Checks a wall, roof or floor build-up for interstitial condensation risk using the
Glaser method: the temperature and vapour-pressure profile through every layer, and where (if
anywhere) vapour pressure would exceed saturation.
Method per BS EN ISO 13788:2012. Single winter design condition, not monthly cycling.
Single design-month check only: not a substitute for monthly-cycling hygrothermal simulation. For design guidance only. Always verify with a qualified building physicist.
What the Glaser method checks
Interstitial condensation happens when moist air diffusing through a wall, roof or floor build-up cools enough, at some point inside the construction, that its vapour pressure exceeds the saturation vapour pressure at that point's temperature, water then condenses out of the vapour inside the construction rather than on a visible surface. Left unchecked this can rot timber, corrode fixings and degrade insulation performance long before any surface symptom is visible.
The Glaser method (BS EN ISO 13788:2012) is the standard steady-state hand-calculation check: it walks two profiles through the construction from the warm (internal) side to the cold (external) side under one fixed winter design condition, a temperature profile driven by each layer's thermal resistance, and a straight-line vapour-pressure profile driven by each layer's equivalent still-air thickness (sd = μ·d, where μ is the material's water vapour resistance factor). Wherever the vapour-pressure line would sit above the saturation curve, that interface is at risk: the interface with the largest exceedance is reported as the condensation plane, and the tool estimates the accumulated condensation mass there over your chosen design period using the standard two-line flux-difference construction.
Reading the result
- Layer order matters. Enter layers interior → exterior (room side first). Glaser is direction-sensitive in a way a plain U-value sum is not.
- A vapour control layer on the warm side (high μ, e.g. foil-backed plasterboard or a polythene membrane) combined with a breathable outer layer (low μ) is the standard way to keep the dew point outside the insulation. Getting this the wrong way round, a vapour-tight outer layer trapping moisture behind it. Is the most common cause of a failed check.
- PASS means vapour pressure stays at or below saturation at every interface for the design condition entered. It is not a guarantee against surface condensation, interstitial condensation under different climate data, or moisture ingress from other sources (rain penetration, rising damp, plumbing leaks).
- This is a single design-month check (steady-state, one winter condition). It does not model summer drying-out or annual moisture balance. Where the result is close to the threshold, or the construction is unusual, commission a full monthly-cycling hygrothermal assessment (e.g. WUFI).
Limitations
The calculator covers material layers only (thickness, thermal conductivity λ and vapour resistance factor μ). It does not currently model airgap/cavity layers, thermal bridging, or surface condensation (a separate ISO 13788 check based on internal surface temperature and humidity). External design relative humidity has no single standardised fixed value; the 90% default is a common conservative simplification for a single winter design point. Replace it with actual site/regional climate data for real project design.
About this interstitial condensation calculator
This free interstitial condensation calculator checks a wall, roof or floor build-up for condensation forming inside the construction, using the Glaser method. It is aimed at architects, building physicists and fabric designers verifying that a proposed layer make-up will not trap moisture that could rot timber, corrode fixings or degrade insulation long before any surface symptom shows. Enter each layer interior → exterior (thickness, thermal conductivity λ and vapour resistance factor μ) plus the internal and external design temperature and humidity, and it returns the temperature and vapour-pressure profile through every interface, the condensation plane if any, and an estimated accumulation mass. Everything runs in your browser. Nothing is uploaded.
How interstitial condensation is calculated
The Glaser method (BS EN ISO 13788:2012) walks two straight-line profiles from the warm side to the cold side. Temperature at each interface is Tk = Ti − (Ti−Te)·(Rsi+ΣR)÷Rtotal, using ISO 6946 Table 7 surface resistances. The vapour-pressure profile is driven by cumulative equivalent air-layer thickness sd = μ·d, with surface films given zero vapour resistance. Saturation pressure at each temperature comes from the Alduchov–Eskridge Magnus form, branched over ice below 0°C. The condensation plane is the interface where vapour pressure most exceeds saturation; where it does, the accumulation rate is the two-line flux difference δ0·(Pv−Psat)÷sd across the plane, integrated over your design period.
Frequently asked questions
Does the order I enter the layers matter?
Yes: critically. Glaser is direction-sensitive, so list layers from the warm room side first to the cold outer face last (the opposite of some U-value tools). A vapour control layer (high μ) on the warm side with a breathable (low μ) outer layer is the standard way to keep the dew point out of the insulation; reversing them is the most common cause of a failed check.
What external humidity should I use?
There is no single standardised fixed value, so the 90% default is a conservative single-point simplification. Replace it with actual site or regional climate data for real design work. The check is also a single steady-state winter design condition, not a monthly-cycling analysis; where the result is marginal, commission a full hygrothermal simulation such as WUFI.
What does a PASS actually guarantee?
Only that vapour pressure stays at or below saturation at every interface for the one design condition entered. It is not a guarantee against surface condensation, condensation under different climate data, or moisture from rain penetration, rising damp or leaks. The tool models plane material layers only, not air cavities, thermal bridges or surface condensation. Always verify with a qualified building physicist.