Interstitial condensation is a form of structural damping that occurs when warm, moist air penetrates inside a wall, roof or floor structure, reaches the dew point and condenses into liquid water. Interstitial condensation is differentiated from surface condensation in buildings sometimes called "cold-bridge condensation" or "warm front condensation" where the condensation forms on the interior or exterior surfaces of a building rather than inside the wall or roof cavities. The warm, damp air may penetrate a wall from the inside of a heated building in a cold climate or from the exterior of an air-conditioned building in a warm climate. This condensation may allow mold growth, rotting of timber, corrosion of metal components and/or a reduction in the thermal insulation's effectiveness. The resulting structural damage may occur without visible indications until significant damage has occurred. Most building materials are permeable so the common method of controlling interstitial condensation is to control indoor moisture at its sources or through dehumidification and/or ventilation and add an impermeable barrier on the warm side of the wall.
Buildings always contain moist air. The recommended indoor relative humidity of air is a range of 40% to 60%. The sources of interior moisture are people, appliances such as dishwashers, cooking, showers, wet basements, and roof/wall leaks. Exterior moisture of concern to condensation is from the warm, humid air. Leaks of liquid water into the building envelope is a different problem than condensation.
Protection against interstitial condensation is necessary in heated or cooled buildings depending on the climate where they are located and other factors. This is done primarily by means of preventing exfiltration (air leakage into the assemblies); controlling indoor moisture at its sources through ventilation, heating, insulation, dehumidification; and using an impervious vapour barrier (vapour check) on the warm side of the insulation, i.e., inside the assembly on a heated building and outside on a cooled building. Buildings that are both heated and cooled should not have vapour barriers on both sides of the assemblies. Vapour barriers are problematic because they difficult to install perfectly and greatly reduce the ability of a cavity to dry out when it does get wet. Vapour barriers are used in conjunction with a housewrap, a vapour permeable but water resistant membrane, so that one side of the cavity is permeable to allow drying. Spray foams may also act as a vapour barrier.
Historically, most buildings built before the twentieth century were not intended to be kept comfortably warm, were naturally well ventilated and built with very permeable materials. The increase in interstitial condensation problems relate to the modern prevalence of central heating and air conditioning, the construction of air-tight, heavily insulated buildings with indoor plumbing.
The problem may also occur in other structures with enclosed air spaces in the presence of high humidity and a large temperature difference between exterior and interior, including refrigerated vehicles.
The process may cause further problems if freezing is involved. Condensed water expands when frozen, possibly causing further structural damage.
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