diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 5ab3334d..21619ed4 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -4616,11 +4616,25 @@ _WHC_NO_WATER_HEATING_SYSTEM: Final[int] = 999
 # "Immersion Heater Type: Single" so the single-immersion path is used.
 _CYLINDER_SIZE_CODE_NORMAL_110L: Final[int] = 2
 # RdSAP 10 Table 29 (PDF p.56) "Hot water cylinder insulation if not
-# accessible" — the §10.7 default cylinder uses the age-band insulation,
-# same rule as the inaccessible-cylinder path: A-F → 12 mm loose jacket
-# (not yet plumbed — strict-raise), G/H → 25 mm foam, I-M → 38 mm foam.
-_TABLE_29_DEFAULT_CYLINDER_INSULATION_MM_BY_AGE: Final[dict[str, int]] = {
-    "G": 25, "H": 25, "I": 38, "J": 38, "K": 38, "L": 38, "M": 38,
+# accessible" — the §10.7 default cylinder uses the age-band insulation:
+# "Age band of main property A to F: 12 mm loose jacket", G/H → 25 mm
+# foam, I-M → 38 mm foam. Each entry is (cylinder_insulation_type,
+# thickness_mm); the loose-jacket branch is now plumbed (S0380.224) so
+# A-F resolves instead of raising.
+_TABLE_29_DEFAULT_CYLINDER_INSULATION_BY_AGE: Final[dict[str, tuple[int, int]]] = {
+    "A": (_CYLINDER_INSULATION_TYPE_LOOSE_JACKET, 12),
+    "B": (_CYLINDER_INSULATION_TYPE_LOOSE_JACKET, 12),
+    "C": (_CYLINDER_INSULATION_TYPE_LOOSE_JACKET, 12),
+    "D": (_CYLINDER_INSULATION_TYPE_LOOSE_JACKET, 12),
+    "E": (_CYLINDER_INSULATION_TYPE_LOOSE_JACKET, 12),
+    "F": (_CYLINDER_INSULATION_TYPE_LOOSE_JACKET, 12),
+    "G": (_CYLINDER_INSULATION_TYPE_FACTORY, 25),
+    "H": (_CYLINDER_INSULATION_TYPE_FACTORY, 25),
+    "I": (_CYLINDER_INSULATION_TYPE_FACTORY, 38),
+    "J": (_CYLINDER_INSULATION_TYPE_FACTORY, 38),
+    "K": (_CYLINDER_INSULATION_TYPE_FACTORY, 38),
+    "L": (_CYLINDER_INSULATION_TYPE_FACTORY, 38),
+    "M": (_CYLINDER_INSULATION_TYPE_FACTORY, 38),
 }
 
 
@@ -4643,9 +4657,8 @@ def _apply_rdsap_no_water_heating_system_default(
     Elmhurst engine's worksheet header for the corpus "no system" cert
     (WHS 903, Single immersion, 110 L cylinder, 25 mm foam at age G).
 
-    Raises `UnmappedSapCode` for age bands A-F (12 mm loose jacket) —
-    no corpus member exercises that combination and the SAP 10.2 Table 2
-    loss-factor dispatch only has the factory-foam path plumbed.
+    Raises `UnmappedSapCode` only when the main dwelling's age band is
+    absent / outside A-M (no Table 29 row to apply).
     """
     if epc.sap_heating.water_heating_code != _WHC_NO_WATER_HEATING_SYSTEM:
         return epc
@@ -4654,17 +4667,18 @@ def _apply_rdsap_no_water_heating_system_default(
         if epc.sap_building_parts else None
     )
     band = (age_band or "")[:1].upper()
-    thickness_mm = _TABLE_29_DEFAULT_CYLINDER_INSULATION_MM_BY_AGE.get(band)
-    if thickness_mm is None:
+    default = _TABLE_29_DEFAULT_CYLINDER_INSULATION_BY_AGE.get(band)
+    if default is None:
         raise UnmappedSapCode(
             "rdsap_10_7_default_cylinder_insulation_age_band", age_band
         )
+    insulation_type_code, thickness_mm = default
     sap_heating = replace(
         epc.sap_heating,
         water_heating_code=_WHC_ELECTRIC_IMMERSION,
         water_heating_fuel=_STANDARD_ELECTRICITY_FUEL_CODE,
         cylinder_size=_CYLINDER_SIZE_CODE_NORMAL_110L,
-        cylinder_insulation_type=_CYLINDER_INSULATION_TYPE_FACTORY,
+        cylinder_insulation_type=insulation_type_code,
         cylinder_insulation_thickness_mm=thickness_mm,
         cylinder_thermostat="Y",
     )
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 899a99f5..3be0ea4f 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -4032,6 +4032,38 @@ 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_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,
+    and "Age band of main property A to F: 12 mm loose jacket". Bands
+    A-F previously raised UnmappedSapCode because the loose-jacket storage
+    loss branch wasn't plumbed (now it is, S0380.224). A band-B cert must
+    resolve to a 12 mm loose-jacket cylinder; band G stays 25 mm factory.
+    """
+    from domain.sap10_calculator.rdsap.cert_to_inputs import _apply_rdsap_no_water_heating_system_default  # pyright: ignore[reportPrivateUsage]
+
+    def _no_dhw_epc(age_band: str) -> EpcPropertyData:
+        return make_minimal_sap10_epc(
+            total_floor_area_m2=_TYPICAL_TFA_M2,
+            habitable_rooms_count=4,
+            country_code="ENG",
+            sap_building_parts=[make_building_part(construction_age_band=age_band)],
+            sap_heating=make_sap_heating(water_heating_code=999),
+        )
+
+    # Act — band B (A-F band) + band G (factory band, regression guard).
+    band_b = _apply_rdsap_no_water_heating_system_default(_no_dhw_epc("B"))
+    band_g = _apply_rdsap_no_water_heating_system_default(_no_dhw_epc("G"))
+
+    # Assert — band B → 12 mm loose jacket (type 2); band G → 25 mm
+    # factory (type 1). Both gain the immersion + 110 L cylinder default.
+    assert band_b.has_hot_water_cylinder is True
+    assert band_b.sap_heating.cylinder_insulation_type == 2  # loose jacket
+    assert band_b.sap_heating.cylinder_insulation_thickness_mm == 12
+    assert band_g.sap_heating.cylinder_insulation_type == 1  # factory
+    assert band_g.sap_heating.cylinder_insulation_thickness_mm == 25
+
+
 def test_cert_with_hot_water_cylinder_computes_primary_loss_59m_from_sap_table_3() -> None:
     """SAP 10.2 §4 line 7700 + Table 3 (PDF p.159) define the primary
     circuit loss for an indirect cylinder: