropertyData) -> HeatTransmissio
         insulated_door_count=epc.insulated_door_count,
         insulated_door_u_value=epc.insulated_door_u_value,
         exposure=exposure,
+        corridor_door_count=_corridor_door_count(epc),
     )
 
 
+def _corridor_door_count(epc: EpcPropertyData) -> int:
+    """RdSAP §3.7 + Table 26 — number of doors opening to an unheated
+    corridor/stairwell (each billed at U=1.4 on the sheltered wall).
+
+    The presence of a SHELTERED alternative wall (`is_sheltered`, the
+    RdSAP §5.9 wall-to-unheated-corridor surface) is the evidence that the
+    dwelling is accessed via an unheated corridor, so its entrance door
+    opens to that corridor. RdSAP convention assumes one such access door
+    when the sheltered wall is present and the cert lodges at least one
+    door; the remainder are external. Returns 0 when no sheltered alt wall
+    is lodged (houses, exposed-gable flats) so the door channel is
+    unchanged for every non-corridor dwelling.
+    """
+    has_sheltered_alt = any(
+        (bp.sap_alternative_wall_1 is not None and bp.sap_alternative_wall_1.is_sheltered)
+        or (bp.sap_alternative_wall_2 is not None and bp.sap_alternative_wall_2.is_sheltered)
+        for bp in (epc.sap_building_parts or [])
+    )
+    return 1 if has_sheltered_alt and epc.door_count > 0 else 0
+
+
 def _rooflight_total_area_m2_from_cert(epc: EpcPropertyData) -> float:
     """Σ area of `epc.sap_roof_windows` for §5 daylight-factor L2a +
     §6 horizontal solar gain. Returns 0.0 when none are lodged.

domain/sap10_calculator/worksheet/heat_transmission.py

@@ -117,6 +117,11 @@ def _decimal_round_half_up_product(a: float, b: float, dp: int) -> float:
 
 _WALL_INSULATION_NONE: Final[int] = 4
 _DEFAULT_DOOR_AREA_M2: Final[float] = 1.85
+# RdSAP 10 Table 26 (PDF p.51): a door opening to an unheated corridor or
+# stairwell takes U=1.4 W/m²K for any age band (vs 3.0 for an external door
+# A-J). The door sits on the sheltered wall (RdSAP §3.7 p.18) and its area
+# deducts from that wall, not the main wall.
+_CORRIDOR_DOOR_U_W_PER_M2K: Final[float] = 1.4
 _DEFAULT_STOREY_HEIGHT_M: Final[float] = 2.5
 # SAP10.2 §3.2 curtain/blind thermal resistance applied to windows (and
 # roof windows) — turns raw window U into the worksheet's (27) effective U.
@@ -569,6 +574,7 @@ def heat_transmission_from_cert(
     insulated_door_count: int = 0,
     insulated_door_u_value: Optional[float] = None,
     exposure: Optional[DwellingExposure] = None,
+    corridor_door_count: int = 0,
 ) -> HeatTransmission:
     """Conduction HLC + thermal-bridging contribution, summed across every
     sap_building_part in the cert. Windows and doors are apportioned to the
@@ -590,7 +596,18 @@ def heat_transmission_from_cert(
     floor_description = _joined_descriptions(epc.floors)
 
     # RdSAP10 §15 — door area rounds to 2 d.p. before entering the calc.
-    door_area = _round_half_up(max(0, door_count) * _DEFAULT_DOOR_AREA_M2, _AREA_ROUND_DP)
+    # A door to an unheated corridor (RdSAP Table 26 / §3.7) is billed at
+    # U=1.4 and its area deducts from the sheltered wall, not the main wall.
+    # Only the remaining EXTERNAL doors stay on the main wall at the
+    # age-default U; the corridor door area is tracked separately and
+    # assigned to the first sheltered alt wall in the BP loop below.
+    corridor_door_count = max(0, min(corridor_door_count, door_count))
+    external_door_count = max(0, door_count - corridor_door_count)
+    door_area = _round_half_up(external_door_count * _DEFAULT_DOOR_AREA_M2, _AREA_ROUND_DP)
+    corridor_door_area = _round_half_up(
+        corridor_door_count * _DEFAULT_DOOR_AREA_M2, _AREA_ROUND_DP,
+    )
+    corridor_door_area_remaining = corridor_door_area
     # SAP10.2 §3.2: effective window U includes the 0.04 m²K/W curtain
     # resistance — `(27)` worksheet column applies it per-window. When
     # sap_windows have per-window U lodgements (mixed glazing types in
@@ -1009,12 +1026,20 @@ def heat_transmission_from_cert(
                 continue
             # RdSAP10 §15 — alt wall area rounded to 2 d.p.
             alt_walls_total_area += _round_half_up(alt_wall.wall_area, _AREA_ROUND_DP)
+            alt_opening = alt_window_area if idx == 0 else 0.0
+            # RdSAP §3.7 (p.18): the door to an unheated corridor deducts
+            # from the SHELTERED wall area. Attach the corridor door to the
+            # first sheltered alt wall (its U=1.4 contribution is billed in
+            # the door channel below, not here).
+            if alt_wall.is_sheltered and corridor_door_area_remaining > 0.0:
+                alt_opening += corridor_door_area_remaining
+                corridor_door_area_remaining = 0.0
             alt_walls_contribution += _alt_wall_w_per_k(
                 alt_wall=alt_wall,
                 country=country,
                 age_band=age_band,
                 wall_description=wall_description,
-                opening_area_m2=alt_window_area if idx == 0 else 0.0,
+                opening_area_m2=alt_opening,
             )
         # Main wall net adds back the alt-wall windows that were initially
         # deducted from the BP's total gross — those openings should have
@@ -1263,6 +1288,13 @@ def heat_transmission_from_cert(
         total_external_area += part_external_area
         bridging += y * part_external_area
 
+    # RdSAP Table 26 — the unheated-corridor door's heat loss (U=1.4). Its
+    # area was deducted from the sheltered alt wall above, so the alt wall
+    # net (and hence its W/K) already excludes it; bill it here at the
+    # corridor U. (31) is unaffected: the door area stays counted in the
+    # alt-wall gross contribution, equivalent to the worksheet's separate
+    # door line.
+    doors += _CORRIDOR_DOOR_U_W_PER_M2K * corridor_door_area
     roof_windows_w_per_k = roof_windows_w_per_k_total
     fabric_heat_loss = (
         walls + roof + floor + party + windows + roof_windows_w_per_k + doors  # (33)

tests/domain/sap10_calculator/worksheet/test_heat_transmission.py

@@ -1228,6 +1228,53 @@ def test_sheltered_alternative_wall_applies_table4_0p5_resistance() -> None:
     assert sheltered_wpk < exposed_wpk
 
 
+def test_corridor_door_on_sheltered_alt_wall_uses_table26_u_1p4() -> None:
+    # Arrange — RdSAP 10 Table 26 (PDF p.51): a door opening to an unheated
+    # corridor/stairwell has U=1.4 (any age), versus 3.0 for an external
+    # door (age A-J). RdSAP §3.7 (p.18): "the door of a flat/maisonette to
+    # an unheated stairwell or corridor ... is deducted from the sheltered
+    # wall area" — i.e. the corridor door sits on the sheltered alt wall,
+    # not the main wall. Simulated case 34: a flat with a sheltered alt
+    # (corridor) wall + 2 doors → 1 corridor door (U=1.4 on the alt wall) +
+    # 1 external door (U=3.0 on the main wall).
+    from dataclasses import replace
+
+    main = replace(
+        make_building_part(
+            construction_age_band="B",
+            wall_construction=4, wall_insulation_type=4,
+            party_wall_construction=1, roof_construction=4,
+            floor_dimensions=[
+                make_floor_dimension(
+                    total_floor_area_m2=50.0, room_height_m=2.5,
+                    party_wall_length_m=0.0, heat_loss_perimeter_m=28.0, floor=0,
+                ),
+            ],
+        ),
+        sap_alternative_wall_1=SapAlternativeWall(
+            wall_area=12.5, wall_dry_lined="N", wall_construction=4,
+            wall_insulation_type=4, wall_thickness_measured="Y",
+            wall_insulation_thickness="NI", is_sheltered=True,
+        ),
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=50.0, country_code="ENG", sap_building_parts=[main],
+    )
+
+    # Act
+    no_corridor = heat_transmission_from_cert(epc, door_count=2, corridor_door_count=0)
+    with_corridor = heat_transmission_from_cert(epc, door_count=2, corridor_door_count=1)
+
+    # Assert — no-corridor: both doors external at the age-default U.
+    door_u = no_corridor.doors_w_per_k / (2 * 1.85)
+    # with-corridor: 1 external @door_u + 1 corridor @1.4 (both 1.85 m²).
+    assert abs(with_corridor.doors_w_per_k - (1.85 * door_u + 1.85 * 1.4)) <= 0.02
+    # The corridor door (U=1.4) is cheaper than an external door (U≈3.0) and
+    # its area moves off the main wall onto the sheltered alt wall, so the
+    # net fabric heat loss drops.
+    assert with_corridor.fabric_heat_loss_w_per_k < no_corridor.fabric_heat_loss_w_per_k
+
+
 def test_window_uses_effective_u_value_with_curtain_resistance_per_sap10_2_section_3_2() -> None:
     """SAP10.2 §3.2: the window U-value used for heat-transmission is the
     effective form `U_eff = 1/(1/U_raw + 0.04)` — the 0.04 m²K/W is the