Condensation in Buildings: Causes and Prevention

Water vapor condensation never happens by chance. It happens whenever the surface temperature drops below the dew point of the surrounding air. This condition is unambiguous, predictable and — if you know it in advance — you can take preventive measures. That is exactly why psychrometrics has a direct economic impact in building science: a badly designed vapor retarder or missing insulation causes defects whose repair costs many times more than a correct design.

The physics of condensation: from vapor to droplet

Water vapor is an invisible component of air under normal conditions. Condensation occurs as soon as the air is supersaturated — the amount of vapor exceeds the saturation value for a given temperature. This happens either by adding more vapor (a shower, cooking, breathing) or by cooling the air below its dew point. The rate of condensation depends on the difference between the partial vapor pressure in the air and the saturation pressure over the surface — the colder the surface, the faster the condensate builds up.

Surface condensation is visible and immediate

It forms on the inner surface of an assembly when its temperature drops below the dew point of the indoor air. Typical examples: a fogged-up window in winter, droplets on a cold pipe, a damp corner behind furniture in an unheated spot.

The condition for it: $t_{\text{surface}} < t_d$. The dew point for ordinary winter conditions (68°F / 50% RH) is about 48.8°F — any surface colder than that (a thermal bridge, an unheated corner, a poorly insulated window) will be wet in winter.

How to assess the risk

On the psychrometric chart we find the indoor-air state, draw a horizontal line from it (constant humidity ratio) left to the saturation curve — the intersection gives the dew point $t_d$. We compare it with the lowest surface temperature in the room. The temperature factor at the internal surface per ISO 13788, $f_{Rsi} = (t_{si} - t_e)/(t_i - t_e)$, places the internal surface temperature $t_{si}$ between the outdoor temperature ($f_{Rsi} = 0$) and the indoor temperature ($f_{Rsi} = 1$) — a property of the construction that does not depend on the weather. It must satisfy $f_{Rsi} \ge f_{Rsi,\min}$, where the minimum follows from the indoor humidity and the outdoor design conditions.

Interstitial condensation is hidden and more dangerous

The more insidious form occurs inside the assembly, for example within an insulation layer, in a cavity wall, under a roof covering. Water vapor penetrates by diffusion from the warm side (interior) to the cold side (exterior), and where the temperature drops below the dew point of the vapor at its given partial pressure, it condenses. It is invisible until it shows up as mold, wood rot or a structural defect. The repair is then many times more costly.

The Glaser method vs. dynamic simulation

The classic Glaser method (embodied in ISO 13788) works with steady-state conditions: for each month it computes the distribution of temperature and partial vapor pressure across the section and identifies the layers where vapor condenses. Simple, conservative, standardized. Dynamic simulation (WUFI, EnergyPlus) tracks the heat and moisture flow throughout the year, including the assembly’s ability to dry out — it is more accurate but more demanding on inputs.

A practical example

An exterior wall of 12 in masonry (thermal resistance assumed at R-3.4) with 6 in of mineral wool (R-24.7) and a thin render. Interior 68°F / 55% RH (dew point 51.3°F), exterior 10°F. With the surface resistances $R_{si} = 0.74$ and $R_{se} = 0.23\ \text{h·ft}^2\text{·°F/Btu}$ the total is $R_T = 29.1\ \text{h·ft}^2\text{·°F/Btu}$ and U ≈ 0.034 Btu/(h·ft²·°F), whether the insulation goes outside or inside. What changes is where the cold ends up. In steady state, the temperature at any plane in the wall follows from the resistance $R_n$ between the indoor air and that plane:

$$t_n = t_i - (t_i - t_e) \cdot \frac{R_n}{R_T}$$

Plane in the wall	Insulation outside	Insulation inside
Interior surface	66.5°F	66.5°F
Masonry/wool interface	59.7°F	17.2°F
Exterior surface	10.4°F	10.4°F

• Insulation outside: the masonry stays on the warm side. The interface at 59.7°F is well above the 51.3°F dew point; almost the whole temperature drop happens inside the insulation, which is why the insulation and the finish should be vapor-open, so that any vapor that gets through dries out to the exterior.
• Insulation inside: the same wool on the inner face leaves the wool/masonry interface at 17.2°F. Mineral wool offers almost no resistance to vapor diffusion, so the vapor reaches the cold masonry at close to its indoor partial pressure (dew point about 51.3°F) and condenses — or freezes — there. Interior insulation therefore needs a continuous vapor retarder on its warm side, sealed at every joint.

(The values are indicative and come from a steady-state temperature profile; the design check with monthly climate data is done per ISO 13788 or in hygrothermal software.)

Ventilation as a moisture-control tool

Ventilation is the most effective way to reduce indoor humidity and push the dew point down. An air change rate of 0.5 h⁻¹ removes most of the moisture produced by occupants. In rooms with high moisture production (bathroom, kitchen, natatorium) passive ventilation is not enough and mechanical exhaust with sufficient capacity is required; in a bathroom that means local extraction on the order of 30–55 CFM, with ASHRAE 62.2 governing residential ventilation rates. In practice it is important to keep the air state far enough from the saturation curve, which can be tracked visually in PsychroView.

Recommendations by type of space

• Windows — double glazing ($U_g \approx 0.18\ \text{Btu/(h·ft}^2\text{·°F)}$) keeps the inner pane at about 59–61°F when it is 10°F outside (and 68°F inside); triple glazing ($U_g \approx 0.09\ \text{Btu/(h·ft}^2\text{·°F)}$) raises the glass temperature above 63–64°F and gives a larger margin. The actual value depends on the indoor conditions and the chosen surface resistance layer (Rsi).
• Basements, crawlspaces, garages — insulation from the ground and controlled ventilation are essential; warm humid air brought into a cold basement in summer condenses immediately.
• Bathrooms — a vapor retarder on the ceiling, direct air extraction from the shower enclosure, tile surfaces above the dew point.

Frequently asked questions

What is the unambiguous condition for condensation to occur? A surface temperature lower than the dew point of the surrounding air. You read the dew point off the psychrometric chart or a calculator and compare it with the coldest point of the assembly.

Why is interstitial condensation more dangerous than surface condensation? It is hidden inside the assembly and only shows up as biological or structural damage, often after years. It requires a calculation per ISO 13788 or a dynamic simulation.

Does reducing humidity help against condensation on windows? Yes. Lower indoor humidity lowers the dew point, so even colder glass stays dry. An alternative is to raise the glass surface temperature with better glazing (triple glazing).