diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index ceb086ce..5af20455 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -812,6 +812,12 @@ def heat_transmission_from_cert(
                 # code feeds the documentary-evidence R-value calc when a
                 # measured wall thickness is also present (else ignored).
                 wall_insulation_thermal_conductivity=part.wall_insulation_thermal_conductivity,
+                # RdSAP 10 §5.7/§5.8 (PDF p.40-41), Table 14 — a dry-lined
+                # (incl. lath-and-plaster) uninsulated wall adds R=0.17.
+                # The alt-wall path already passes this; the main wall must
+                # too, else every lodged `wall_dry_lined=Y` main wall is
+                # billed at the un-adjusted U.
+                dry_lined=bool(part.wall_dry_lined),
             )
         # When the per-bp `roof_insulation_thickness` is explicitly lodged
         # as 0 (uninsulated — e.g. cert 001479 Ext2 PS sloping ceiling
diff --git a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
index c1632a39..b6a21d0a 100644
--- a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
+++ b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
@@ -1275,6 +1275,50 @@ def test_corridor_door_on_sheltered_alt_wall_uses_table26_u_1p4() -> None:
     assert with_corridor.fabric_heat_loss_w_per_k < no_corridor.fabric_heat_loss_w_per_k
 
 
+def test_main_wall_dry_lining_applies_table_14_resistance() -> None:
+    # Arrange — RdSAP 10 §5.7/§5.8 (PDF p.40-41), Table 14: a dry-lined
+    # (including lath-and-plaster) uninsulated wall adds R=0.17 m²K/W:
+    # U_adj = 1/(1/U₀ + 0.17). A solid-brick (construction 3) age-A wall
+    # with a measured 230 mm thickness has U₀=1.70 (Table 13, 200-280 mm
+    # band) → dry-lined U=1/(1/1.70+0.17)=1.32 (2 d.p.). The alt-wall path
+    # already applies this; the MAIN wall dropped the `dry_lined` kwarg, so
+    # every lodged `wall_dry_lined=Y` main wall was billed at the un-adjusted
+    # U — under-rating solid-brick stock (API wall_construction=3 cohort:
+    # 48 dry-lined certs at 10% within-0.5, signed -1.33).
+    from dataclasses import replace
+
+    base_part = make_building_part(
+        construction_age_band="A",
+        wall_construction=3,        # solid brick
+        wall_insulation_type=0,     # uninsulated (as-built)
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=50.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=28.0, floor=0,
+            ),
+        ],
+    )
+    not_dry = replace(base_part, wall_thickness_mm=230, wall_dry_lined=False)
+    dry = replace(base_part, wall_thickness_mm=230, wall_dry_lined=True)
+    epc_not_dry = make_minimal_sap10_epc(
+        total_floor_area_m2=50.0, country_code="ENG", sap_building_parts=[not_dry],
+    )
+    epc_dry = make_minimal_sap10_epc(
+        total_floor_area_m2=50.0, country_code="ENG", sap_building_parts=[dry],
+    )
+
+    # Act
+    ht_not_dry = heat_transmission_from_cert(epc_not_dry, door_count=0)
+    ht_dry = heat_transmission_from_cert(epc_dry, door_count=0)
+
+    # Assert — same net wall area, so the W/K ratio is the U ratio: the
+    # dry-lined wall is 1.32/1.70 = 0.776× the as-built wall.
+    assert ht_not_dry.walls_w_per_k > 0.0
+    expected_ratio = 1.32 / 1.70
+    assert abs(ht_dry.walls_w_per_k / ht_not_dry.walls_w_per_k - expected_ratio) <= 0.005
+    assert ht_dry.fabric_heat_loss_w_per_k < ht_not_dry.fabric_heat_loss_w_per_k
+
+
 def test_window_uses_effective_u_value_with_curtain_resistance_per_sap10_2_section_3_2() -> None:
     """SAP10.2 §3.2: the window U-value used for heat-transmission is the
     effective form `U_eff = 1/(1/U_raw + 0.04)` — the 0.04 m²K/W is the