Mould, moisture, and the building envelope

by arslan_ahmed | January 2, 2023 9:00 am

[1]

By David Muise and Gord Rajewski

The term “mould” refers to a large group of microorganisms that grow on organic material and reproduce through the formation and spread of spores. These spores germinate within a few days of finding the right combination of surface, moisture, and temperature, and can form new spores to produce the next generation of growth within five to 10 days.

Mould growth in buildings can occur when mould spores, which are always present in buildings, are exposed to wet or humid conditions for a sufficient length of time and on an appropriate surface to support growth. Water sources can include plumbing leaks, spills, foundation seepage, roof or wall leaks, condensation on cool surfaces such as inside ductwork and on windowsills, etc. In the presence of wet or highly humid conditions, mould spores can germinate within 48 to 72 hours (about 3 days).

Health risks and effects

Exposure to airborne mould can have varying effects on building occupants depending on their individual sensitivities. Those who are affected may have increased risk for respiratory symptoms, asthma, and respiratory infections in some individuals. Some common symptoms associated with exposure may include: runny nose and congestion, eye irritation, coughing, and headache.

There are no set exposure limits for indoor airborne spore concentrations. Health impacts are more broadly related to the presence or absence of dampness and mould growth and not a specific spore concentration or type of mould. Further, a combination of level of exposure, individual sensitivity, and extent and location of mould growth are all factors that may impact risk. Virtually, all guidelines reviewed indicate all mould can be harmful, so if discovered indoors, it should be eliminated.

Controlling mould growth in buildings 

To remove all spores from a building, or to have a building free of finishes which promote mould growth, is simply not possible. The only way to truly prevent mould growth in a building is with the careful management of humidity, condensation, and water intrusion via building envelope leaks, floods, or plumbing failures. For this reason, the
risk of recurring mould growth must be addressed, not only by prompt repairs to address sources of water damage, but through a comprehensive mould and water damage management program. Key elements of a mould and water damage management plan are:

• The training of facilities maintenance staff in the causes and how to identify indoor mould growth.
• Development of a surveillance program to ensure all instances of water/moisture infiltration are discovered and mitigated in a timely manner.
• Regular building inspections to examine the condition of interior finishes which could be susceptible to water/moisture infiltration issues.
• Routine inspection of the interior accessible HVAC components in the building.
• Timely review of exterior envelope conditions to look for deterioration which could lead to future water infiltration and subsequent mould growth.

Given all the science and expertise available to draw from, even a regular educated employee can become a trained set of eyes, ears, and nose to identify potential problem areas.

[2]

Building science and mould control

In general, the buildings envelope is a key line of defence for mitigating water and moisture infiltration into a building.  General building envelope methodologies include the concept of providing both waterproofing and shedding water away from the buildings elements.  The key building envelope concepts can be categorized into four components: deflection, drainage, durability, and drying.  A properly designed, installed, and maintained envelope should be able to effectively deflect moisture from precipitation events, drain roof components efficiently away from the building, provide long-lasting materials and assemblies, and be implemented using three primary processes for drying.  These processes include:

• Diffusion—movement of moisture through materials at the molecular level.
• Evaporation—conversion from liquid to a gas at the surface.
• Convection—transport of moisture via air currents.

To manage the negative effects of deleterious moisture within building enclosures, proper building envelope design must provide control layers that include water shedding surfaces, the design of building form and features to enhance drainage, employment of water-resistive barrier (WRB)  continuous air barrier systems, thermal insulation, and vapour retarder/barriers. These concepts and designs must be effectively communicated (including effective details, drawings, specification, and construction review services) during the construction and/or restoration of buildings in order to ensure continuity of building envelope concepts.

Moisture control and durability of building envelopes is of particular concern in northern climates which are subjected to large temperature gradients, extreme weather, and require indoor heat and comfortable humidity levels.  In addition to the moisture infiltration from exterior sources, such as precipitation, controlling indoor temperatures can also impact the production of moisture within a building.  Similar to the concept of an aluminum can cooler sleeve, when warm air (indoor heated air) reaches a cool object (exterior/perimeter components of a building), it condenses and creates moisture.  The concept that takes this into consideration is thermal bridging. This is simply a pathway (bridge) to move cold temperatures from an outdoor surface to an indoor surface. An example of this would be a wooden wall stud on an exterior wall. The stud gets cold and acts as a “bridge” to transport the cold temperature to an interior wall. When the interior wall cools, there is potential for condensation and mould growth.

When moisture becomes a problem and steps towards resolution

Mould is considered a symptom of a problem, its roots are most often linked to a failure in the design and execution of the building envelope assembly and the associated air handling systems provided.

The process towards resolution of moisture issues within a building enclosure involves three phases, broken down into nine key elements.

Phase I

Condition assessment

Investigators should undertake building envelope condition assessments as a first step towards the remediation of moisture problems in affected building enclosures. These assessments can include: review of architectural as-built drawings and details, interviews with occupants, on site visual review, air leakage testing, thermography, water testing, vapour transmission testing, destructive testing, monitoring, and sample material testing.

Evaluation

Once the sources of problematic moisture are identified and understood, a conceptual remedial repair plan with design options is created and evaluated. In this second step, inspectors will also draw upon their experience and industry knowledge, and consult with representatives from manufacturers to source appropriate options. Inspectors may also use analytical software, such as WUFI, THERM, Window, and Energy Star.

[3]

System selection

Once the design options have been successfully evaluated, pre-construction estimates or quotations are provided for client approval. Obtaining a definitive estimate at this early stage is critical for the success of the project.

Phase II

Design 

Architects and specifiers develop drawings and specifications based on the client’s selected system. The client reviews the design and confirms it is in line with their intent.

Tender

The most common form of project delivery is a method called “Design, Bid, Build.” It occurs once the prime consultants have prepared a tender package including a detailed scope of service and technical specifications. To ensure the intent of the design is communicated with clarity, a pre-bid meeting is often arranged with a group of qualified bidders. Following the bid period, the tender is closed and the competitive bids are reviewed, and a bid is chosen.

Phase III

Once the owner has selected the project construction company, and a contract has been executed between parties, preconstruction services commence.

Preconstruction   

Services may include a project startup meeting with owner representatives, design consultants, and subcontractors. These meetings are critical in the process, as it may be the first time the owner, design consultant, and those tasked with executing the construction will meet. Making sure everyone is on the same page before construction commences in the field is vital to the project’s successful outcome.

At this stage product approvals and shop drawings are also obtained for review by the consultant’s team.

Mock-ups and testing   

Mock-ups may be used for complex or unique conditions in advance of construction to enhance the understanding of the construction team.  Mock-ups provide the project team with additional clarity on trade sequencing and understanding of critical continuity detailing.

Project management and oversight

Oversight and project management should be undertaken through the duration of the project to ensure the construction aligns with the intent of the design and is in compliance with best practices identified in the contract documents. The architect’s review of critical detailing at key intervals ensures the intent of the design is actualized.

On many projects quality control (QC)  will include in-situ testing, using field applied standard test methods involving smoke, water, and air leakage. It is recommended that third party review and testing is conducted at the different phases of the project to ensure the design intent has been carried out through to construction completion. Projects involving significant restoration of building envelope are best served by thermography, in conjunction with air leakage testing, following relevant ASTM and CSA guidelines, if applicable.

For projects being constructed following performance-based code standards, building envelope commissioning may be required. Building envelope commissioning involves more rigorous compliance testing, including whole building air leakage testing, among others.

Project close out

The last element in the project cycle involves ensuring all deficiencies identified during the construction sequence are corrected. Project records, reporting and contract close out documentation is gathered and provided to the client. Warranties for various systems are also provided.

Conclusion

As climate continues to change, and more severe weather events and temperature fluctuations occur, susceptibility to water intrusion through the building envelope will increase. Builders have to understand these moisture control concepts going forward so they can be integrated into renovations or new builds. It is important to stay ahead of this curve so the potential for health and safety concerns and building degradation stay at a minimum.

Authors

[4]David Muise is the national practice leader with the Indoor Environmental Quality (IEQ) group. As national practice leader, David’s focus is on the technical performance of his team, to ensure his clients receive consistent
and responsive superior service in every region and across Pinchin’s Indoor Environmental Quality offerings. Muise works closely with clients in several sectors, including: commercial, institutional, health care, education, and construction. He works closely with the firm’s local offices to support national and local
clients, as an integral partner in ongoing maintenance and compliance.

[5]Gord Rajewski is the national practice leader and business leader at Building Science and Sustainability(BSS) group and has been employed by Pinchin Ltd. since 2018. Rajewski holds an honours diploma in building construction engineering technology from the Northern Alberta Institute of Technology. He has more than 39 years of building envelope and roof consulting experience and has undertaken projects throughout Canada, parts of the U.S., and completed one assignment in Africa.

Source URL: https://www.constructioncanada.net/mould-moisture-and-the-building-envelope/

---