Methodology: how our calculations work

Reviewed and updated 22 October 2026 · How we assess condensation

We publish every equation and assumption so you — or a surveyor — can check our reasoning. Our advantage is meant to be transparent reasoning and better measurements, not a long questionnaire or a precise-looking score.

Engine version 1.0 (October 2026). Changes to equations, default values or guidance are recorded in the changelog at the bottom of this page.

What the tool is — and isn't

It's a screening and planning tool for a single room. It's not a validated diagnostic instrument or a substitute for a survey. Wording such as "consistent with", "possible" and "needs investigation" is deliberate: we don't give percentage likelihoods for causes, because those would need validation against independently surveyed homes. We also never give a single overall "home health" score — a dry window can't cancel out a leaking roof.

1. Psychrometrics

Saturation vapour pressure over water (Magnus form, coefficients from Alduchov & Eskridge):

es(T) = 6.112 · exp(17.62·T / (243.12 + T)) hPa, T in °C

• Vapour pressure: e = RH/100 · es(T)
• Dew point: Td = 243.12·γ / (17.62 − γ), γ = ln(e / 6.112)
• Absolute humidity: v = 216.7 · e / (T + 273.15) g/m³
• Humidity at a surface: RHs = 100 · e / es(Ts)

We compare water content or vapour pressure — never relative humidity at different temperatures. Air at 5 °C and 90% warmed to 18 °C is about 38%.

2. Surface temperatures

Measured surface temperatures (infrared thermometer, ±0.5 °C assumed) are always used when given. Otherwise we estimate them with the temperature factor used in BS EN ISO 13788:

Ts = Te + fRsi · (Ti − Te)

For a plane element with thermal transmittance U (stated with standard surface resistances 0.13 and 0.04 m²K/W), re-solved for an internal surface resistance Rsi:

fRsi = 1 − Rsi / (Rsi + (1/U − 0.17) + 0.04)

U-value ranges used (W/m²K):

Steady-state factors ignore thermal mass and solar gain, so they tend to describe the coldest part of a cold night. A whole-window U-value can't reliably give the coldest point at a frame edge or a poorly insulated reveal — which is why we treat edges separately and recommend measuring.

3. Room moisture balance

Steady state, well-mixed room: v_in = v_out + G / (n · V), where G is moisture generation (g/h), n the air-change rate (per hour) and V the room volume (m³).

These are typical published estimates, used as ranges. Room humidity is capped at 95%: above that, cold surfaces condense the excess out of the air, so a steady state at saturation is not physical. If you enter a measured humidity, we calibrate the model to it and scale scenarios from your reading instead.

Air-change rates come from the draught answer — none 0.2–0.5, slight 0.4–0.8, noticeable 0.7–1.2, strong 1.0–2.0, not sure 0.35–1.2 per hour — plus trickle vents (+0.1–0.3), windows open overnight (+0.5–2.0), continuous extract (20–45 m³/h), MVHR (+0.3–0.5) and positive input ventilation (+0.2–0.6). Infiltration really does vary with wind, shelter and temperature, as the government's Home Energy Model documentation notes, so a "slightly draughty" answer is never turned into one precise number.

4. Uncertainty

Every uncertain input is a low–high range, and we evaluate the corners of those ranges to report the lowest and highest plausible result. "Not sure" answers widen the range rather than defaulting to a favourable value. Each input is labelled as measured, user, estimated or default in your results.

5. Scenarios

• Ventilation: +0.3–0.8 air changes/h (living rooms and bedrooms) or a 25–45 m³/h continuous fan (bathrooms, kitchens). Heat cost = 0.33 · Δn · V · ΔT Wh/h.
• Dehumidifier: rated capacity de-rated for room temperature and humidity — compressor 5–20% of rating below 10 °C, 15–35% at 10–15 °C, 25–50% at 15–20 °C, 35–60% above 20 °C, further scaled for humidity; desiccant 50–85%. It is assumed to run during the problem period and stop at 50% humidity.
• Heating +2 °C: air and surfaces warmed through their temperature factors; water content unchanged. Cost from fabric and ventilation heat loss.
• Laundry elsewhere, furniture gap, glazing upgrade: remove the laundry load; change the furniture surface resistance; change the glass U-value and reduce air changes by 0–0.1/h (never below 0.2/h) to reflect tighter new windows.
• Combinations are recalculated together, never summed.

6. Costs

Electricity: watts × hours × your unit rate. Heat: kWh of heat ÷ efficiency (gas boiler 78–90%; heat pump COP 2.5–3.5; direct electric 100%) × unit rate. Default rates are examples only — enter yours from your bill.

7. Cause screening rules

Rules flag possibilities from the pattern and timing of symptoms: moisture between panes → failed sealed unit; worse after rain or known external defect → rain penetration; near plumbing or following use → leak check; low-level band or raised ground → ground moisture needing a proper survey (never "rising damp" from a meter reading); recent wet work → drying out; recent new windows or draught-proofing → reduced ventilation; unflued heater → safety and moisture.

8. Validation plan

Before we describe the tool as accurate we intend to: check calculations against independent reference calculations; compare scenarios with logged data from real homes across construction types, occupancy patterns and seasons; test screening rules against independent survey findings; publish prediction errors and failure cases; and use sensitivity testing to decide which measurements to ask for first. If you've logged temperature and humidity in your home and would share it, please get in touch.

Sources and further reading

• BS EN ISO 13788 — hygrothermal performance of building components: surface temperature to avoid critical surface humidity.
• Historic England, RICS and the Property Care Association — joint guidance on investigating moisture in buildings.
• Historic England — energy efficiency and traditional buildings.
• Energy Saving Trust — guidance on windows, condensation and solid-wall insulation.
• UK Health Security Agency / DLUHC — Understanding and addressing the health risks of damp and mould in the home (2023).
• Scottish Government building standards technical handbook — guidance on condensation.
• DESNZ — Home Energy Model technical documentation (ventilation and infiltration).
• Approved Document F (England) and Welsh ventilation guidance — note that each nation publishes separate documents with their own effective dates.
• US National Weather Service humidity equations; ENERGY STAR dehumidifier test conditions.

Changelog

• 1.0 — October 2026: first public release.