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, a measure of how much heating its climate needs, and its winter design temperature, the cold day heating systems are sized for, is −12.8°C. Those figures come from the climate data for St-John's International in HOT2000, the home-energy software Natural Resources Canada (NRCan) gives energy advisors. 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
Atlantic Canada Maritimes and Newfoundland
What this means in St. John's’s climate
Damp, windy, icy winters and costly oil: switch to heat pumps, seal, test for radon.
Atlantic winters are wet, windy and icy rather than extremely cold, and summers are mild. Heating oil is still common and its price rose sharply in 2026, while heat pumps already heat many homes. An EnerGuide evaluation here should plan the move off oil, seal wind-driven leaks, fix moisture before insulating, and include a radon test.
- Fix moisture before you insulate. Halifax, St. John's, Saint John and Charlottetown get enough wind-driven rain that new walls must have a rainscreen. Stop water getting into walls and basements before adding insulation. Ask: Ask the advisor to check the basement, attic sheathing and walls under windows for damp, and how bath fans vent.
What else homes in St. John's need →
Sources for this climate guide · checked October 26, 2026
- National Building Code of Canada 2020, climate data (NRC)
- Canadian Climate Normals 1991–2020 (ECCC)
- Keeping the Heat In (NRCan)
- Heat pumps, Canada Greener Homes (NRCan)
- Heat pump basics (NRCan)
- Heating and Cooling With a Heat Pump (NRCan)
- Primary heating systems by household, Table 38-10-0286-01 (Statistics Canada)
- Household heating oil prices, Table 18-10-0001-01 (Statistics Canada)
- Electricity price comparison, April 2025 (Hydro-Québec)
- Prevent oil tank spills (Government of Nova Scotia)
- Canada's top weather stories of 2019 (ECCC)
- Canada's top weather stories of 2022 (ECCC)
- Canada's top weather stories of 2023 (ECCC)
- 2024 Cross-Canada Survey of Radon Exposure
- Guide for Radon Measurements in Homes (Health Canada)
- Conditions for thermal scans, ASTM C1060 summary (Cx Associates)
- Canada's top weather stories of 2020 (ECCC)
- Canada's top weather stories of 2021 (ECCC)
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.