diff --git a/domain/modelling/generators/glazing_recommendation.py b/domain/modelling/generators/glazing_recommendation.py
index ac6c371c..b6ec54d0 100644
--- a/domain/modelling/generators/glazing_recommendation.py
+++ b/domain/modelling/generators/glazing_recommendation.py
@@ -21,7 +21,7 @@ scoring (ADR-0016).
 from dataclasses import dataclass
 from typing import Final, Optional
 
-from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from datatypes.epc.domain.epc_property_data import EpcPropertyData, SapWindow
 from domain.geospatial.planning_restrictions import PlanningRestrictions
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import EpcSimulation, WindowOverlay
@@ -47,6 +47,7 @@ class _GlazingTarget:
     glazing_type: int
     u_value: float
     solar_transmittance: float
+    frame_factor: float
 
 
 # Unrestricted: replace the units with double glazing (gt=5 "Double post 2022";
@@ -57,6 +58,10 @@ _DOUBLE: Final[_GlazingTarget] = _GlazingTarget(
     glazing_type=5,
     u_value=1.40,
     solar_transmittance=0.72,
+    # Replacement double units re-lodge a standard FF=0.70 (cert 001431's
+    # after), overriding the panes they replace (e.g. FF 1.00 / 0.50 on the
+    # "single glazing, known data" windows) — feeds §6 solar gains.
+    frame_factor=0.70,
 )
 # Protected (conservation/listed/heritage): fit an internal secondary pane
 # (gt=11 "Secondary glazing - Normal emissivity", what cert 001431 re-lodges;
@@ -69,9 +74,61 @@ _SECONDARY: Final[_GlazingTarget] = _GlazingTarget(
     glazing_type=11,
     u_value=2.90,
     solar_transmittance=0.85,
+    frame_factor=0.70,  # cert 001431's after re-lodges FF=0.70.
 )
 
 
+def _is_draught_proofed(window: SapWindow) -> bool:
+    """`SapWindow.draught_proofed` is `Union[bool, str]` (bool from the
+    site-notes mapper, the string "true"/"false" from the API). Normalise
+    to a bool."""
+    flag = window.draught_proofed
+    if isinstance(flag, bool):
+        return flag
+    return flag.strip().lower() in {"true", "yes"}
+
+
+def _recompute_percent_draughtproofed(
+    epc: EpcPropertyData, upgraded_indices: tuple[int, ...]
+) -> Optional[int]:
+    """RdSAP 10 §8.1 draught-proofing percentage after a glazing upgrade.
+
+    §8.1: "[(number of draughtproofed openable windows & doors) / (total
+    number of openable windows & doors)] × 100", as an integer. Sealed
+    double/secondary units are draught-proofed, so every upgraded window
+    that was NOT draught-proofed flips into the numerator.
+
+    Anchored on the lodged dwelling-level `percent_draughtproofed` (the
+    value the §2 cascade reads) rather than re-deriving the before-count
+    from per-window flags: the unchanged openings are already folded into
+    that aggregate, so this is robust to incomplete window extraction.
+
+        d0    = round(before% / 100 × N)        # draught-proofed before
+        after = round((d0 + flips) / N × 100)    # RdSAP 10 §8.1, integer
+
+    N counts every openable window (vertical + roof) plus external doors.
+    Returns None when no before% is lodged or there are no openings.
+    """
+    before = epc.percent_draughtproofed
+    if before is None:
+        return None
+    n_openings = (
+        len(epc.sap_windows)
+        + len(epc.sap_roof_windows or [])
+        + (epc.door_count or 0)
+    )
+    if n_openings <= 0:
+        return None
+    flips = sum(
+        1
+        for index in upgraded_indices
+        if not _is_draught_proofed(epc.sap_windows[index])
+    )
+    draught_proofed_before = round(before / 100 * n_openings)
+    after = round((draught_proofed_before + flips) / n_openings * 100)
+    return max(0, min(100, after))
+
+
 def recommend_glazing(
     epc: EpcPropertyData,
     products: ProductRepository,
@@ -98,9 +155,15 @@ def recommend_glazing(
                 glazing_type=target.glazing_type,
                 u_value=target.u_value,
                 solar_transmittance=target.solar_transmittance,
+                frame_factor=target.frame_factor,
             )
             for index in single_indices
-        }
+        },
+        # Sealed units draught-proof the panes they replace — RdSAP 10
+        # §8.1 re-lodges the dwelling's percentage (cert 001431: 84 → 100).
+        percent_draughtproofed=_recompute_percent_draughtproofed(
+            epc, single_indices
+        ),
     )
     cost = Cost(
         total=len(single_indices) * product.unit_cost_per_m2,
diff --git a/domain/modelling/scoring/overlay_applicator.py b/domain/modelling/scoring/overlay_applicator.py
index 8bd5dfb0..bde9794d 100644
--- a/domain/modelling/scoring/overlay_applicator.py
+++ b/domain/modelling/scoring/overlay_applicator.py
@@ -50,6 +50,8 @@ def apply_simulations(
             )
         if simulation.lighting is not None:
             _fold_lighting(result, simulation.lighting)
+        if simulation.percent_draughtproofed is not None:
+            result.percent_draughtproofed = simulation.percent_draughtproofed
 
     return result
 
@@ -66,11 +68,13 @@ def _fold_lighting(epc: EpcPropertyData, overlay: LightingOverlay) -> None:
 
 def _fold_window(window: SapWindow, overlay: WindowOverlay) -> None:
     """Write a `WindowOverlay`'s non-``None`` fields onto a (copied) window:
-    ``glazing_type`` flat on the window, ``u_value`` / ``solar_transmittance``
-    into its `WindowTransmissionDetails` (where the cascade reads them), starting
-    a fresh one when the window lodged none."""
+    ``glazing_type`` and ``frame_factor`` flat on the window, ``u_value`` /
+    ``solar_transmittance`` into its `WindowTransmissionDetails` (where the
+    cascade reads them), starting a fresh one when the window lodged none."""
     if overlay.glazing_type is not None:
         window.glazing_type = overlay.glazing_type
+    if overlay.frame_factor is not None:
+        window.frame_factor = overlay.frame_factor
     if overlay.u_value is None and overlay.solar_transmittance is None:
         return
     details: Optional[WindowTransmissionDetails] = window.window_transmission_details
diff --git a/domain/modelling/simulation.py b/domain/modelling/simulation.py
index 94e43c44..2db98cd2 100644
--- a/domain/modelling/simulation.py
+++ b/domain/modelling/simulation.py
@@ -57,12 +57,16 @@ class WindowOverlay:
     are written into the window's `WindowTransmissionDetails` — where the
     calculator reads heat loss and solar gain from — because our calculator
     consumes the lodged values directly rather than deriving them from
-    `glazing_type`. A `None` field means "leave the baseline value unchanged".
+    `glazing_type`. `frame_factor` is written flat on the window (the §6
+    solar-gain area factor); a replacement unit re-lodges its own FF, which
+    can differ from the pane it replaced. A `None` field means "leave the
+    baseline value unchanged".
     """
 
     glazing_type: Optional[int] = None
     u_value: Optional[float] = None
     solar_transmittance: Optional[float] = None
+    frame_factor: Optional[float] = None
 
 
 @dataclass(frozen=True)
@@ -105,3 +109,8 @@ class EpcSimulation:
     windows: Mapping[int, WindowOverlay] = field(default_factory=_no_windows)
     ventilation: Optional[VentilationOverlay] = None
     lighting: Optional[LightingOverlay] = None
+    # Whole-dwelling RdSAP 10 §8.1 draught-proofing percentage, when a
+    # Measure changes it (e.g. glazing: sealed units draught-proof the
+    # panes they replace). The §2 cascade reads this dwelling-level value,
+    # so the overlay sets it directly. `None` leaves the baseline's value.
+    percent_draughtproofed: Optional[int] = None
diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 6bba36ef..3aa20c29 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -17,8 +17,6 @@ from __future__ import annotations
 from dataclasses import replace
 from typing import Final
 
-import pytest
-
 from datatypes.epc.domain.epc_property_data import (
     BuildingPartIdentifier,
     EpcPropertyData,
@@ -486,24 +484,13 @@ def test_double_glazing_overlay_reproduces_the_relodged_after_windows() -> None:
     assert _window_spec(applied) == _window_spec(after)
 
 
-_GLAZING_DRAUGHT_COUPLING_REASON: Final[str] = (
-    "Blocked on the glazing measure's draught-proofing coupling. The window "
-    "U/g overlay reproduces the after's 14 windows EXACTLY (all four single-"
-    "glazed panes — codes 1 and 15 — become the relodged double/secondary "
-    "spec). The residual ~0.7 SAP is a secondary effect the overlay does not "
-    "model: replacing the single-glazed (lodged draught_proofed=No) windows "
-    "with sealed units re-lodges percent_draughtproofed 84->100 (~0.3 SAP) and "
-    "lowers fabric heat loss by ~+150 kWh space heating (~0.4 SAP) not yet "
-    "isolated. Flips green once the glazing overlay propagates draught-proofing "
-    "(and the residual fabric coupling is modelled)."
-)
-
-
-@pytest.mark.xfail(strict=True, reason=_GLAZING_DRAUGHT_COUPLING_REASON)
 def test_double_glazing_overlay_reproduces_the_relodged_after() -> None:
     # Arrange — cert 001431 lodges four single-glazed windows (codes 1 and 15,
     # "single glazing, known data"); the after re-lodges every one as double
-    # (gt=5, U=1.40, g=0.72).
+    # (gt=5, U=1.40, g=0.72). The overlay now also models the two secondary
+    # effects of fitting sealed units: RdSAP 10 §8.1 draught-proofing
+    # 84 → 100 (`percent_draughtproofed`) and the re-lodged FF=0.70 on the
+    # panes that lodged FF 1.00 / 0.50 — so the full-SAP pin closes.
     before: EpcPropertyData = parse_recommendation_summary(
         "double_glazing_001431_before.pdf"
     )
@@ -519,7 +506,6 @@ def test_double_glazing_overlay_reproduces_the_relodged_after() -> None:
     )
 
 
-@pytest.mark.xfail(strict=True, reason=_GLAZING_DRAUGHT_COUPLING_REASON)
 def test_secondary_glazing_overlay_reproduces_the_relodged_after() -> None:
     # Arrange — a planning protection forces secondary glazing; the after
     # re-lodges every single-glazed window as secondary (gt=11, U=2.90, g=0.85).
diff --git a/tests/domain/modelling/test_glazing_recommendation.py b/tests/domain/modelling/test_glazing_recommendation.py
index 993052f0..52063b5e 100644
--- a/tests/domain/modelling/test_glazing_recommendation.py
+++ b/tests/domain/modelling/test_glazing_recommendation.py
@@ -67,8 +67,12 @@ def test_single_glazed_dwelling_yields_a_double_glazing_recommendation() -> None
     option = recommendation.options[0]
     assert option.measure_type == "double_glazing"
     assert dict(option.overlay.windows) == {
-        0: WindowOverlay(glazing_type=5, u_value=1.40, solar_transmittance=0.72),
-        2: WindowOverlay(glazing_type=5, u_value=1.40, solar_transmittance=0.72),
+        0: WindowOverlay(
+            glazing_type=5, u_value=1.40, solar_transmittance=0.72, frame_factor=0.70
+        ),
+        2: WindowOverlay(
+            glazing_type=5, u_value=1.40, solar_transmittance=0.72, frame_factor=0.70
+        ),
     }
 
 
@@ -121,6 +125,10 @@ def test_planning_protection_picks_secondary_glazing_over_double() -> None:
     option = recommendation.options[0]
     assert option.measure_type == "secondary_glazing"
     assert dict(option.overlay.windows) == {
-        0: WindowOverlay(glazing_type=11, u_value=2.90, solar_transmittance=0.85),
-        2: WindowOverlay(glazing_type=11, u_value=2.90, solar_transmittance=0.85),
+        0: WindowOverlay(
+            glazing_type=11, u_value=2.90, solar_transmittance=0.85, frame_factor=0.70
+        ),
+        2: WindowOverlay(
+            glazing_type=11, u_value=2.90, solar_transmittance=0.85, frame_factor=0.70
+        ),
     }