Methodology & formulas

Home Moisture Lab combines room air conditions with a cold-surface check. The goal is to compare rooms and test assumptions — not to turn one humidity value into a building diagnosis.

1. Dew point

For room temperature T in °C and relative humidity RH, the calculator uses a Magnus-type approximation:

Dew point is then compared with the selected or measured surface temperature. If the surface is at or below dew point, saturation can occur at that surface under the modeled steady conditions.

2. Surface temperature

If you enter a measured surface temperature, the model uses it directly. If you leave that field blank, it estimates the inside surface from indoor temperature, outdoor temperature, an approximate U-value and an internal surface resistance of 0.13 m²K/W.

These presets cannot model spacer bars, frames, reveals, corners, curtains, local air movement, thermal bridges, wind, solar gain or the exact build-up of your assembly. That is why the UI marks estimated surface temperatures as estimates.

3. Surface relative humidity

The calculator first derives actual vapor pressure from room temperature and room RH. It then divides that vapor pressure by saturation vapor pressure at the surface temperature. The result is the estimated RH at that surface.

The site uses 80% surface RH as a caution band for comparison, not as a health threshold or a statement that growth is present.

4. Reverse room-RH limits

Instead of only asking “what is the surface RH now?”, the tool solves the equation backwards. It reports the approximate room RH that would correspond to 80% RH at the selected surface and the room RH that would bring that surface to saturation.

This is why there is no single “safe humidity for every window”: the limit changes with indoor temperature, outside temperature, construction and the coldest local surface.

5. Cold-snap outdoor breakpoints

For preset surfaces only, the tool also solves the simplified surface-temperature model backwards. It first finds the surface temperature that would correspond to 80% surface RH under the current indoor vapor pressure, then calculates the outdoor temperature that would produce that surface temperature in the preset model. The same method is used with dew point to estimate the outdoor temperature at which the preset surface reaches saturation.

This is a scenario threshold, not a forecast. It assumes indoor temperature and RH stay the same and inherits all limitations of the preset U-value model.

6. Moisture attention score

The 0–100 score is a site-specific prioritization index. It is not an industry certification. It adds points for elevated room RH, high calculated surface RH and a small or negative dew-point margin. The whole-home score blends the average room score with the worst room so a single cold problem area is not hidden by several normal rooms.

7. Driver label

The label “room moisture,” “cold surface,” “mixed,” or “lower concern” is based only on the current modeled snapshot. It is intended to tell you what to measure next. It does not identify the physical cause of a leak, damp wall or material damage.

8. Saved history

Snapshots are stored in your browser using local storage. They are not uploaded by the calculator. The CSV export contains your saved room readings and calculated values so you can compare repeated measurements outside the site.

Important limitations

The model does not know moisture duration, rain intrusion, plumbing leaks, capillary moisture, material moisture content, airflow behind furniture, exact U-values, pressure differences or hidden cavities. Persistent dampness, visible growth or damaged materials require source investigation beyond this tool.

General humidity and dampness guidance is separated from the formulas and listed on the sources page.