How St. John's is built
- Before 1961 22%
- 1961–1990 42%
- 1991–2000 11%
- 2001 or later 26%
Why mould shows up in St. John's homes
St. John's gets about 4,578 heating degree-days a year and has a winter design temperature of −12.8°C, according to NRCan's HOT2000 climate data for St-John's International. Mild, wet winters keep materials damp, so poorly vented bathrooms, basements and crawlspaces are the usual suspects.
Of St. John's's 42,120 houses (single-detached, semi-detached, row houses and duplexes), 22% were built before 1961 and 64% before 1991, according to the 2021 Census. Older basements without a moisture barrier and houses that were air-sealed without adding ventilation are common trouble spots.
Mould inspection: when sampling is worth it
Start by looking and smelling: stains, discoloured patches, peeling paint and a musty odour, especially in basements, bathrooms, window frames and closets on outside walls. Health Canada says measuring mould concentrations or identifying species is usually unnecessary.
No Canadian province has a mandatory mould standard for homes. Health Canada's guideline sets no numerical limit: it says to fix moisture problems and clean up visible mould.
Air or surface sampling helps in three cases:
- You smell mustiness but can't find the source.
- After a leak or flood, to check that cleanup worked.
- Before buying an older house with signs of past water damage.
Cleanup and remediation: the size rules
CMHC and Health Canada sort mould by size, and the size decides who should clean it up:
| Size | What counts | Who should clean it |
|---|---|---|
| Small | One to three patches, each smaller than 1 m² | A healthy homeowner, with water and unscented dish detergent (CMHC) |
| Medium | More than three patches, or patches between 1 and 3 m² (32 square feet) | Trained help; homeowners only with training and precautions |
| Large | A patch larger than 3 m², or many patches through the house | A qualified mould remediation contractor (CMHC) |
- Fix the moisture source first: a leak, poor drainage, or condensation from high humidity. Health Canada recommends 30–50% relative humidity.
- For a small cleanup, wear safety glasses or goggles, an N95 mask if possible and household rubber gloves. People at risk from mould shouldn't clean it and should stay away.
- Health Canada suggests professional help for extensive mould, especially growth that keeps coming back after cleaning.
Choosing a tester, lab or remediation contractor
Ask any St. John's tester which lab analyzes their samples, and look for one accredited under AIHA's EMLAP program for environmental microbiology. Pair sampling with a ventilation and humidity assessment, which finds the cause. For remediation, get two or three written quotes that describe containment, what will be removed and how the area is cleaned.
Ventilation and humidity assessments in St. John's · Our mould and indoor air quality guide
Mould and indoor air quality testing in St. John's: common questions
How much does mould testing cost in St. John's?
We found no reputable Canadian source for the cost of a professional mould assessment, so get two or three written quotes and ask how many samples are included, which lab analyzes them and what the report covers.
Do I need a test if I can see mould?
Usually not. Visible mould needs cleanup and a fix for the moisture source. Testing helps most when you suspect mould you can't see.
How much mould can I clean myself?
CMHC says a healthy homeowner can clean one to three patches, each under 1 m², with water and unscented dish detergent. Larger areas need trained help or a remediation contractor.
When should I hire a mould remediation contractor?
For a single patch over 3 m², many patches through the house, or mould that keeps coming back after cleaning (CMHC, Health Canada).
Is there a legal mould limit for homes in Canada?
No. Health Canada sets no numerical limit; its guidance is to control moisture and clean up mould. Quebec has a voluntary handling standard.