Mould in a home is a moisture problem first and a cleaning problem second. Health Canada sets no "safe" level and does not recommend routine air testing, so the job for a housing department is to find the water, fix it, size the mould correctly, and match the cleanup to that size. Mould remediation and the repairs behind it are eligible expenses under ISC's On-Reserve Housing Program and CMHC's On-Reserve RRAP.
This guide draws on Health Canada, Indigenous Services Canada (ISC), CMHC and the Auditor General of Canada. It is general information, not health, legal or engineering advice: confirm program details with your ISC regional office or CMHC specialist, and bring in your Environmental Public Health Officer (EPHO) or a trained professional for anything beyond a small patch.
Why does mould keep coming back in some homes?
Because the water is still there. CMHC's resource guide for First Nations housing providers puts it plainly: "Mould problems indoors are always caused by moisture or water" (Mould in Housing: A Resource Guide for First Nations Housing Providers, CMHC). ISC lists three common sources: condensation from "excessive humidity, lack of ventilation, or low temperature"; steam from baths, showers and cooking; and leaks from a roof, plumbing, a cracked basement or flooding (Mould and your health in First Nations communities, ISC).
Crowding is part of the picture. CMHC's guide lists "many people living in one home" first among causes of high indoor humidity, followed by firewood stored indoors, clothes dried indoors, cooking, and missing range hoods and bathroom fans. CMHC's mould page notes that, according to the 2021 Census, people in First Nations communities are four times more likely than the non-Indigenous population to live in crowded housing, and six times more likely to live in homes needing major repairs (Mould in housing, CMHC).
Cold surfaces matter as much as humidity. The CMHC guide names missing insulation, older windows, thermal bridges, drafts, wind-washed attic insulation, covered or blocked heaters and cold water pipes as places where condensation forms. As the guide warns, there is "little point in completing a cleanup before the moisture problem has been solved." For the ventilation side of the problem, see our separate guide to HRVs and indoor air in First Nations housing.
What does Health Canada say about mould and health?
Health Canada says people living with mould and damp are more likely to have eye, nose and throat irritation, coughing and phlegm, wheezing and shortness of breath, worsening asthma, and other allergic reactions. Children, seniors and people with conditions like asthma and severe allergies can be more susceptible, and "there is no 'safe' limit for mould." Some airborne moulds can cause severe lung infections in people with very weakened immune systems (Mould, Health Canada).
Health Canada's formal guideline explains why there is no number to test against. It considers that mould growth in homes "may pose a health hazard," but the range of species and the variation in how people react "preclude the derivation of exposure limits." Its recommendations are to control humidity, repair water damage diligently, and clean thoroughly any visible or concealed mould, regardless of species (Residential Indoor Air Quality Guideline: Moulds, Health Canada).
ISC adds that the level of concern depends on how much mould there is, how long it has been present, and who is exposed, naming infants, children, seniors, pregnant people and people with asthma as at greater risk (ISC appearance before the INAN committee, 2024).
Should you pay for mould testing? Usually not. Health Canada says that in most cases there is no need to measure mould in the air or identify the species: "You just need to find and fix the moisture and mould problems" (Health Canada). Its guideline adds that without exposure limits, air test results "cannot be used to assess risks to the health of building occupants."
CMHC's guide is just as direct. Testing "should not be the first step," air sampling alone tells you nothing about the extent of the problem, its cause or how to fix it, and results "may take several weeks to come back," which can delay cleanup. A complete investigation of the house by a qualified person is usually more useful. Where sampling is justified, trained staff or EHOs can send samples to an accredited laboratory (CMHC).
How should housing staff inspect a home for mould?
CMHC's guide sets out a whole-house investigation, outside first, then inside, and includes a Household Investigation Tool with a Mould Assessment Summary that records the size and location of mould room by room. Across many homes, it documents the extent of mould in the community over time. A practical inspection sequence drawn from the guide:
- Outside. Check grading slopes away from the house, standing water, roof and shingles, flashing, chimney, cladding, caulking around windows, window wells, and ice damming in winter. Downspouts should extend at least 3 m (10 ft.) from any wall.
- Attic. Shine a light on the underside of the roof for stains, smell for mustiness, and look for air leaks from the house that cause winter condensation.
- Living areas. Look at exterior wall corners near the ceiling and baseboard, and behind furniture, pictures and curtains. Lift carpet edges. Push gently on walls below windows to feel for soft spots.
- Bathrooms, kitchen and laundry. A working bath fan should hold a sheet of tissue to the grille. Range hoods and dryers should vent outside; dryers through short, straight runs of aluminum duct with sealed joints. Check under sinks, behind toilets and around tub caulking.
- Basement and crawl space. Look for leaks, rot, damp carpet or vinyl on the slab, and "dried riverbeds" leading to the floor drain.
- Mechanical. Check the furnace service date, humidifiers, filters, ducts, the water heater, pipe insulation and the HRV.
- Humidity. Measure relative humidity with a hygrometer in every home you inspect.
The guide also describes quick field checks. Shine a flashlight across a surface in a darkened room: mould can show as a shadow of fine fuzz. Rub a coffee filter across a dark patch: if powder comes off, it may be mould (soot from candles does the same). A drop of household bleach that lightens a spot within two hours suggests mould. White fuzz on concrete that fizzes or dissolves with vinegar is efflorescence, a salt deposit and a sign of a damp foundation, not mould (CMHC).
How big is the problem, and who should clean it?
Size decides who does the work. Health Canada classifies mould as small (1 to 3 patches, each under 1 m², or 10 sq. ft.), medium (more than 3 patches, or patches larger than 1 m² but under 3 m², or 32 sq. ft.), and extensive (a patch larger than 3 m²) (Health Canada). CMHC uses the same breakpoints and treats patches close together as one patch.
- Small: occupants or maintenance staff can usually clean it with an N95 respirator, eye protection and gloves, using water and unscented dish detergent. Health Canada says "There's no need to use bleach." Porous materials such as ceiling tiles, carpet and upholstery that are mouldy should be replaced (ISC).
- Medium: CMHC says professional help is often needed, though trained occupants may manage with proper precautions. Health Canada suggests isolating the area with plastic sheeting and HEPA vacuuming before and after cleaning.
- Large: leave it to a trained mould remediation contractor (CMHC).
ISC tells occupants to contact their housing manager or local EHO for any patch over 1 m², more than three smaller patches, mould that keeps coming back after cleaning, or a problem they cannot solve on their own (ISC). Your housing policy should say who answers that call and how fast. For setting up the wider maintenance system, see our guide to a housing maintenance program on reserve.
What does a well-run remediation look like?
CMHC's guide treats remediation as a small project with a plan, not a cleaning visit. The steps a housing department can run:
- Fix the moisture first. Stop leaks, regrade, repair foundations or add exhaust as needed. Complex work, such as excavating a foundation, may run alongside the cleanup.
- Write the scope. Agreements with the cleanup contractor and any renovator should describe the work, costs and schedule, and how unforeseen work will be priced, in writing. An independent inspector trained in mould remediation can confirm the plan and check quality as work proceeds.
- Hold a start-up meeting. CMHC suggests bringing together the EHO, community health nurse or representative, occupants and contractor to agree on roles.
- Protect people. Isolate the work area. If a large area cannot be isolated, the household may need to move out during the work. Health Canada advises that people with breathing difficulties, mould allergies or chemical sensitivities stay out of the work area.
- Clean, dry, then rebuild. Store debris in closed containers, remove it promptly, and dry the area completely before refinishing.
- Confirm before re-occupancy. Consult the EHO or community health nurse to confirm the house is fit to live in, show occupants any new equipment, and leave written maintenance instructions.
- Monitor. Revisit the house afterward. As CMHC notes, "With enough available moisture, mould problems will return."
Sometimes the right answer is replacement. CMHC lists houses that may be too costly to clean, such as those with mould likely hidden in every cavity or older homes with many layers of renovations. It advises judging each case on cost estimates and building any replacement "to improved standards" (CMHC).
Who pays, and what support does ISC offer?
ISC told the House of Commons Indigenous and Northern Affairs committee in 2024 that mould remediation, and the repairs needed to address its underlying causes, "are eligible expenses under ISC's First Nations On-Reserve Housing Program" and under many CMHC programs, including the Residential Rehabilitation Assistance Program. ISC also said its Environmental Public Health Program "is not funded for home renovations or repairs to address moulds." EPHOs inspect homes on request and advise on remediation, cleaning and prevention, but ISC cannot inspect a home without the occupant's permission (ISC).
CMHC's On-Reserve Residential Rehabilitation Assistance Program lists "Major or Emergency Repairs" for health and safety, "including mould remediation." The page states up to $60,000 per unit, possibly 25% more in northern or remote areas, paid as a contribution to the First Nation, with household income limits, work starting within 3 months of approval and finishing within 12 (On-Reserve RRAP, CMHC). Program terms change, so confirm with your CMHC Indigenous and the North Housing Solutions Specialist. For the wider funding picture, see our funding guide for First Nations modular buildings.
Good records help make the case. The Auditor General's 2024 report found that ISC and CMHC "do not know how bad it is": only one ISC regional office collected mould data, and the 2008 national mould strategy was no longer in use (Report 2: Housing in First Nations Communities, Auditor General of Canada). ISC said that EPHO inspection data "are not representative" because roughly 1 to 3% of homes on reserve are inspected each year, on request (ISC).
Recording each home's Mould Assessment Summary, the cause found, the fix and the follow-up date gives the housing department its own evidence for prioritizing repairs and replacements.
What should you specify in new homes to prevent mould?
CMHC's mould resources set out measures for builders and renovators (CMHC). Put them in your tender and drawings:
- Foundation: a well-drained site, grading sloped away from the foundation, and a sealed and insulated foundation to keep the basement warm and dry.
- Roof and attic: air leakage points between house and attic sealed and insulated, and raised heel trusses so full attic insulation reaches over the exterior walls.
- Interior: quiet, energy-efficient exhaust fans and controls, a heat recovery ventilation system, and durable materials that resist moisture and are easy to clean.
- Windows and doors: energy-efficient windows to avoid condensation, moisture-resistant vinyl, fibreglass or metal-clad frames, and good weatherstripping.
- Venting: range hood, bathroom fans and dryer all vented outdoors.
Inspection is part of prevention. For housing funded under ISC's Capital Facilities and Maintenance Program, the department's protocol requires housing capital projects to be inspected by certified inspectors at least at site, foundation, framing and completion stages (Protocol for ISC-Funded Infrastructure). Finally, plan a move-in walkthrough: CMHC's guide says to have someone explain new heating and ventilating equipment to occupants and leave printed instructions.
Where does a builder fit in?
A builder cannot fix how a home is lived in, but it can deliver the envelope, drainage details and ventilation that make mould far less likely. Herauf Modular, a division of Herauf Enterprises, builds homes and community buildings as finished sections indoors in its Saskatchewan and BC plants, then delivers and sets them on site. Every unit meets or exceeds CSA A277 and the National Building Code for climate zones 6 through 8. Most builds are manufactured and delivered in about 16 weeks, with foundations, pads and services prepared on site at the same time.
If you are planning replacement homes and want to talk through specifications, send us an enquiry. We reply within one business day.
This guide is general information, not legal or financial advice. Program rules change: confirm the details with the program, your Indigenous Services Canada regional office or CMHC before relying on them. Herauf does not administer any program or apply on a community's behalf.