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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Water condenses on any surface colder than the air's dew point. Enter the room temperature and relative humidity for the dew point, then the outdoor temperature and the R-value of a wall or window to see how cold its inside surface gets, whether it will sweat, and the highest indoor humidity it can take.
With these inputs:
| Result | Value |
|---|---|
| Dew point | 44.6 °F |
| Inside surface temperature | 53 °F |
| Condensation on that surface | No — 8.4 °F above the dew point |
| Highest indoor humidity before it condenses | 55% RH |
Why the surface temperature matters more than the air. Condensation forms where a surface is at or below the dew point, not where the air is. Room air at 70 °F and 40% RH has a dew point of about 44.6 °F, so it only wets a surface that cold. On a 20 °F night, a surface with R-2 overall — roughly an older double-pane window — sits near 53 °F inside, above that dew point. A single-pane window, at about R-1, drops to about 36 °F and runs with water. The inside surface temperature comes from the ratio of the inside air film (R-0.68 in the ASHRAE Handbook of Fundamentals) to the whole assembly's R-value. More insulation raises the inside surface temperature; a cold spot such as a thermal bridge, a missing batt or a window frame lowers it locally. Inside the wall. The same rule applies to layers inside a wall: warm, moist indoor air that leaks into the cavity condenses on the first surface colder than its dew point, often the back of the exterior sheathing. The fix is air sealing first, a vapor retarder where your climate calls for one, and enough exterior insulation to keep the sheathing warm. The wall assembly R-value calculator totals the layers. Formula and accuracy. The calculator uses the Magnus approximation with Alduchov and Eskridge's 1996 coefficients (17.625 and 243.04 °C), which stays within about 0.1 °C of the full saturation vapor pressure equations between −40 and 50 °C. Surface temperatures are steady-state estimates; real surfaces vary with air movement, sun and thermal bridges.
γ = ln(RH/100) + 17.625 × T / (243.04 + T), with T in °C · Dew point (°C) = 243.04 × γ / (17.625 − γ) (Magnus form, Alduchov & Eskridge 1996) · Inside surface °F = Tin − (Tin − Tout) × 0.68 / R-total (0.68 = inside air film, ASHRAE Fundamentals) · Max RH = 100 × exp(17.625·Ts/(243.04+Ts) − 17.625·Tin/(243.04+Tin)), temperatures in °C
| Room temp | 30% RH | 40% RH | 50% RH | 60% RH |
|---|---|---|---|---|
| 65 °F | 32.8 °F | 40.1 °F | 45.9 °F | 50.8 °F |
| 68 °F | 35.4 °F | 42.8 °F | 48.7 °F | 53.6 °F |
| 70 °F | 37.1 °F | 44.6 °F | 50.5 °F | 55.5 °F |
| 72 °F | 38.9 °F | 46.4 °F | 52.3 °F | 57.4 °F |
| 75 °F | 41.5 °F | 49 °F | 55.1 °F | 60.2 °F |
Computed with this calculator's Magnus formula.
| Assembly R-value | Surface °F | Condenses above |
|---|---|---|
| R-1 | 36 °F | 29% RH |
| R-2 | 53 °F | 55% RH |
| R-3.3 | 59.7 °F | 70% RH |
| R-5 | 63.2 °F | 79% RH |
| R-15 | 67.7 °F | 93% RH |
Computed: steady state, inside air film R-0.68. Window edges and frames run colder than the centre of the glass.
About 44.6 °F (7.0 °C). Any surface in the room colder than that, such as a single-pane window on a cold night, will collect condensation.
Convert to °C, compute γ = ln(RH/100) + 17.625·T/(243.04 + T), then dew point = 243.04·γ/(17.625 − γ). At 21.1 °C and 40% RH, γ = 0.492 and the dew point is 7.0 °C (44.6 °F).
The glass is colder than the dew point of the room air. At 70 °F indoors and 20 °F outside, a single-pane window (about R-1) sits near 36 °F, below a 40% RH dew point of 44.6 °F. Lower the humidity, improve the window, or keep air moving across the glass.
It depends on your coldest surface. Enter the outdoor temperature and the R-value of your windows or walls: the calculator gives the highest relative humidity that surface can take before it condenses.
It helps by keeping inside surfaces warmer, but air sealing matters as much. Moist indoor air leaking into a wall condenses on the cold sheathing no matter how much insulation is in the cavity.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.