Interstitial condensation check (Glaser method, BS EN ISO 13788)

R = R_{s i} + \sum_{j = 1}^{4} \frac{d_{j}}{1000 λ_{j}} + R_{s e}

S_{d} = s_{v} + \sum_{j = 1}^{4} \frac{μ_{j} d_{j}}{1000}

p_{v i} = \frac{{RH}_{i}}{100} \times 610.5 \exp (\frac{17.269 T_{i}}{237.3 + T_{i}})

p_{v e} = \frac{{RH}_{e}}{100} \times 610.5 \exp (\frac{17.269 T_{e}}{237.3 + T_{e}})

T_{k} = T_{i} - (T_{i} - T_{e}) \frac{R_{k}}{R}

p_{k} = p_{v i} - (p_{v i} - p_{v e}) \frac{S_{d k}}{S_{d}}

p_{s k} = 610.5 \exp (\frac{17.269 T_{k}}{237.3 + T_{k}})

M = p_{s k} - p_{k}

where
$R$: total thermal resistance (m²K/W)
$R_{s} i, R_{s} e$: internal and external surface resistances (m²K/W)
$d_{j}$: thickness of layer j (mm)
$λ_{j}$: thermal conductivity of layer j (W/mK)
$S_{d}$: total vapour resistance as equivalent air thickness (m)
$s_{v}$: vapour control layer sd (m)
$μ_{j}$: vapour resistance factor of layer j
$p_{v} i, p_{v} e$: internal and external vapour pressures (Pa)
$R H_{i}, R H_{e}$: internal and external relative humidity (%)
$T_{i}, T_{e}$: internal and external air temperatures (°C); below 0 °C the constants 21.875 and 265.5 are used
$T_{k}$: temperature at interface k (°C)
$R_{k}$: resistance from inside air to interface k (m²K/W)
$p_{k}$: vapour pressure at interface k (Pa)
$S_{dk}$: vapour resistance from inside to interface k (m)
$p_{sk}$: saturation vapour pressure at interface k (Pa)
$M$: margin before condensation at interface k (Pa)

Explain this equation

Checks whether moisture from inside a building could condense within a wall, roof or floor build-up, using the Glaser method of BS EN ISO 13788 for one set of conditions.

Temperature drops through the build-up in proportion to each layer's thermal resistance, so the temperature at each interface is the inside temperature less its share of the total drop. Vapour pressure drops in proportion to each layer's vapour resistance (μ times thickness gives an equivalent thickness of still air, S_d). The saturation pressure at each interface follows from its temperature using the Magnus formula. Wherever the actual vapour pressure exceeds saturation (a negative margin), condensation is predicted.

Both heat and vapour flow are treated as steady flows through layers in series, each dropping in proportion to resistance.

Sense check: insulation on the warm side of a vapour-tight outer layer with no vapour control layer often shows condensation at the cold face of the insulation in winter. Adding a VCL on the warm side lowers p_k throughout and usually clears it.

Watch out: this is a single-condition snapshot. It ignores rain, drying, liquid moisture movement and stored moisture, so for solid walls and historic buildings use a full monthly or hygrothermal assessment.

Draws the temperature and vapour-pressure profile through up to four layers and flags any interface where the vapour pressure would exceed the saturation pressure, which is where condensation would form.

Enter your values to see the result.

Good to know

Temperature falls through each layer in proportion to its thermal resistance; vapour pressure falls in proportion to its vapour diffusion thickness sd = μ × d. Saturation pressure follows BS EN ISO 13788 Annex E (over water above 0 °C, over ice below). Interfaces are numbered from the inner face of layer 1 (0) to the outer face of layer 4 (4); a vapour control layer is placed at interface 0. The default is 12.5 mm plasterboard over 50 mm mineral wool on 215 mm brick with 20 mm render, which condenses on the warm face of the brick without a VCL; add a VCL with sd = 10 m to clear it. Typical μ (BS EN ISO 10456): mineral wool 1, plasterboard about 10, brick 10–16, foil-faced PIR is effectively a VCL. BS 5250 relies on this method but says it is a screening tool, not a prediction.

Common mistake

Treating a clear result as proof that internal wall insulation on solid masonry is safe. Glaser ignores rain wetting, built-in moisture, liquid water movement and drying to the inside, which are often the real risk on solid walls.

Limits

Single steady-state check at the conditions you enter, vapour diffusion only, and the simple "flag where p > psat" step rather than the full ISO 13788 tangent construction and monthly accumulation and evaporation. For solid-wall IWI, timber in masonry or heritage fabric, BS 5250 recommends hygrothermal modelling to BS EN 15026. Surface resistances for vapour are neglected.

Sources

Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.

Interstitial condensation check (Glaser method, BS EN ISO 13788)

Good to know

Common mistake

Limits

Sources

Interstitial condensation check (Glaser method, BS EN ISO 13788)

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