diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index b2e76b8f..33b9741c 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -589,6 +589,35 @@ def u_wall(
     ):
         u0 = _u_stone_thin_wall_age_a_to_e(construction, wall_thickness_mm)
         if u0 is not None:
+            # RdSAP 10 §5.8 + Table 14 (PDF p.41-42) — added External/Internal
+            # insulation on a stone wall: U = 1/(1/U₀ + R_ins), with U₀ the
+            # RAW §5.6 stone result (the Table-6 footnote (a) 1.7 cap does NOT
+            # apply to the insulated path — same rule the brick branch below
+            # and the dry-lined granite pin 000565 follow). λ defaults to
+            # 0.04 W/m·K per §5.8 final note. Mirrors the WALL_SOLID_BRICK
+            # insulated branch; without it a stone wall lodging code 1/3 +
+            # a thickness was billed at its UNINSULATED U (e.g. sandstone
+            # 520 mm + 100 mm internal: 1.64 → 0.30), the dominant cause of
+            # the wall_insulation_type=3 corpus under-rate cluster.
+            if (
+                wall_insulation_type in (
+                    _WALL_INSULATION_EXTERNAL, _WALL_INSULATION_INTERNAL,
+                )
+                and insulation_thickness_mm is not None
+                and insulation_thickness_mm > 0
+            ):
+                r_ins = _r_insulation_table_14(
+                    insulation_thickness_mm,
+                    _resolve_wall_insulation_lambda_w_per_mk(
+                        wall_insulation_thermal_conductivity
+                    ),
+                )
+                u_unrounded = 1.0 / (1.0 / u0 + r_ins)
+                return float(
+                    Decimal(str(u_unrounded)).quantize(
+                        Decimal("0.01"), rounding=ROUND_HALF_UP
+                    )
+                )
             if dry_lined:
                 # Round to 2 d.p. — worksheet (29a) A×U product uses
                 # the 2-d.p.-displayed U (cf. 000565 Main alt_wall_1:
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index 8d4e3612..38efc93c 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -725,6 +725,80 @@ def test_u_wall_stone_sandstone_thin_wall_age_a_120mm_uses_5_6_sandstone_formula
     assert abs(result - 3.7408) <= 1e-3
 
 
+def test_u_wall_stone_sandstone_with_internal_insulation_applies_5_8_table_14_r_value() -> None:
+    # Arrange — RdSAP 10 §5.8 + Table 14 (PDF p.41-42): a stone wall lodging
+    # External/Internal insulation (wall_insulation_type 1/3) + a thickness
+    # gets the same R-value adjustment as solid brick, applied to the RAW §5.6
+    # U₀. Mirrors corpus cert 100052159386 (Sandstone, 520 mm, 100 mm internal):
+    #   U₀ = 54.876 × 520^(-0.561) = 1.6433
+    #   R  = 0.025 × 100 + 0.25    = 2.75   (Table 14, λ = 0.04)
+    #   U  = 1 / (1/1.6433 + 2.75) = 0.2977 → 0.30 (2 d.p.)
+    # Before this branch the wall was billed at its UNINSULATED U (≈1.64),
+    # the dominant cause of the wall_insulation_type=3 corpus under-rate cluster.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="A",
+        construction=WALL_STONE_SANDSTONE,
+        insulation_thickness_mm=100,
+        insulation_present=True,
+        wall_insulation_type=3,
+        dry_lined=False,
+        wall_thickness_mm=520,
+    )
+
+    # Assert
+    assert abs(result - 0.30) <= 1e-4
+
+
+def test_u_wall_stone_sandstone_insulated_feeds_raw_u0_not_table_6_cap() -> None:
+    # Arrange — the Table-6 footnote (a) 1.7 cap applies ONLY to the as-built
+    # row; the insulated §5.8 path takes the RAW §5.6 U₀ (same rule the brick
+    # branch and the dry-lined granite pin 000565 follow). At W=120 mm the raw
+    # sandstone U₀ = 3.7408 (> 1.7), so the 100 mm internal result must be
+    #   1 / (1/3.7408 + 2.75) = 0.331 → 0.33   (raw),
+    # NOT the capped 1 / (1/1.7 + 2.75) = 0.30. The 0.33 vs 0.30 split proves
+    # the cap is bypassed on the insulated path.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="A",
+        construction=WALL_STONE_SANDSTONE,
+        insulation_thickness_mm=100,
+        insulation_present=True,
+        wall_insulation_type=3,
+        dry_lined=False,
+        wall_thickness_mm=120,
+    )
+
+    # Assert
+    assert abs(result - 0.33) <= 1e-4
+
+
+def test_u_wall_stone_granite_with_external_insulation_applies_5_8_table_14_r_value() -> None:
+    # Arrange — granite/whinstone §5.6 formula + §5.8 external insulation:
+    #   U₀ = 45.315 × 120^(-0.513) = 3.8871
+    #   R  = 0.025 × 50 + 0.25     = 1.50   (Table 14, λ = 0.04)
+    #   U  = 1 / (1/3.8871 + 1.50) = 0.567 → 0.57 (2 d.p.)
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="A",
+        construction=WALL_STONE_GRANITE,
+        insulation_thickness_mm=50,
+        insulation_present=True,
+        wall_insulation_type=1,
+        dry_lined=False,
+        wall_thickness_mm=120,
+    )
+
+    # Assert
+    assert abs(result - 0.57) <= 1e-4
+
+
 def test_u_wall_stone_granite_age_g_with_wall_thickness_ignores_5_6_formula_per_age_a_to_e_gate() -> None:
     # Arrange — §5.6 (PDF p.40) heading explicitly scopes the formula
     # to "age bands A to E". For age F onwards Table 6 gives literal
diff --git a/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py b/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
index 1cddd87f..8944b576 100644
--- a/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
+++ b/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
@@ -102,10 +102,19 @@ _CORPUS = Path(
 # part → SapConservatory → §6.1 window/rooflight/floor cascade + TFA, MIRRORING
 # the case-44 Summary path pinned to 1e-4) -> 68.6% (MAE 0.942). 5 type-4
 # certs were over-rating (conservatory dropped → too little heat loss).
-_MIN_WITHIN_HALF_SAP = 0.68
-_MAX_SAP_MAE = 0.95
-_MAX_CO2_MAE_TONNES = 0.35      # t CO2 / yr vs co2_emissions_current
-_MAX_PE_PER_M2_MAE = 16.0       # kWh / m2 / yr vs energy_consumption_current
+# STONE WALL + INTERNAL/EXTERNAL INSULATION (RdSAP 10 §5.8 + Table 14, p.41-42):
+# the §5.8 added-insulation R-value adjustment was applied ONLY to WALL_SOLID_
+# BRICK; a stone (granite/sandstone) wall lodging wall_insulation_type 1/3 + a
+# thickness fell through the §5.6 branch and was billed at its UNINSULATED U
+# (e.g. sandstone 520 mm + 100 mm internal: 1.64 instead of 0.30 → 5× wall heat
+# loss). Mirroring the brick branch into the stone block recovered the worst of
+# the wall_insulation_type=3 under-rate cluster (cert 100052159386 -26.2 -> -4.1
+# SAP, walls 300 -> 55 W/K). within-0.5 68.6% -> 68.8% (MAE 0.942 -> 0.888;
+# PE MAE 14.3 -> 13.9; CO2 MAE 0.27 -> 0.26). Unit-pinned in test_rdsap_uvalues.
+_MIN_WITHIN_HALF_SAP = 0.685
+_MAX_SAP_MAE = 0.89
+_MAX_CO2_MAE_TONNES = 0.30      # t CO2 / yr vs co2_emissions_current
+_MAX_PE_PER_M2_MAE = 14.5       # kWh / m2 / yr vs energy_consumption_current
 
 
 def _load_corpus() -> list[dict[str, Any]]: