Interstitial condensation creates structural damping that occurs when moist air penetrates inside the hidden space within an enclosed wall, roof or floor cavity structure. When that moisture laden air reaches a layer inside the interstitial structure that is at dew point temperature, it will condense into liquid water. The moisture laden air can penetrate into hidden interstitial wall cavity from the exterior in warm outdoor temperatures and inside the building during cold outdoor temperatures. Groundwater soaking basement foundation walls from wet soil is common. This can result from a high water table or from improperly drained rainwater runoff soaking into the ground next to the basement walls. Moisture saturated basement walls will add moisture directly into basement interstitial spaces leading to condensation with cool basement temperatures. All interstitial condensation can cause uncontrolled mold and bacteria growth, rotting of wood components, corrosion of metal components and/or a reduction in the thermal insulation's effectiveness. The resulting structural damage,along with mold and bacteria growth may occur without any visible surface indications until significant damage or extensive mold and bacteria growth has occurred. HVAC ducts within interstitial spaces (chases) can leak out cold air through unsealed joints/connections which produces dew point surfaces. Unsealed duct joints/connections can also create suction that pulls humid outdoor air into interstitial spaces and chases. This can promote more mold and bacteria growth on the condensed cool surfaces of the interstitial spaces. In addition, the cool ducts themselves can condense humid air and “sweat” even more liquid water into the interstitial spaces thereby exacerbating mold and bacteria growth.
Since most building materials are permeable and many joints are not completely sealed, it's critical in controlling interstitial condensation to control indoor moisture at its sources (venting out shower vapor), through HVAC dehumidification, ventilation and by adding an impermeable vapor barrier in the interstitial cavity. In addition, since the air in interstitial cavities can communicate with interior spaces through tiny cracks and unsealed joints, any airborne mold, aerosolized fungal fragments and bacteria growth in the interstitial cavity can travel into the building's air to be breathed in by occupants.
Interstitial condensation is differentiated from surface condensation in buildings which is known as "cold-bridge condensation" or "warm front condensation" where the condensation forms on the interior or exterior surfaces of a building rather than inside wall, floor or roof cavities.
It is physically impossible to build envelope assemblies so that they completely prevent air infiltration,exfiltration of water vapor diffusion. Moist air can infiltrate envelope assemblies driven by the pressure differential created by wind and stack effect. Since all buildings contain various levels of moist air, cognizant authorities have recommended maintaining an indoor relative humidity of air between 40% to 60%. The sources of interior moisture are people, appliances such as dishwashers, cooking, showers, wet basements, leaking pipes and roof/wall rainwater leaks. It should be noted that leaks of liquid water into the building envelope are a different problem than interstitial moisture condensation, but this additional water can exacerbate interstitial wetting which can increase mold and bacteria growth.
Preventing interstitial condensation by keeping these hidden spaces dry, is critical in all buildings. This is done by :
a. maintain a slightly positive indoor pressure in warm months and a neutral pressurization in cold months
b. preventing infiltration (exterior air leakage into the building)
c. preventing exfiltration (interior air leakage into the assemblies)
d. controlling indoor moisture at its sources through exhaust ventilation,
f. effective vapor barrier wall sealing
Vapor barriers can be problematic because they difficult to install perfectly and also reduce the ability of a cavity to dry out when it does get wet. Vapor barriers are used in conjunction with a housewrap, a vapor permeable but water resistant membrane, so that one side of the cavity is permeable to allow drying. Spray foam insulation can an effective vapor barrier if applied correctly.
Historically, most buildings built before the twentieth century were not designed to maintain 70F/21C, were naturally well ventilated and built with very permeable materials. The increase in interstitial condensation problems have increased due to the modern prevalence of central heating and air conditioning, the construction of air-tighter enclosures causing buildings to be negatively pressurized along with more heavily insulated buildings and with more indoor plumbing sweating.
Interstitial condensation problems may also occur in other structures with enclosed air spaces along with 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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- Tim Hutton. "Condensation". The Building Conservation Directory, 2004 accessed 2012-05-16
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- McArthur, Hugh, and Duncan Spalding. Engineering Materials Science: Properties, Uses, Degradation and Remediation. Chichester, U.K.: Horwood Pub., 2004. 166. Print.
- McMullan, Randall. Environmental Science in Building. 4th ed. Basingstoke, England: Macmillan, 1998. 98. Print.