Moisture in building envelopes are a natural result of assembling products with varying moisture storage capacity and sensitivity. While moisture is ever-present in our exterior and interior environments, it can cause serious and sometimes hidden problems for building envelope systems, particularly those that have not considered control layers.
Where moisture builds within a building envelope system, increased relative humidity can boost the growth of microorganisms, such as bacteria and mould. When obvious, this can lead to mould on the interior of the building envelope, resulting in both aesthetic and health issues.
Often not considered, the presence of water, due to its high thermal conductivity, can also result in a decrease in the thermal performance of the thermal control layer. In excessive cases, this can lead to material decay and mechanical stress over prolonged periods, endangering the long-term stability of a structure.
When considering the role of moisture within the building envelope, it is subject to complex interplay of connected physical mechanisms, including moisture intrusion, deposition and built-in moisture.
Moisture and Glass
While glazing is an impermeable material, highly resistant to moisture transfer, it can represent a major condensation risk due to its inherently slim profile and low thermal resistance. Understanding this phenomenon is important in the design and construction of new buildings, and in the assessment of existing buildings, as the formation of condensation, can affect comfort and can be a hazard to health.
Condensation occurs when the air cools down, typically overnight. As it is less able to hold moisture, saturation water vapour density falls while relative humidity rises. When the relative humidity reaches 100%, the air is described as being saturated. Known as the ‘dew point’ temperature, when air stays cool, moisture will begin to condense and form on the glazing surface as water droplets.
Glazing frame and/or glazing systems can contribute to the possibility of condensation if they are poor performing for a specific climate. Higher performance glazing creates warmer interior glazing surfaces, reducing condensation and frost risk.
Three types of condensation are likely to occur on glazing, surface condensation on the interior or exterior and inner cavity condensation.
Internal Surface Condensation
Internal surface condensation can occur on the interior side of the glazing when relative humidity is high and/, or the temperature is low. Under normal internal conditions in residential buildings in colder climates, this surface condensation frequently occurs for single glazing due to its inherent low thermal performance and thin profile and is much less frequent on higher-performing double-glazed systems. If combined with thermally broken frames or triple glazing, internal surface condensation should be very rare and only evident under extreme climatic conditions.
The insulative effect of glazing types has a major impact on the point in which surface condensation can occur. For single glazing, when the external environment is 0 °C and the internal environment is 20 °C, condensation will start occurring when the glass surface temperature is 5.6 °C. A double-glazed unit is significantly better at reducing the risk of condensation, with no risk of the occurrence until the glass surface temperature is 12.8 °C.
External Surface Condensation
External surface condensation on the external side of the glazing is infrequent and can occur following clear nights with little to no wind. This is particularly relevant for higher performance glazing systems, which incidentally proves the insulating efficiency of the glazing, where the external pane cools to such a point that condensation forms on the outside.
Internal Cavity Condensation
Based on a defective product, condensation can also occur within the cavity of a double-glazed unit when the seal of the unit is no longer hermetically sealed against vapour and humidity.