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What is Interstitial Condensation?

Interstitial condensation is a phenomenon that plagues construction projects and finished properties. While surface condensation is easy to spot and often easier to treat, interstitial condensation is hidden and much harder to diagnose. We have discussed condensation often in our blogs but seldom focused on interstitial condensation. However, it can be extremely detrimental to your property, as the effects can be structural.

Understanding condensation

To get to the heart of interstitial condensation, it’s essential to first understand the basic principles of condensation. Condensation is a natural process where water vapour in the air transforms into liquid water. This change typically occurs when warm, moisture-laden air comes into contact with a cooler surface, leading the water vapour to cool and change into liquid form. It’s a phenomenon we observe daily. For example, the misting of a bathroom mirror during a hot shower or dew forming on grass on a cool morning.

The science behind condensation is rooted in the concept of relative humidity and the dew point. Relative humidity is the amount of water vapour present in the air. It is expressed as a percentage of the amount needed for saturation at the same temperature. As warm air holds more moisture than cold air, when the air cools, it can reach a point – known as the dew point – where it can no longer hold all its moisture. At this point, excess moisture is released in the form of condensation.

Different types of condensation

Condensation can manifest in various forms:

  1. Surface Condensation: This is the most visible form, occurring on surfaces like windows, walls, or ceilings. It’s often seen in bathrooms or kitchens due to steamy conditions.
  2. Interstitial Condensation: This occurs within the building structure, such as within wall cavities or layers of roofing. It’s less visible and often goes unnoticed until it causes damage.
  3. Cold Bridge Condensation: This happens at ‘cold bridges’ where there is a break in insulation, allowing cold spots.

What is interstitial condensation?

Interstitial condensation is a distinct type of condensation that presents unique challenges in the context of building and construction. Unlike surface condensation, which is visible and often easier to address, interstitial condensation occurs within the internal structures of buildings, often going unnoticed until it causes significant damage. The key characteristic that sets interstitial condensation apart is its location. This type of condensation occurs within the “interstices” or spaces between layers in the building envelope, such as walls, roofs, and floors. These layers can include a range of materials like insulation, vapour barriers, structural components, and finishes.

How it forms
  1. Moisture Penetration: Moist air from either inside or outside the building penetrates the building envelope.
  2. Temperature Gradient: As this moist air moves through different layers, it encounters varying temperatures. In colder climates, the exterior layers are colder compared to the interior layers.
  3. Reaching Dew Point: If the air cools to the dew point within these layers, the moisture in the air condenses into liquid water.
  4. Lack of Evaporation: Unlike surface condensation, where moisture can evaporate back into the air, interstitial condensation is often trapped within the structure, unable to easily escape.
The impact of interstitial condensation

The consequences of interstitial condensation can be severe, particularly because it often remains undetected until it has caused significant damage:

Distinguishing factors

What distinguishes interstitial condensation from other types is the complex interplay of factors that contribute to its development:

  • Building Design and Construction: The way a building is designed and constructed can inadvertently create conditions conducive to interstitial condensation. For example, insufficient insulation, lack of vapour barriers, or poor ventilation can all play a role.
  • Environmental Conditions: The local climate and weather patterns can influence the likelihood of interstitial condensation. Areas with high humidity or significant temperature variations are particularly at risk.
  • Internal Sources of Moisture: Activities within the building, such as cooking, bathing, and even breathing, can increase indoor humidity levels, contributing to the risk of interstitial condensation.

How can you prevent interstitial condensation?

Building design

Preventing interstitial condensation is a comprehensive process that begins right from the design phase and extends to ongoing building maintenance. In the initial stages, architects and engineers play a crucial role in identifying and mitigating potential problem areas where moisture might penetrate or get trapped within the building envelope. The integration of vapour barriers or vapour checks is essential in this regard. These materials, strategically placed on the warm side of the insulation, are crucial in preventing the passage of warm, moist air into cooler parts of the building. Additionally, the correct placement of insulation is vital. It should be continuous and gap-free to avoid cold bridges that are prone to condensation. Where applicable, the use of breathable materials can facilitate the escape of moisture from the structure. Thereby, the risk of condensation is reduced.

Controlling humidity

Managing the levels of humidity inside a building is equally important in preventing interstitial condensation. This management can be achieved through adequate ventilation. This may be natural, like windows and vents, or mechanical, involving extractor fans and ventilation units. In areas known for high humidity, dehumidifiers can be employed to maintain a more controlled indoor humidity level. Furthermore, encouraging changes in occupant behaviour, such as using lids on pots while cooking, drying clothes outdoors, or ensuring bathrooms are well-ventilated, can significantly reduce indoor humidity.

Construction practices

The construction phase is critical, as even the best designs can be compromised if not executed properly. A key aspect is ensuring that all materials, especially vapour barriers, are correctly joined and sealed to prevent the ingress of moist air. Protecting building materials from moisture during construction is also imperative. Wet materials can trap moisture, leading to condensation issues later. Regular quality control inspections during construction can help in identifying and addressing potential problems early, ensuring that the design is accurately implemented in the structure.

Monitoring and maintenance

Lastly, the role of ongoing monitoring and maintenance in preventing interstitial condensation cannot be overstated. Regular inspections, particularly in areas prone to moisture ingress, are essential in identifying and addressing issues before they escalate into interstitial condensation. Installing humidity sensors can provide continuous monitoring, offering early alerts to potential problems. Additionally, the proper functioning and maintenance of ventilation systems are crucial for controlling indoor humidity levels and maintaining a healthy building environment. This holistic approach, encompassing thoughtful design, meticulous construction, and diligent maintenance, is key to effectively preventing interstitial condensation and ensuring the longevity and health of buildings.


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