Annual Drying Analysis

BS EN ISO 13788:2012 §6 — Glaser method, annual moisture balance

How this differs from the winter check: that page is a worst-case design-day snapshot ("could condensation form on a cold day?"). This page integrates 12 months of mean climate and credits drying, answering "does moisture accumulate over a year?" A layer can flag on the winter check yet dry out here — but because this uses monthly means, a pass here does not rule out condensation on a genuine cold snap. Use both, and WUFI for anything marginal.

Known constraints & limitations of this analysis

This is a simplified steady-state annual analysis. It can confirm drying adequacy for vapour-closed assemblies (polythene VCL, foil-faced PIR). It is not appropriate for:

Vapour-open assemblies (wood fibre, hemp, cellulose, lime render) — this tool will show unrealistically high condensation rates. Dynamic simulation (WUFI Pro, Delphin) is required.
Hygrovariable membranes (Intello, MemBrain, DB+) — now approximated: a layer named "smart"/"Intello" gets a month-by-month Sd from an indicative Sd–RH curve (tight when dry, open when humid), crediting inward drying. This is a simplified model, not a substitute for WUFI's measured curves.
Capillary-active materials (lime, AAC, unfaced mineral wool, wood fibre) — liquid transport dominates over vapour diffusion and is not modelled here.
Solar-driven vapour — not modelled (relevant for dark external cladding).
Thermal mass / dynamic temperature effects — not modelled.

Climate data: monthly mean temperature and RH, representative values from CIBSE Guide A Table A2.1. For precise assessment, use site-specific measured climate data. Condensation plane is fixed at the worst-case winter interface for the full year.

Build-up

No build-up loaded. Return to the winter check tool and click "Annual drying analysis →" to transfer your build-up, or load a JSON file.

Climate & occupancy

Location

Interior moisture load

Interior T (°C)

Rsi

Cold-side boundary

Model smart VCL as hygrovariable (Sd varies with RH)

View monthly exterior conditions

Monthly moisture flux at condensation plane

Condensation (g/m²/month)

Evaporation capacity (g/m²/month)

Accumulated moisture (g/m², right axis)

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Peak accumulation

g/m² at condensation plane

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Annual balance

end-of-year residual

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Assessment

Monthly breakdown

Interior: —

Month	T ext	RH int	VCL Sd m	Condensation g/m²	Drying cap. g/m²	Accumulated g/m²

Material tolerance reference (peak accumulation)

For context only. These are indicative thresholds from BRE Digest 369 and Fraunhofer IBP. They are not regulatory limits — always verify against manufacturer specifications and BS 5250.

Material / interface	Threshold	Notes
Timber / OSB (mould risk)	200 g/m²	Surface mould above ~18% MC
Structural timber	300 g/m²	Rot risk above ~20% MC
Mineral wool (general)	1000 g/m²	Low sensitivity; thermal performance loss
Wood fibre insulation	500 g/m²	Hygroscopic buffering partially offsets risk

⚠ For assemblies containing timber, OSB, or wood fibre: if peak accumulation exceeds 200 g/m² under this analysis, commission dynamic simulation before proceeding.

Sources & method

Method: BS EN ISO 13788:2012 Section 6.3 — annual Glaser steady-state moisture balance. Condensation plane fixed from worst winter month. δ₀ = 2×10⁻¹⁰ kg/(m·s·Pa) per EN 13788.

Climate data: Monthly mean temperature and relative humidity for UK sites, from CIBSE Guide A Table A2.1 / BS EN ISO 15927-4. These are representative monthly means; site-specific measured data is preferable for detailed design.

Interior moisture loads: BS EN ISO 13788 §4.2 internal humidity classes. Interior vapour pressure = exterior + vapour-pressure excess Δp, where Δp is constant (class peak) for external temperature ≤ 0 °C and ramps linearly to 0 at +20 °C. Δp peaks: class 1 = 270 Pa, class 3 = 810 Pa, class 4 = 1080 Pa.

Material tolerances: BRE Digest 369; Künzel & Kiessl (Fraunhofer IBP); BS 5250:2021.

Hygrovariable smart VCL (approximate): a layer named "smart"/"Intello" is given a month-by-month Sd from an indicative Sd–RH curve (≈10 m dry → 0.25 m humid), using the mean RH across the membrane — and it opens when the structure behind it is wet, crediting summer inward drying. Below-grade option: holds the cold side at a stable saturated ground temperature while the interior moisture load still follows seasonal air. Both are simplifications.

Still not modelled: capillary transport / wicking, hygroscopic buffering, ISO 13370 ground coupling, solar-driven vapour, thermal mass, and measured product Sd–RH curves. For rigorous work — especially below grade — use WUFI Pro or Delphin.