diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index ec770e72b..0f20e7a39 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -6365,6 +6365,92 @@ def test_loose_jacket_cylinder_computes_storage_loss_via_table2_loose_jacket_bra
     assert got_jan_kwh > 36.9530  # loose jacket loses more than factory
 
 
+def test_uninsulated_cylinder_computes_storage_loss_via_table2_jacket_t0() -> None:
+    """SAP 10.2 Table 2 (PDF p.158): an UNINSULATED cylinder takes the
+    loose-jacket loss factor at t = 0 — L = 0.005 + 1.76 / 12.8 = 0.1425
+    kWh/L/day, the worst storage loss SAP knows. The EPB API lodges
+    cylinder_insulation_type=0 = no insulation (1 = factory, 2 = loose
+    jacket) with no thickness field. Before this fix
+    `_cylinder_storage_loss_override` returned None for type 0, so the
+    worksheet kept the zero-storage-loss combi default and the dwelling
+    OVER-RATED — the opposite of what "no insulation" means. Portfolio
+    814's audit (2026-07-03) found all 26 such properties over-rated
+    +1..+18 SAP vs lodged (avg +12.7 for type 0); same-lodged WD5 0DP
+    neighbours (pids 742215/742216, both lodged 54, identical building
+    parts) split effective 50 vs 69 because one lodged a 12 mm jacket and
+    the other lodged no insulation.
+    """
+    # Arrange — identical to the loose-jacket storage-loss test but
+    # cylinder_insulation_type=0 (none) and no lodged thickness.
+    from domain.sap10_calculator.worksheet.water_heating import (
+        cylinder_storage_loss_factor_table_2,
+        cylinder_temperature_factor_table_2b,
+        cylinder_volume_factor_table_2a,
+    )
+
+    hp_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2206,
+        main_heating_category=4,
+        sap_main_heating_code=None,
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        has_hot_water_cylinder=True,
+        sap_building_parts=[make_building_part()],
+        sap_heating=make_sap_heating(
+            main_heating_details=[hp_main],
+            water_heating_code=901,
+            cylinder_size=3,  # Medium → 160 L
+            cylinder_insulation_type=0,  # no insulation
+            cylinder_insulation_thickness_mm=None,
+            cylinder_thermostat="Y",
+        ),
+    )
+    # Expected (56)m Jan from the Table 2 loose-jacket branch at t=0 —
+    # the SAP uninsulated-cylinder factor (same V / VF / TF as the
+    # loose-jacket test; only the thickness differs).
+    loss_factor = cylinder_storage_loss_factor_table_2(
+        insulation_type="loose_jacket", thickness_mm=0.0
+    )
+    vol_factor = cylinder_volume_factor_table_2a(160.0)
+    temp_factor = cylinder_temperature_factor_table_2b(
+        has_cylinder_thermostat=True, separately_timed_dhw=True
+    )
+    expected_jan_kwh = 160.0 * loss_factor * vol_factor * temp_factor * 31
+
+    # Act
+    wh_result, _ = _water_heating_worksheet_and_gains(
+        epc=epc,
+        water_efficiency_pct=1.7,
+        is_instantaneous=False,
+        primary_age="D",
+        pcdb_record=None,
+    )
+
+    # Assert — non-None (was the zero-loss combi default) and equal to
+    # the loose-jacket branch at t=0; an uninsulated cylinder must lose
+    # MORE than the 50 mm loose-jacket case, never less.
+    assert wh_result is not None
+    got_jan_kwh = wh_result.solar_storage_monthly_kwh[0]
+    assert abs(got_jan_kwh - expected_jan_kwh) < 1e-4
+    jacket_50mm_jan_kwh = (
+        160.0
+        * cylinder_storage_loss_factor_table_2(
+            insulation_type="loose_jacket", thickness_mm=50.0
+        )
+        * vol_factor
+        * temp_factor
+        * 31
+    )
+    assert got_jan_kwh > jacket_50mm_jan_kwh
+
+
 def test_no_water_heating_default_age_a_to_f_uses_12mm_loose_jacket_per_table_29() -> None:
     """RdSAP 10 §10.7 + Table 29 (PDF p.55-56): when no water heating
     system is lodged, the default cylinder takes the age-band insulation,