Building science: The process of mitigating mold and moisture

by arslan_ahmed | April 27, 2023 6:00 pm

[1]By David Muise and Gord Rajewski

The term “mold” refers to a large group of microorganisms which 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.

Mold growth in buildings can occur when mold 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, and condensation on cool surfaces such as inside ductwork and on windowsills, etc. In the presence of wet or highly humid conditions, mold spores can germinate within 48 to 72 hours (about three days).

Health risks and effects

Exposure to airborne mold can have varying effects on building occupants, depending on their individual sensitivities. Those who are affected may have an 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 to name a few.

There are no set exposure limits for indoor airborne spore concentrations. Currently, the Environmental Protection Agency (EPA) does not have standards or threshold limit values (TLVs) for airborne concentrations of mold or mold spores. For example, in Canada, Health Canada considers mold growth/exposure to pose a health risk. The specific health risk is dependent on multiple factors, including the health of the building occupant(s), allergic sensitivities, degree of exposure, and the combination of mold species identified. As a result of this uncertainty and variability in human response, the derivation of exposure limits is difficult and therefore has not been completed.

Health impacts are more broadly related to the presence or absence of dampness and mold growth, but not to a specific spore concentration or type of mold. Further, a combination of level of exposure, individual sensitivity, and extent and location of the mold are all factors which may impact risk.

Virtually, all guidelines reviewed indicate all mold can be harmful, so if discovered indoors, it should be eliminated. Some of the guidelines reviewed included the 2001 Health Canada document, “Construction-related Nosocomial Infections in Patients in Health Care Facilities,” the CSA Standard Z317.13.17, Infection control during construction, renovation, and maintenance of health care facilities, the document “Mould guidelines for the Canadian construction industry” by the Canadian Construction Association (CCA), and the Institute of Inspection, Cleaning and Restoration Certification (IICRC)’s ANSI/IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration to name a few.

Among other things, each of these standards have in common very specific procedures to follow when cleaning or remediating mold from a building. These remediation procedures are typically in three levels, from low to high, with the distinguishing difference in each being the surface area and the identified mold covers. As an example of that, the CCA’s mold guideline has Levels 1, 2, and 3. Specifically Level 1 (small) would be < 0.9 m2 (10 sf), Level 2 (intermediate) is between 0.9 and 9.2 m2 (10 and 100 sf), and Level 3 (large) is > 9.2 m2 (100 sf).

Controlling mold growth in buildings 

To remove all mold spores from a building, or to have a building without finishes that promote mold growth, is simply not possible. The only way to truly prevent mold growth in a building is with careful management of humidity, condensation, and water intrusion via building envelope leaks, floods, or plumbing failures. For this reason, the risk of recurring mold growth must be addressed, not only by prompt repairs to address sources of water damage, but through a comprehensive mold and water damage management program. Key elements of a mold and water damage management plan are:

By regularly providing training for employees, they can become proficient in being able to see, hear, and smell potential problem areas that may require attention.

Building science and mold control

In general, the buildings envelope is a key line of defense 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 building’s elements. The key building envelope concepts can be categorized into four components (can also be known as the 4D’s): 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:

Proper building envelope design must provide control layers which have water shedding surfaces, the design of building form and features to enhance drainage, employment of water-resistive barriers, continuous air barrier systems, thermal insulation, and vapor 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 to ensure the continuity of building envelope concepts.

A building envelope condition assessments is the first step toward the remediation of moisture problems in affected building enclosures. It can include multiple reviews, interviews, and testing.

Moisture control and durability of building envelopes is of particular concern in 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, creating moisture. The concept taking 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 mold growth.

When moisture becomes a problem

Mold is symptomatic to a problem, of which the root cause is most often linked to a failure in the design and execution of the building envelope assembly, and the associated air handling systems provided.

Resolving moisture issues within a building enclosure involves three phases, broken down into nine key elements.

Phase I

Element one: Condition assessment

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

 Element two: Evaluation

Once the sources of the problematic moisture is or 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.

Element three: 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

Element four: 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.

Careful management is needed to truly prevent mold growth in a building.

Element five: Tender

The most common form of project delivery is a method called “design, bid, build.” With this methodology, prime consultants prepare a tender package inclusive of a detailed scope of services 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, 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, pre-construction services commence.

Element six: Pre-construction

Services may include a project start-up 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(s), 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 for a successful outcome.

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

Element seven: 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 give a better understanding of critical continuity detailing.

Element eight: 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 the 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 reviews and testing are conducted at different phases of the project, to ensure the design intent has been carried out through to construction completion. For projects involving significant restoration of the building envelope, thermography is recommended, in conjunction with air leakage testing following ASTM guidelines. Third-party reviews and field testing also needs to be conducted by qualified firms and representatives as relative to the work.

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.

Element nine: Project close-out

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


As the climate continues to change and cities start to witness more severe weather events and temperature fluctuations, the susceptibility to water intrusion through the building envelope will increase. A project should, at a minimum, have a basic understanding of moisture control concepts and practices for integration into facility renovations or new builds. It is important to stay ahead of this curve so the potential for health and safety concerns and building degradation are kept at a minimum.


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