Dew Point Calculator

About Dew Point Calculation

This dew point calculator calculates temperature and water vapor pressure distribution through the layers of an insulated wall. It determines thermal resistance, evaluates the risk of condensation within the wall and on its interior surface, and identifies conditions that may promote mold formation.

The calculation is useful for selecting insulation, comparing different insulation arrangements, and evaluating combinations of building materials. It considers the thickness, thermal conductivity, and water vapor diffusion resistance of each layer, along with air temperature and relative humidity on both sides of the wall.

Guidelines and Recommendations

Thermal Resistance and Heat Transfer

Thermal resistance of a layer is determined by its thickness d in millimeters and thermal conductivity λ in W/(m·K). The lower the thermal conductivity and the greater the material thickness, the higher its thermal resistance.

Rlayer = d / (1000 × λ)

The result is expressed in m²·K/W. For the entire wall, the thermal resistances of all layers are added to the internal and external surface resistances:

R = Rsi + ΣRlayer + Rse

For a vertical wall with horizontal heat flow, the assumed values are R_si = 0.13 m²·K/W and R_se = 0.04 m²·K/W. These values correspond to standard heat transfer conditions specified in EN ISO 6946:2017, "Building Components and Building Elements - Thermal Resistance and Thermal Transmittance - Calculation Methods".

Thermal transmittance U is the reciprocal of the total thermal resistance:

U = 1 / R

U is expressed in W/(m²·K). A lower U-value indicates less heat transfer through the wall under the same temperature difference.

Temperature Distribution Through the Wall

The calculation assumes steady-state, one-dimensional heat flow. Air temperatures and material thermal conductivities are treated as constant. Heat flow density is calculated by dividing the temperature difference between indoors and outdoors by the total thermal resistance of the wall:

q = (Ti − Te) / R

Here, T_i and T_e are the indoor and outdoor air temperatures in °C, and q is the heat flow density in W/m².

Internal surface temperature is calculated using the internal surface thermal resistance:

Tsi = Ti − q × Rsi

The temperature then decreases or increases across each successive layer in proportion to its thermal resistance. The temperature at each layer boundary is determined from the accumulated thermal resistance between the indoor air and that boundary. Within a homogeneous layer, the temperature changes linearly.

Water Vapor Diffusion

Relative humidity is the ratio of the actual partial pressure of water vapor to its saturation vapor pressure at the same temperature. For typical living spaces, a range of 40–60% is often used as a general reference, although the calculation should use values representing the conditions being evaluated.

First, saturation vapor pressure p_sat in pascals is calculated from temperature. The following approximations describe the relationship between saturation vapor pressure and temperature T in °C:

At temperatures of 0 °C and above:

psat = 610.5 × exp(17.269 × T / (237.3 + T))

At temperatures below 0 °C:

psat = 610.5 × exp(21.875 × T / (265.5 + T))

For temperatures below freezing, the saturation vapor pressure relationship over ice is used. The actual partial vapor pressures indoors and outdoors are obtained by multiplying the corresponding saturation vapor pressures by relative humidity expressed as a fraction:

p = psat × RH / 100

Water vapor diffusion resistance is characterized by the dimensionless factor μ. For each layer, the equivalent air layer thickness s_d is calculated in meters:

sd = μ × d / 1000

Here, d is entered in millimeters. The greater the s_d value, the more strongly the layer restricts water vapor diffusion. For vapor control films and membranes, using s_d directly is often more practical because their physical thickness is small. The calculator's material database includes indicative s_d values ranging from 0.05 to 1500 m for different types of membranes and vapor barriers.

Partial vapor pressure within the wall is calculated from the accumulated equivalent air layer thickness s_d. Moving from the indoor side toward the outdoor side, pressure changes linearly with the cumulative vapor diffusion resistance:

p(x) = pi + (pe − pi) × sd(x) / Σsd

Here, p_i and p_e are the indoor and outdoor partial vapor pressures in Pa, s_d(x) is the accumulated equivalent air layer thickness up to the position being examined, and Σs_d is the sum of these values for all layers.

Dew Point and Condensation Risk Assessment

The dew point of indoor air is the temperature at which saturation vapor pressure equals the actual partial pressure of water vapor in the room. It is calculated using the inverse relationship between saturation vapor pressure and temperature.

Condensation risk within the wall is evaluated by comparing the calculated partial vapor pressure p(x) with the saturation vapor pressure p_sat(T(x)) at each position within the wall assembly. If partial vapor pressure exceeds saturation vapor pressure, the corresponding section is identified as a potential condensation zone:

p(x) > psat(T(x))

The boundaries of each zone are determined where the two pressure curves intersect and are expressed in millimeters from the internal wall surface. If several zones are detected, they are reported separately. The calculation also identifies the maximum excess vapor pressure in Pa and the distance to the position where it occurs.

This comparison uses continuous temperature and vapor pressure profiles within each layer. The result is an assessment of potential condensation risk under the specified constant conditions, based on a simplified heat transfer and water vapor diffusion model.

Surface Condensation and Mold Formation

To assess unfavorable conditions at the interior wall surface, a separate temperature calculation uses an increased internal surface thermal resistance of R_si = 0.25 m²·K/W. This value is specified in EN ISO 13788:2012, "Hygrothermal Performance of Building Components and Building Elements - Internal Surface Temperature to Avoid Critical Surface Humidity and Interstitial Condensation - Calculation Methods", to account for unfavorable surface heat transfer conditions, particularly in corners and behind furniture.

For this calculation, R_si = 0.13 is replaced with 0.25 m²·K/W in the total thermal resistance. The relative humidity directly at the internal surface is then calculated:

RHsi = 100 × pi / psat(Tsi)

Two threshold conditions are used to generate warnings:

• RH_si ≥ 80%. A warning indicates that mold formation may be possible if these conditions persist for an extended period. The 80% threshold is used as a reference for critical surface humidity according to EN ISO 13788:2012.
• RH_si ≥ 100%. An additional warning indicates the possibility of condensation on the internal wall surface.

If both conditions are met, both warnings are displayed. If surface relative humidity is below 80%, no additional warnings are shown. This assessment is performed independently of the condensation risk calculation within the wall, so the results may differ.

Material Properties and Calculation Assumptions

Reference values of thermal conductivity λ and water vapor diffusion resistance factor μ are used for building materials. These values can be adjusted according to the technical specifications of particular products. When μ is changed, s_d is recalculated automatically, and when s_d is changed, μ is recalculated.

Heat flow and water vapor diffusion are assumed to occur in one direction, perpendicular to the wall surface. Each layer is treated as homogeneous, and its properties are assumed to remain independent of temperature and humidity. Air temperatures and relative humidity are considered constant throughout each calculation.

The assessment follows the principles of thermal calculations in EN ISO 6946:2017 and hygrothermal analysis in EN ISO 13788:2012. The resulting thermal resistance, temperature distribution, and partial vapor pressure distribution are determined using the formulas provided above and the selected material properties.

FAQs

Why does the indoor air dew point differ from the location of possible condensation within the wall?

The dew point of indoor air depends on its temperature and relative humidity. Within an insulated wall, both temperature and partial vapor pressure change across the layers. Therefore, condensation risk is assessed by comparing these values at each position within the wall assembly rather than using indoor air temperature alone.

Why does placing insulation on the outside or inside of a wall affect the result?

When the same materials and thicknesses are used, the total thermal resistance may remain unchanged regardless of their order. However, the temperature and vapor pressure profiles change, so the location and extent of potential condensation zones can differ considerably between internal and external wall insulation.

Can condensation risk be assessed using thermal transmittance U alone?

No. The U-value describes heat transfer but does not account for how water vapor diffusion resistance is distributed across the wall layers. Two wall assemblies with the same U-value may have different condensation risks because of differences in μ factors, s_d values, and material arrangement.

Why can there be a risk of mold formation when no condensation is detected?

Liquid water condensation is not required for mold to develop. If relative humidity at the surface remains around 80% or higher for an extended period, conditions may become favorable for mold formation, while actual surface condensation is associated with relative humidity reaching 100%.

Why can the μ factor of thin vapor barrier membranes be extremely high?

The μ factor describes a material's resistance to water vapor diffusion relative to an air layer of the same thickness. Because membranes are very thin, even a moderate equivalent air layer thickness s_d can correspond to a μ factor of tens of thousands or more. When comparing vapor control materials, using the s_d value in meters is generally more convenient.