diff --git a/domain/modelling/simulation.py b/domain/modelling/simulation.py
index c39d960e..deafd66c 100644
--- a/domain/modelling/simulation.py
+++ b/domain/modelling/simulation.py
@@ -22,6 +22,10 @@ class BuildingPartOverlay:
     """
 
     wall_insulation_type: Optional[int] = None
+    # Added solid-wall insulation depth (mm) — drives the calculator's Table 6
+    # bucket / §5.8 documentary U-value for EWI (`wall_insulation_type=1`) and
+    # IWI (`wall_insulation_type=3`); λ defaults to 0.04 W/m·K in the calculator.
+    wall_insulation_thickness: Optional[int] = None
     roof_insulation_thickness: Optional[int] = None
     floor_insulation_thickness: Optional[int] = None
     floor_insulation_type_str: Optional[str] = None
diff --git a/tests/domain/modelling/fixtures/solid_brick_ewi_001431_after.pdf b/tests/domain/modelling/fixtures/solid_brick_ewi_001431_after.pdf
new file mode 100644
index 00000000..566c1e79
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diff --git a/tests/domain/modelling/fixtures/solid_brick_ewi_001431_before.pdf b/tests/domain/modelling/fixtures/solid_brick_ewi_001431_before.pdf
new file mode 100644
index 00000000..3d0c7396
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diff --git a/tests/domain/modelling/fixtures/solid_brick_iwi_001431_after.pdf b/tests/domain/modelling/fixtures/solid_brick_iwi_001431_after.pdf
new file mode 100644
index 00000000..9f135a34
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diff --git a/tests/domain/modelling/fixtures/solid_brick_iwi_001431_before.pdf b/tests/domain/modelling/fixtures/solid_brick_iwi_001431_before.pdf
new file mode 100644
index 00000000..3d0c7396
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diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index dd53df22..b02d1f97 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -16,13 +16,16 @@ from __future__ import annotations
 
 from typing import Final
 
-from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from datatypes.epc.domain.epc_property_data import (
+    BuildingPartIdentifier,
+    EpcPropertyData,
+)
 from domain.modelling.scoring.package_scorer import PackageScorer, Score
 from domain.modelling.product import Product
 from domain.modelling.recommendation import Recommendation
 from domain.modelling.generators.floor_recommendation import recommend_floor_insulation
 from domain.modelling.generators.roof_recommendation import recommend_loft_insulation
-from domain.modelling.simulation import EpcSimulation
+from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from domain.modelling.generators.wall_recommendation import recommend_cavity_wall
 from domain.sap10_calculator.calculator import Sap10Calculator, SapResult
 from repositories.product.product_repository import ProductRepository
@@ -35,6 +38,14 @@ from tests.domain.modelling._elmhurst_recommendation import (
 # e2e tolerance.
 _PIN_ABS: Final[float] = 1e-4
 
+# RdSAP wall_insulation_type codes for solid-wall insulation (Elmhurst
+# Summary "E External" / "I Internal"); cf. domain/sap10_ml/rdsap_uvalues.py.
+_WALL_INSULATION_EXTERNAL: Final[int] = 1
+_WALL_INSULATION_INTERNAL: Final[int] = 3
+# Recommended solid-wall insulation depth (mm); the calculator's λ default
+# (0.04 W/m·K) matches Elmhurst's lodged thermal conductivity.
+_SOLID_WALL_INSULATION_MM: Final[int] = 100
+
 
 class _AnyProduct(ProductRepository):
     """In-memory ProductRepository returning a fixed Product for any Measure
@@ -83,6 +94,49 @@ def test_cavity_wall_overlay_reproduces_the_relodged_after() -> None:
     )
 
 
+def test_solid_brick_ewi_overlay_reproduces_the_relodged_after() -> None:
+    # Arrange — 100 mm external wall insulation on a solid-brick main wall.
+    before: EpcPropertyData = parse_recommendation_summary(
+        "solid_brick_ewi_001431_before.pdf"
+    )
+    after: EpcPropertyData = parse_recommendation_summary(
+        "solid_brick_ewi_001431_after.pdf"
+    )
+    overlay = EpcSimulation(
+        building_parts={
+            BuildingPartIdentifier.MAIN: BuildingPartOverlay(
+                wall_insulation_type=_WALL_INSULATION_EXTERNAL,
+                wall_insulation_thickness=_SOLID_WALL_INSULATION_MM,
+            )
+        }
+    )
+
+    # Act / Assert
+    _assert_overlay_reproduces_after(before, after, overlay)
+
+
+def test_solid_brick_iwi_overlay_reproduces_the_relodged_after() -> None:
+    # Arrange — 100 mm internal wall insulation on a solid-brick main wall
+    # (also lowers the thermal-mass parameter, unlike EWI).
+    before: EpcPropertyData = parse_recommendation_summary(
+        "solid_brick_iwi_001431_before.pdf"
+    )
+    after: EpcPropertyData = parse_recommendation_summary(
+        "solid_brick_iwi_001431_after.pdf"
+    )
+    overlay = EpcSimulation(
+        building_parts={
+            BuildingPartIdentifier.MAIN: BuildingPartOverlay(
+                wall_insulation_type=_WALL_INSULATION_INTERNAL,
+                wall_insulation_thickness=_SOLID_WALL_INSULATION_MM,
+            )
+        }
+    )
+
+    # Act / Assert
+    _assert_overlay_reproduces_after(before, after, overlay)
+
+
 def test_loft_overlay_reproduces_the_relodged_after() -> None:
     # Arrange
     before: EpcPropertyData = parse_recommendation_summary(