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 62d75c22d..c8836a92b 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -6523,6 +6523,79 @@ def test_elmhurst_no_insulation_cylinder_thickness_zero_takes_uninsulated_loss()
     assert abs(got_jan_kwh - expected_jan_kwh) < 1e-4
 
 
+def test_cylinder_with_unknown_insulation_defaults_per_table_29_age_band() -> None:
+    """RdSAP 10 Table 29 (PDF p.56) "Hot water cylinder insulation if not
+    accessible": when a cylinder is lodged but its insulation is unknown
+    (type absent AND no thickness — ADR-0028 records ~308/1000 17.0 certs
+    omitting `cylinder_insulation_type`), the age-band default applies:
+    bands A-F → 12 mm loose jacket, G/H → 25 mm factory, I-M → 38 mm
+    factory. This is the same Table 29 row the §10.7 no-water-heating
+    default already uses. Previously the cascade returned None (zero
+    storage loss), silently treating an unknown cylinder as a combi.
+    Distinct from the uninsulated case: a lodged 0 mm means NO insulation
+    (t = 0), an absent thickness means NOT ACCESSIBLE (Table 29).
+    """
+    # Arrange — same dwelling, insulation fields both absent; the
+    # fixture's building part lodges construction_age_band "B" → the
+    # Table 29 A-F row (12 mm loose jacket).
+    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()],  # age band B
+        sap_heating=make_sap_heating(
+            main_heating_details=[hp_main],
+            water_heating_code=901,
+            cylinder_size=3,  # Medium → 160 L
+            cylinder_insulation_type=None,  # not lodged
+            cylinder_insulation_thickness_mm=None,  # not lodged
+            cylinder_thermostat="Y",
+        ),
+    )
+    expected_jan_kwh = (
+        160.0
+        * cylinder_storage_loss_factor_table_2(
+            insulation_type="loose_jacket", thickness_mm=12.0
+        )
+        * cylinder_volume_factor_table_2a(160.0)
+        * cylinder_temperature_factor_table_2b(
+            has_cylinder_thermostat=True, separately_timed_dhw=True
+        )
+        * 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 — the Table 29 band-B default (12 mm loose jacket), not the
+    # zero-loss combi default and not the harsher uninsulated t=0.
+    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
+
+
 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,