slice S-B21: wind shelter factor on infiltration (SAP §2)

Per SAP 10.3 §2 worksheet line 22 / RdSAP10 §4.1: effective infiltration =
raw_ACH × (1 - 0.075 × sheltered_sides). Default 2 sheltered sides for
typical UK terraced/semi-detached layout (the cert doesn't lodge a
sheltered-sides count, so we apply the spec's typical default).

infiltration_ach() gains a `sheltered_sides` kwarg defaulting to 0
(spec-pure intermediate result; existing unit tests keep that contract).
cert_to_inputs passes sheltered_sides=2.

Found via energy decomposition: our predicted total energy was running
+15.7 kWh/m² over cert (10% over) — wind shelter knocks ~15% off
infiltration, contributing to closing that gap.

300-cert parity probe:
  MAE 5.43 → 5.34 (-0.09)
  bias -0.52 → +0.29 (back near zero)
  within ±10: 86.3% → 86.7%

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -720,6 +720,10 @@ def cert_to_inputs(
         passive_vents=vc.passive_vents,
         flueless_gas_fires=vc.flueless_gas_fires,
         window_pct_draught_proofed=float(epc.percent_draughtproofed or 0),
+        # SAP §2 shelter factor: 2 sheltered sides default per typical
+        # UK terraced/semi-detached layout. The cert doesn't lodge a
+        # sheltered-sides count, so we apply the spec's typical default.
+        sheltered_sides=2,
     )
 
     main = _first_main_heating(epc)

packages/domain/src/domain/sap/worksheet/ventilation.py

@@ -70,9 +70,14 @@ def infiltration_ach(
     suspended_timber_floor_sealed: bool = False,
     has_draught_lobby: bool = False,
     window_pct_draught_proofed: float = 0.0,
+    sheltered_sides: int = 0,
 ) -> InfiltrationBreakdown:
     """Air-change rate (ach) per SAP 10.3 §2 / RdSAP10 §4.1, no pressure
-    test path."""
+    test path. `sheltered_sides` defaults to 0 (no shelter; spec-pure
+    intermediate value). Callers can pass 2 (typical UK terraced /
+    semi-detached) to apply the SAP §2 shelter factor
+    (1 - 0.075 × sheltered_sides) so the returned total_ach is the
+    effective rate after wind shelter."""
     if volume_m3 <= 0:
         raise ValueError(f"volume_m3 must be > 0, got {volume_m3}")
     openings_m3_h = (
@@ -95,7 +100,11 @@ def infiltration_ach(
         floor = 0.0
     draught_lobby = 0.0 if has_draught_lobby else 0.05
     window = 0.25 - 0.2 * (window_pct_draught_proofed / 100.0)
-    total = openings + additional + structural + floor + draught_lobby + window
+    raw_total = openings + additional + structural + floor + draught_lobby + window
+    # SAP §2 worksheet line 22 shelter factor: 1 - 0.075 × sheltered_sides.
+    # 2 sheltered sides → multiply by 0.85.
+    shelter_factor = 1.0 - 0.075 * max(0, min(4, sheltered_sides))
+    total = raw_total * shelter_factor
     return InfiltrationBreakdown(
         openings_ach=openings,
         additional_ach=additional,