diff --git a/backend/documents_parser/tests/test_heating_systems_corpus.py b/backend/documents_parser/tests/test_heating_systems_corpus.py
index cfde8c83..46c80a79 100644
--- a/backend/documents_parser/tests/test_heating_systems_corpus.py
+++ b/backend/documents_parser/tests/test_heating_systems_corpus.py
@@ -233,7 +233,7 @@ _EXPECTATIONS: tuple[_CorpusExpectation, ...] = (
     _CorpusExpectation(variant='oil pcdb 1', block='11a', expected_sap_resid=+0.4239, expected_cost_resid_gbp=-9.7668, expected_co2_resid_kg=-35.9551, expected_pe_resid_kwh=-83.8239),
     _CorpusExpectation(variant='oil pcdb 2', block='11a', expected_sap_resid=+0.4239, expected_cost_resid_gbp=-9.7668, expected_co2_resid_kg=-35.9551, expected_pe_resid_kwh=-83.8239),
     _CorpusExpectation(variant='oil pcdb 3', block='11a', expected_sap_resid=+1.1597, expected_cost_resid_gbp=-26.7204, expected_co2_resid_kg=-53.1709, expected_pe_resid_kwh=-271.4351),
-    _CorpusExpectation(variant='pcdb 1', block='11a', expected_sap_resid=+2.8556, expected_cost_resid_gbp=-63.2154, expected_co2_resid_kg=-328.7435, expected_pe_resid_kwh=-1257.9712),
+    _CorpusExpectation(variant='pcdb 1', block='11a', expected_sap_resid=+0.5677, expected_cost_resid_gbp=-12.5482, expected_co2_resid_kg=-51.1912, expected_pe_resid_kwh=-109.4555),
     # Slice S0380.133 unblocked 10 solid-fuel variants by routing the
     # Elmhurst §14.0 "Main Heating EES Code" through the new
     # `_ELMHURST_MAIN_HEATING_EES_TO_FUEL_CODE` dict. Pre-slice the
diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index d3bf0822..5b7267e7 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -383,6 +383,56 @@ def test_summary_001431_pcdb_1_inaccessible_cylinder_resolves_to_normal_per_rdsa
     assert epc.sap_heating.cylinder_size == 2
 
 
+def test_summary_001431_pcdb_1_inaccessible_cylinder_resolves_insulation_to_25mm_foam_per_rdsap_10_table_29() -> None:
+    # Arrange — Heating-systems corpus fixture 001431 / "pcdb 1" lodges
+    # §15.1 "Cylinder Size: No Access" alongside age band G (1983-1990).
+    # Per RdSAP 10 Specification §10.11 Table 29 page 56 "Hot water
+    # cylinder insulation if not accessible":
+    #
+    #   - Age band of main property A to F: 12 mm loose jacket
+    #   - Age band of main property G, H: 25 mm foam
+    #   - Age band of main property I to M: 38 mm foam
+    #
+    # pcdb 1 lodges construction_age_band = "G 1983-1990" → 25 mm foam.
+    # The SAP10 `cylinder_insulation_type` enum 1 maps to "factory-
+    # applied" (foam) per `_ELMHURST_CYLINDER_INSULATION_LABEL_TO_SAP10`;
+    # `cylinder_insulation_thickness_mm` carries the literal millimetre
+    # value the cascade feeds into SAP 10.2 Table 2 Note 1's smooth
+    # formula L = 0.005 + 0.55 / (t + 4) for the storage loss factor
+    # (worksheet pcdb 1 (51) = 0.024 ≡ 25 mm).
+    #
+    # Pre-slice the mapper left both fields as None on "No Access"
+    # lodging because `_elmhurst_cylinder_insulation_code` and the
+    # thickness field both look up only the §15.1 measured labels —
+    # which the Summary doesn't carry when the cylinder is
+    # inaccessible. The §4 (56)m storage-loss cascade then skipped the
+    # cylinder loss entirely (`_cylinder_storage_loss_override` requires
+    # insulation_type=factory + thickness to fire), driving worksheet
+    # (56)m sum ~695 kWh missing from cert pcdb 1's (62)m demand.
+    summary_pdf = (
+        Path(__file__).parents[3]
+        / "sap worksheets/heating systems examples/pcdb 1/Summary_001431.pdf"
+    )
+    pages = _summary_pdf_to_textract_style_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_heating.cylinder_insulation_type == 1, (
+        f"pcdb 1 cylinder_insulation_type: got "
+        f"{epc.sap_heating.cylinder_insulation_type!r}, want 1 "
+        f"(factory-applied / foam) per RdSAP 10 §10.11 Table 29 age G "
+        f"row."
+    )
+    assert epc.sap_heating.cylinder_insulation_thickness_mm == 25, (
+        f"pcdb 1 cylinder_insulation_thickness_mm: got "
+        f"{epc.sap_heating.cylinder_insulation_thickness_mm!r}, want 25 "
+        f"per RdSAP 10 §10.11 Table 29 age G row (25 mm foam)."
+    )
+
+
 def test_summary_001431_electric_1_underfloor_heating_resolves_to_in_screed_per_rdsap_10_section_10_11() -> None:
     # Arrange — Heating-systems corpus fixture 001431 / "electric 1" lodges
     # §14.0 "Heat Emitter: Underfloor Heating" (bare form, no subtype
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 0258da83..0878f2ad 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -4319,6 +4319,54 @@ def _elmhurst_cylinder_insulation_code(
     return code
 
 
+def _resolve_elmhurst_inaccessible_cylinder_insulation(
+    age_band: str,
+) -> tuple[int, int]:
+    """RdSAP 10 §10.11 Table 29 page 56 — derive cylinder insulation
+    type + thickness when §15.1 lodges "No Access" / Inaccessible.
+
+    Spec rule verbatim ("Hot water cylinder insulation if not
+    accessible"):
+
+      - Age band of main property A to F: 12 mm loose jacket
+      - Age band of main property G, H:   25 mm foam
+      - Age band of main property I to M: 38 mm foam
+
+    Returns `(insulation_type_code, thickness_mm)` where the SAP10
+    `cylinder_insulation_type` enum value 1 means "factory-applied"
+    (foam) per `_ELMHURST_CYLINDER_INSULATION_LABEL_TO_SAP10`. The
+    cascade's SAP 10.2 Table 2 dispatch (worksheet (51) storage-loss
+    factor) reads thickness as a millimetre integer.
+
+    Age bands A-F (loose jacket) are deferred until a fixture lodges
+    that combination; no current Elmhurst corpus member is age A-F
+    with §15.1 = "No Access". The cascade has no loose-jacket SAP10
+    enum value plumbed (only factory=1 is exercised in
+    `cylinder_storage_loss_factor_table_2`), so raising
+    `UnmappedElmhurstLabel` is the spec-correct strict-fallback per
+    [[reference-unmapped-sap-code]] pattern.
+    """
+    code = age_band[0] if age_band else ""
+    if code in {"G", "H"}:
+        return (1, 25)
+    if code in {"I", "J", "K", "L", "M"}:
+        return (1, 38)
+    if code in {"A", "B", "C", "D", "E", "F"}:
+        raise UnmappedElmhurstLabel(
+            "cylinder_insulation",
+            (
+                f"age band {code!r} (No Access) → 12 mm loose jacket "
+                f"per RdSAP 10 §10.11 Table 29 — loose-jacket SAP10 "
+                f"enum not yet exercised (no corpus member at age A-F "
+                f"with inaccessible cylinder)"
+            ),
+        )
+    raise UnmappedElmhurstLabel(
+        "cylinder_insulation",
+        f"unrecognised age-band code {code!r} for No Access cylinder",
+    )
+
+
 # Elmhurst Summary §11 "Windows" lodged glazing-type strings mapped to
 # the SAP 10.2 Table U2 glazing-type enum that
 # `domain/sap10_calculator/worksheet/internal_gains._G_LIGHT_BY_GLAZING_CODE`
@@ -4638,6 +4686,36 @@ def _map_elmhurst_sap_heating(survey: ElmhurstSiteNotes) -> SapHeating:
         if main_2_detail is not None
         else [main_1_detail]
     )
+    # RdSAP 10 §10.11 Table 29 (p.56) — when the Summary lodges §15.1
+    # "Cylinder Size: No Access" the cylinder is inaccessible during
+    # the survey, so the Summary doesn't carry the cylinder insulation
+    # label / thickness either. Per Table 29 the cascade defaults to
+    # the age-band lookup (G/H = 25 mm foam, I-M = 38 mm foam, A-F =
+    # 12 mm loose jacket). For accessible cylinders the Summary
+    # carries the measured label + thickness and the existing helpers
+    # apply unchanged.
+    is_inaccessible_cylinder = (
+        survey.water_heating.hot_water_cylinder_present
+        and survey.water_heating.cylinder_size_label == "No Access"
+    )
+    if is_inaccessible_cylinder:
+        ins_type_code, ins_thickness_mm = (
+            _resolve_elmhurst_inaccessible_cylinder_insulation(
+                survey.construction_age_band,
+            )
+        )
+        cylinder_insulation_type_field: Optional[int] = ins_type_code
+        cylinder_insulation_thickness_mm_field: Optional[int] = ins_thickness_mm
+    else:
+        cylinder_insulation_type_field = _elmhurst_cylinder_insulation_code(
+            survey.water_heating.cylinder_insulation_label,
+            survey.water_heating.hot_water_cylinder_present,
+        )
+        cylinder_insulation_thickness_mm_field = (
+            survey.water_heating.cylinder_insulation_thickness_mm
+            if survey.water_heating.hot_water_cylinder_present
+            else None
+        )
     return SapHeating(
         instantaneous_wwhrs=InstantaneousWwhrs(),
         main_heating_details=main_heating_details,
@@ -4649,15 +4727,8 @@ def _map_elmhurst_sap_heating(survey: ElmhurstSiteNotes) -> SapHeating:
             water_heating_fuel_label=survey.water_heating.water_heating_fuel_type,
             meter_type_label=survey.meters.electricity_meter_type,
         ),
-        cylinder_insulation_type=_elmhurst_cylinder_insulation_code(
-            survey.water_heating.cylinder_insulation_label,
-            survey.water_heating.hot_water_cylinder_present,
-        ),
-        cylinder_insulation_thickness_mm=(
-            survey.water_heating.cylinder_insulation_thickness_mm
-            if survey.water_heating.hot_water_cylinder_present
-            else None
-        ),
+        cylinder_insulation_type=cylinder_insulation_type_field,
+        cylinder_insulation_thickness_mm=cylinder_insulation_thickness_mm_field,
         # Cascade reads `cylinder_thermostat == "Y"` (string compare) per
         # `cert_to_inputs.py:2252` / `:2218`. Map the bool to the Y/N
         # string the cascade expects; None when no cylinder is present.