Identify the right “dew point calculation” for your job straight away
Ambient air dew point: risk of condensation on cold surfaces
On site, the dew point calculation links air temperature and relative humidity. From this, you derive the surface temperature you must not go below to avoid fogging on cold walls, chilled-water pipework, or cooled floors. This is the value that sets the lower temperature limit for cooling supply water.
Condensation within a wall: check through the thickness, not just at the surface
Here, we are no longer talking about the surface. You check the interstitial risk by comparing vapor pressures through the layers, using a Glaser-type approach. The vapor barrier is installed on the warm side, and its continuity is tracked on drawings and site photos.
Flue gas dew point: condensation in the boiler and flue
Last case: flue gases. Their dew point depends on the fuel and excess air. It determines the choice of a condensing boiler, condensate drainage, and the durability of the flue (material, airtightness, slope, corrosion resistance). For sizing and installation constraints, also see the specifics of a concentric flue duct.
Ambient air dew point calculation: anticipate condensation on walls, pipework and cooling systems
Record temperature and relative humidity: what this changes in the result
For a reliable reading and a usable dew point calculation, measure dry-bulb temperature and relative humidity as close as possible to the risk zone, away from direct radiation and supply air. An error of 1 °C or 5% RH is enough to shift the dew point and change your decision on insulation, a setpoint, or resealing. Record the location, time and instrument.
Reading from a psychrometric chart and using an approximate formula: methods you can use on site
On a psychrometric chart, start from the dry-bulb temperature, follow the relative humidity curve, then read the dew point on the saturation curve. For a quick calculation, the Magnus formula works well. Td = (b·γ)/(a-γ), with γ = ln(RH/100) + a·T/(b+T), a = 17.62 and b = 243.12.
Using the dew point calculation: cold wall, chilled water, cooled floor and minimum supply temperature
Simple rule. Keep a 2 °C margin between the surface temperature, or the cooling supply temperature, and Td. That is what prevents condensation on cold walls, chilled-water pipework and cooled floors. To better understand what happens at the level of a cold wall, and its effects on comfort, explore the notion of a cold wall in more depth.
| Air temperature (°C) | Relative humidity (%) | Dew point Td (°C) | Recommended minimum surface or supply temp. (°C) |
|---|---|---|---|
| 26 | 50 | 14.8 | 16.8 |
| 26 | 60 | 17.6 | 19.6 |
| 28 | 70 | 22.0 | 24.0 |
Condensation within a wall assembly: check the risk with vapor pressures
Principle of vapor pressure comparison: where and why condensation occurs
The approach is based on diffusion. At each layer interface, you track the partial pressure of water vapor and compare it with the saturated vapor pressure at the local temperature. If the partial pressure curve exceeds the saturation curve, interstitial condensation occurs. The dew point calculation is used here to set the indoor conditions, then unfold the temperature and vapor profile through the assembly.
Role of the vapor barrier on the warm side: continuity, junctions and common mistakes
A vapor barrier on the warm side is only effective if it is continuous. On site, performance is decided at overlaps, penetrations (service ducts, boxes), junctions with internal partitions and the window-to-wall connection. Classic mistake: confusing a variable-permeability vapor retarder with a vapor barrier, or installing an interrupted membrane behind a perforated lining.
Renovation case studies: internal insulation, cold zones and moisture transfer
With internal insulation, the masonry becomes colder in winter. The cold zones (floor junctions, reveals, cross walls) weaken the assessment if they are not modeled. Set the assumptions (temperature, RH, critical month), address thermal bridges, and document the choice of membrane and junction details to secure the trade during inspection.
Flue gas dew point calculation: avoid condensation damage on the appliance and flue
Why flue gases condense: return temperature, heat exchanger and efficiency
Flue gases condense when a surface in the appliance, often the heat exchanger, drops below their dew point. The on-site trigger is an excessively low return temperature or an overly aggressive heat transfer. In a condensing boiler, this is intentional to recover latent heat. Outside a design intended for condensation, condensates attack metals, load deposits and quickly degrade airtightness.
Dependence on fuel and excess air: what changes the flue gas dew point
The dew point calculation for flue gases depends on the water vapor content, CO2, and the excess air that dilutes the combustion products. Also watch, depending on the fuel, for acid condensation.
| Fuel | Flue gas dew point, order of magnitude (°C) | Sensitivity |
|---|---|---|
| Natural gas | 50 to 60 | Excess air, return temp |
| Oil | 40 to 55 | Sulfur, excess air |
| Biomass | 45 to 65 | Fuel moisture |
Choosing the flue and materials: airtightness, condensate resistance and drainage slopes
If condensation is possible, use a Class W flue with corrosion resistance and proper condensate management. Provide slope, collection, trap and access for inspection. During renovation, check the liner, joints and material compatibility (suitable stainless steel, PP, ceramic) with the nature of the condensates; see also the DTU points and best practices for lining and flue compliance.
Avoid site mistakes: do not mix up the values and size for the worst-case scenario (2026)
Do not confuse ambient air dew point and flue gas dew point: order of magnitude and impact
Ambient air dew point is used to prevent condensation on a cold surface (chilled-water circuit, cooled floor). Flue gas dew point is used to define the condensing regime of a generator and the requirements for the flue. Keep the order of magnitude in mind.
| Case | Typical dew point (°C) | Site impact |
|---|---|---|
| Indoor air 20 °C, 50 to 60% RH | 9 to 12 | Lower limit in cooling |
| Natural gas boiler flue gases (H2O) | 55 | Choice of condensing/non-condensing unit |
Anti-condensation insulation: size it for the most humid conditions of the year
For insulation, size for the worst case. High RH, minimum surface temperature, and vapor barrier continuity. Aim for a 2 K margin between calculated surface temperature and dew point.
- Calculation inputs. Air temp, RH, fluid temp, insulation λ, thickness, vapor barrier Sd.
- Critical points. Flanges, supports, valves, penetrations, uninsulated areas.
Simple checks to put in place: measurements, safety margins and operating adjustments
During commissioning, impose a minimum cooling supply temperature via the control system, adjusted using the control strategy, based on air temperature and RH measurements. Record the readings, and lock in setpoints during operation.
- Measure. Thermo-hygrometer, surface temperature, fluid temperature, and seasonal maximum humidity.
- Adjust. RH alarm, cooling shutdown, gradual restart after a humidity peak.
