u_floor: route age A,B unknowns to suspended-timber branch (Table 19 fn 1)

RdSAP10 §5.12 Table 19 footnote (1): when floor_construction is unknown,
age bands A and B default to suspended timber, not solid. Previously
u_floor always used the BS EN ISO 13370 solid-floor formula, which
under-counted ~14% on pre-1929 dwellings.

Elmhurst worksheet U985-0001-000490 Main Dwelling (A=14.85, P=7.42,
w=0.400, age B) records floor U=0.71 W/m²K — the suspended-floor formula
on §5.12 page 46 reproduces this exactly. The solid branch returned 0.66.

Description prefixes "Solid, ..." / "Suspended, ..." take precedence over
the age-band default since they're explicit assessor observations.

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

packages/domain/src/domain/ml/rdsap_uvalues.py

@@ -460,6 +460,54 @@ _FLOOR_INSULATION_DEFAULT_MM: Final[dict[str, int]] = {
     "H": 0, "I": 25, "J": 75, "K": 100, "L": 100, "M": 140,
 }
 
+# Table 19 footnote (1): age bands whose default floor_construction is
+# suspended timber (when unknown). All other bands default to solid.
+_SUSPENDED_TIMBER_DEFAULT_BANDS: Final[frozenset[str]] = frozenset({"A", "B"})
+
+
+def _floor_is_suspended_from_description(description: Optional[str]) -> Optional[bool]:
+    """Parse the cert's floor description prefix ("Solid, ..." vs
+    "Suspended, ...") into a tri-state: True if explicitly suspended,
+    False if explicitly solid, None if the description carries no
+    construction signal. `EpcFloorDescriptions` in `datatypes.epc.floor`
+    enumerates the canonical prefixes."""
+    if description is None:
+        return None
+    desc = description.lower().lstrip()
+    if desc.startswith("suspended"):
+        return True
+    if desc.startswith("solid"):
+        return False
+    return None
+
+
+def _u_floor_suspended(
+    *,
+    area_m2: float,
+    perimeter_m: float,
+    wall_thickness_mm: Optional[int],
+    insulation_thickness_mm: int,
+) -> float:
+    """Suspended ground-floor U-value per RdSAP10 §5.12 (page 46). Uses
+    BS EN ISO 13370 with the suspended-floor adjustments — underfloor
+    ventilation Ux is added to the soil Ug term before inverting.
+
+    Parameter defaults are pinned by the spec: thermal resistance of an
+    uninsulated deck Rf=0.2 m²K/W (adds insulation R when present);
+    underfloor height h=0.3 m; mean wind speed v=5 m/s; wind shielding
+    fw=0.05; ventilation openings ε=0.003 m²/m; wall-to-underfloor U_w=1.5.
+    """
+    w = (wall_thickness_mm or 300) / 1000.0
+    soil_g = 1.5
+    r_si = 0.17
+    r_se = 0.04
+    r_f = 0.2 + (insulation_thickness_mm / 1000.0) / 0.035
+    d_g = w + soil_g * (r_si + r_se)
+    b = 2.0 * area_m2 / perimeter_m
+    u_g = 2.0 * soil_g * log(pi * b / d_g + 1.0) / (pi * b + d_g)
+    u_x = (2.0 * 0.3 * 1.5 / b) + (1450.0 * 0.003 * 5.0 * 0.05 / b)
+    return 1.0 / (2.0 * r_si + r_f + 1.0 / (u_g + u_x))
+
 
 def u_floor(
     country: Optional[Country],
@@ -512,6 +560,25 @@ def u_floor(
         # Table 19 footnote (2): "use the greater of 50 mm and the
         # thickness according to the age band".
         ins_mm = max(50, age_default_mm)
+    # Table 19 footnote (1): if floor_construction is unknown, age bands
+    # A and B default to suspended timber (the rest default to solid).
+    # A description prefix of "Solid, ..." or "Suspended, ..." takes
+    # precedence over the age-band default since it's an explicit assessor
+    # observation about the construction.
+    band_upper = age_band.upper() if age_band else None
+    described_suspended = _floor_is_suspended_from_description(description)
+    use_suspended_branch = (
+        described_suspended
+        if described_suspended is not None
+        else (construction is None and band_upper in _SUSPENDED_TIMBER_DEFAULT_BANDS)
+    )
+    if use_suspended_branch:
+        return _u_floor_suspended(
+            area_m2=area_m2,
+            perimeter_m=perimeter_m,
+            wall_thickness_mm=wall_thickness_mm,
+            insulation_thickness_mm=ins_mm or 0,
+        )
     r_f = ((ins_mm or 0) / 1000.0) / 0.035
     d_t = w + soil_g * (r_si + r_f + r_se)
     b = 2.0 * area_m2 / perimeter_m

packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py

@@ -742,6 +742,29 @@ def test_u_floor_solid_uninsulated_typical_geometry_returns_iso_13370_value() ->
     assert result == pytest.approx(0.68, abs=0.05)
 
 
+def test_u_floor_age_b_unknown_construction_uses_suspended_timber_per_table_19_footnote_1() -> None:
+    # Arrange — RdSAP10 §5.12 Table 19 footnote (1) routes age A, B with
+    # unknown floor_construction to the suspended-timber branch. Geometry
+    # is taken from Elmhurst worksheet U985-0001-000490 Main Dwelling
+    # (A=14.85, P=7.42, w=0.400) — the worksheet records U=0.71 W/m²K,
+    # confirming the suspended-floor formula on §5.12 (page 46) is the
+    # one Elmhurst applies for this fixture.
+
+    # Act
+    result = u_floor(
+        country=Country.ENG,
+        age_band="B",
+        construction=None,
+        insulation_thickness_mm=None,
+        area_m2=14.85,
+        perimeter_m=7.42,
+        wall_thickness_mm=400,
+    )
+
+    # Assert
+    assert result == pytest.approx(0.71, abs=0.01)
+
+
 def test_u_floor_with_insulation_lowers_u_value() -> None:
     # Arrange — same geometry but with 100mm insulation -> R_f = 0.1/0.035 = 2.857.