diff --git a/domain/sap10_calculator/worksheet/mean_internal_temperature.py b/domain/sap10_calculator/worksheet/mean_internal_temperature.py
index cddd8a08..9a5e2d76 100644
--- a/domain/sap10_calculator/worksheet/mean_internal_temperature.py
+++ b/domain/sap10_calculator/worksheet/mean_internal_temperature.py
@@ -32,6 +32,66 @@ _MONTHS: Final[range] = range(12)
 _TIME_CONSTANT_DIVISOR_KJ_TO_WH: Final[float] = 3.6
 
 
+# SAP 10.2 PDF p.107 Table N5 — additional days at longer heating duration
+# for variable heating duration packages. Each row gives (PSR, N24,9, N16,9):
+# days per year operating at 24 / 16 hours respectively, instead of the
+# standard 9 hours/day. "Use linear interpolation for intermediate values
+# of plant size ratio, rounding the result to the nearest whole number of
+# days." Clamped to the table's bounds per the same convention as PSR
+# efficiency interpolation (PDF p.101 lines 6007-6008).
+_TABLE_N5_VARIABLE_HEATING_DAYS: Final[tuple[tuple[float, int, int], ...]] = (
+    (0.2, 218, 6),
+    (0.3, 191, 22),
+    (0.4, 168, 29),
+    (0.5, 128, 56),
+    (0.6, 94, 74),
+    (0.7, 50, 95),
+    (0.8, 26, 103),
+    (0.9, 14, 92),
+    (1.0, 8, 77),
+    (1.1, 4, 55),
+    (1.2, 3, 38),
+)
+
+
+def extended_heating_days_from_psr_variable(*, psr: float) -> tuple[int, int]:
+    """SAP 10.2 Appendix N3.5 + Table N5 (PDF p.107) — for heat-pump
+    packages with `heating_duration_code = "V"` (Variable), linearly
+    interpolate annual N24,9 and N16,9 totals between the bracketing
+    Table N5 rows at the dwelling's plant size ratio, rounding the
+    result to the nearest whole number of days.
+
+    Clamps to the table's bounds (PSR ≤ 0.2 → first row; PSR ≥ 1.2 →
+    last row) per the same convention as PSR efficiency interpolation
+    in Appendix N (PDF p.101 lines 6007-6008).
+
+    For the legacy fixed durations:
+      "24" → (365, 0)
+      "16" → (0, 365)
+      "9"  → (0, 0)
+    Those branches are caller responsibilities (Table N4) — this helper
+    only covers the Variable case (the only duration lodged on modern
+    PCDB Table 362 records per footnote 48).
+    """
+    if psr <= _TABLE_N5_VARIABLE_HEATING_DAYS[0][0]:
+        return (_TABLE_N5_VARIABLE_HEATING_DAYS[0][1], _TABLE_N5_VARIABLE_HEATING_DAYS[0][2])
+    if psr >= _TABLE_N5_VARIABLE_HEATING_DAYS[-1][0]:
+        return (_TABLE_N5_VARIABLE_HEATING_DAYS[-1][1], _TABLE_N5_VARIABLE_HEATING_DAYS[-1][2])
+    for low, high in zip(
+        _TABLE_N5_VARIABLE_HEATING_DAYS,
+        _TABLE_N5_VARIABLE_HEATING_DAYS[1:],
+    ):
+        low_psr, low_n24, low_n16 = low
+        high_psr, high_n24, high_n16 = high
+        if low_psr <= psr <= high_psr:
+            span = high_psr - low_psr
+            t = (psr - low_psr) / span if span > 0 else 0.0
+            n24_f = low_n24 + (high_n24 - low_n24) * t
+            n16_f = low_n16 + (high_n16 - low_n16) * t
+            return (round(n24_f), round(n16_f))
+    raise AssertionError("PSR bracket not found despite range check")
+
+
 def elsewhere_heating_temperature_c(
     *,
     heat_loss_parameter: float,
diff --git a/domain/sap10_calculator/worksheet/tests/test_mean_internal_temperature.py b/domain/sap10_calculator/worksheet/tests/test_mean_internal_temperature.py
index e1923346..b32f8093 100644
--- a/domain/sap10_calculator/worksheet/tests/test_mean_internal_temperature.py
+++ b/domain/sap10_calculator/worksheet/tests/test_mean_internal_temperature.py
@@ -18,6 +18,7 @@ from domain.sap10_calculator.climate.appendix_u import external_temperature_c
 from domain.sap10_calculator.worksheet.mean_internal_temperature import (
     MeanInternalTemperatureResult,
     elsewhere_heating_temperature_c,
+    extended_heating_days_from_psr_variable,
     mean_internal_temperature_monthly,
     off_period_temperature_reduction_c,
 )
@@ -296,3 +297,64 @@ def test_long_off_period_temperature_reduction_uses_linear_branch() -> None:
     assert result == pytest.approx(8.02, abs=0.05)
 
 
+def test_extended_heating_days_from_psr_variable_clamps_low() -> None:
+    """SAP 10.2 PDF p.107 Table N5: PSR ≤ 0.2 uses the first row's
+    (N24,9, N16,9) = (218, 6).
+
+    Per spec PDF p.101 lines 6007-6008 the PSR is clamped to the
+    table's range (the same clamp policy already applied to PSR
+    efficiency lookup in slice 102c.2)."""
+    # Arrange / Act
+    n24_9, n16_9 = extended_heating_days_from_psr_variable(psr=0.1)
+
+    # Assert
+    assert (n24_9, n16_9) == (218, 6)
+
+
+def test_extended_heating_days_from_psr_variable_clamps_high() -> None:
+    """SAP 10.2 PDF p.107 Table N5: PSR ≥ 1.2 uses the last row's
+    (N24,9, N16,9) = (3, 38). Cert 0380's PSR ≈ 1.43 lands here —
+    worksheet shows N24,9 = 3 (Jan) and N16,9 = 28 + 10 = 38 (Jan
+    + Dec) for cert 0380, which is exactly this row."""
+    # Arrange / Act
+    n24_9, n16_9 = extended_heating_days_from_psr_variable(psr=1.4266)
+
+    # Assert
+    assert (n24_9, n16_9) == (3, 38)
+
+
+def test_extended_heating_days_from_psr_variable_interpolates_midpoint() -> None:
+    """SAP 10.2 PDF p.107 Table N5: "Use linear interpolation for
+    intermediate values of plant size ratio, rounding the result to
+    the nearest whole number of days."
+
+    Midpoint between PSR 0.5 (128, 56) and PSR 0.6 (94, 74):
+      N24,9 at PSR 0.55 = (128 + 94) / 2 = 111 (exact)
+      N16,9 at PSR 0.55 = (56 + 74) / 2 = 65 (exact)
+    """
+    # Arrange / Act
+    n24_9, n16_9 = extended_heating_days_from_psr_variable(psr=0.55)
+
+    # Assert
+    assert (n24_9, n16_9) == (111, 65)
+
+
+def test_extended_heating_days_from_psr_variable_rounds_to_nearest_day() -> None:
+    """SAP 10.2 PDF p.107 Table N5: "Use linear interpolation … rounding
+    the result to the nearest whole number of days."
+
+    Between PSR 0.5 (128, 56) and PSR 0.6 (94, 74) at PSR 0.53:
+      t = 0.3
+      N24,9 = 128 + 0.3 × (94 − 128) = 128 − 10.2 = 117.8 → 118
+      N16,9 = 56  + 0.3 × (74 − 56)  = 56  +  5.4 =  61.4 →  61
+
+    PSR 0.53 lands clear of any half-step where IEEE-754 rounding is
+    ambiguous; the production helper just calls Python `round`.
+    """
+    # Arrange / Act
+    n24_9, n16_9 = extended_heating_days_from_psr_variable(psr=0.53)
+
+    # Assert
+    assert n24_9 == 118
+    assert n16_9 == 61
+