Opening windows in winter is a common practice that many homeowners do to let fresh air in and prevent moisture buildup inside the home. However, there is an ongoing debate about whether this practice actually helps prevent mould growth or makes it worse.
Mould growth is a significant problem in many homes, especially in damp and cold climates. Excess moisture inside a home can allow mould spores that are always present in the air to germinate and grow. Mould can cause a variety of health problems, damage materials in the home, and be very difficult to fully eliminate once a significant growth has occurred. Homeowners are often looking for ways to control moisture and humidity levels to prevent mould growth. The question is, does opening windows in cold weather help or hurt this goal?
How Mould Grows
Mould is a type of fungus that produces microscopic spores that are constantly floating through the air, both indoors and out. When these spores land on a damp spot, they can begin to grow and multiply, forming the fuzzy mould growth that is commonly seen. Mould can grow on many materials, including wood, drywall, carpeting, tile, and fabric when moisture is present.
Some key factors that influence mould growth include:
– Moisture – Mould spores need moisture to germinate and thrive. Dampness allows mould to grow much faster.
– Humidity – The optimal relative humidity level for indoor air is 30-50 percent. Consistently higher humidity provides moisture that enables mould growth.
– Temperature – Most mould species prefer warmer temperatures of around 70-90°F. Heat combined with moisture accelerates growth.
– Time – Mould needs time to establish itself and spread. Prolonged damp conditions allow colonies to grow larger.
– Nutrients – Materials that contain cellulose, such as wood and drywall, provide food sources for mould. Dust and dirt also supply nutrients.
– Air circulation – Stagnant air allows moisture levels to rise in localized areas, creating prime areas for mould. Good airflow helps prevent this.
Why Do People Open Windows in Winter?
During the colder winter months, homeowners are advised to open windows periodically as part of their strategy to control indoor moisture levels and prevent mould growth. This practice is recommended because:
– It brings in fresh, dry air – In most climates, outdoor air contains lower moisture levels in winter than heated indoor air. Replacing some indoor air helps lower indoor humidity.
– It reduces moisture buildup – Daily activities like showering, cooking, and even breathing add a lot of water vapor to the air. If the moist air cannot escape, it can condense into dampness.
– It prevents stagnant conditions – Stale, stagnant air allows localized moisture buildup in certain areas of a home. Fresh air circulation helps prevent this.
– It offsets “tight home” issues – Modern, well-insulated homes tend to trap moisture more than older, drafty homes. Periodic air exchange helps control this.
– It provides a sense of air quality – Stale indoor air just feels stuffy and can take on odors. Airing out a home makes it feel fresher.
So in theory, letting in outdoor air through open windows should lower indoor humidity and moisture, creating an environment less conducive to mould growth. But is this really effective in practice?
Potential Problems with Open Windows
While the concept of opening windows to exchange air and lower moisture seems sensible, there are some potential problems with relying on this practice in winter:
– Very cold air may not improve humidity – If outdoor air is extremely cold, such as below freezing, it may not be dry enough to lower indoor humidity. The cold air becomes warmer inside, lowering its relative humidity and negating the intended moisture reduction.
– Temperature differences can create condensation – Large temperature differences between indoor and outdoor air can cause condensation on cold surfaces inside the home, such as windows. This added moisture undermines the strategy.
– Moisture can be drawn into the home – If colder outdoor air contacts warm, damp surfaces like glass, tile, or walls, the moisture can condense on those cooler surfaces and be reintroduced indoors.
– Ventilation may be inadequate – Just opening one window may provide insufficient air exchange to meaningfully lower humidity, especially in larger homes. And air pockets can form that do not interact with the fresh air.
– Outdoor air may be polluted – Depending on outdoor conditions and sources near the home, incoming air may contain pollutants, chemicals, or allergens. These may counteract the positives of fresh air.
– Heat will escape from the home – Exchanging heated indoor air with frigid outdoor air can tax the home’s heating system as it works to regain lost heat. This wastes energy.
– Pipes may freeze – In extremely cold weather, letting very cold air inside increases the risk of freezing vulnerable water pipes. Burst pipes obviously cause serious water damage issues.
So while the idea of opening windows sounds good, the realities of winter air conditions may undermine or negate the humidity-lowering benefits in practice. Poor execution can even make problems worse in some cases.
Tips for Opening Windows in Winter
If you do want to try limited, strategic window opening to manage indoor humidity in winter, here are some tips:
– Check outdoor temperature and humidity levels – Only open windows on days dry enough to potentially lower indoor humidity. Use a hygrometer to compare indoor and outdoor readings.
– Open windows for short time periods – Exchange air for 30 minutes at most. Longer can chill the home too much and waste energy reheating it.
– Open multiple windows – Opening windows on opposite sides of a room or home improves air exchange. Try to create cross ventilation.
– Use fans – Placing fans in open windows pulls old air out and draws new air in faster and more efficiently.
– Prioritize rooms that need ventilation – Target bathrooms, laundry rooms, basements and other damp areas for air exchange.
– Avoid periods of high outdoor humidity – Early mornings on sunny winter days are often lower humidity times to open windows.
– Watch for condensation and frost indoors – If windows or surfaces get icy, the strategy is backfiring. Close windows.
– Monitor conditions and adjust – Pay attention to humidity trends and whether open windows are helping. Adapt as needed.
– Use other moisture control methods too – Window opening should be part of a larger humidity control plan, not the only strategy.
Alternative Moisture and Mould Control Strategies
While limited window opening may help, it is likely not sufficient on its own to control winter humidity and prevent mould growth. Some other key methods to employ include:
– Use exhaust fans – Routinely run bath and kitchen exhaust fans to remove humidity from cooking, bathing, etc. Vent fans outside.
– Avoid moisture sources – Limit moist activities like laundry drying, houseplants, humidifiers and non-vented activities that add indoor moisture.
– Dehumidify – Use dedicated dehumidifiers to actively remove excess moisture from indoor air when needed to maintain 30-50% relative humidity.
– Monitor humidity – Use hygrometers to monitor indoor humidity. Ideally maintain levels under 50%.
– Manage air circulation – Improve air flow by adjusting vents, running fans and opening interior doors to prevent moist air pockets.
– Insulate and air seal – Properly insulate attics, walls and basements. Seal noticeable air leaks around windows, ducts, wiring holes etc. to prevent condensation issues.
– Maintain negatives air pressure – Ensure attic and crawl space areas have negative pressure to draw excess air and humidity outside, rather than allowing it to enter living spaces.
– Keep spaces clean and dry – Clean up moisture and spills promptly. Avoid overcrowding to minimize humidity from occupants. Manage gutters to prevent foundation leaks.
– Be proactive – Watch for early signs of condensation, high humidity and mould growth. Take corrective action at the first signs of trouble spots. An ounce of prevention is worth a pound of cure!
Ventilation Strategies Summary Table
| Strategy | How it Works | Benefits | Drawbacks |
|---|---|---|---|
| Opening windows | Exchanges indoor and outdoor air |
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| Using exhaust fans | Vents moist air from bathrooms, kitchen, etc. outside |
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| Dehumidifiers | Actively condense and collect moisture from air |
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Health Concerns With Mould Exposure
It’s understandable why homeowners want to prevent mould growth within their homes. In addition to causing structural damage, mould can negatively impact health in several ways:
– Allergic reactions – Inhaling airborne mould spores or particles can trigger allergic reactions in sensitive individuals, causing respiratory issues like sneezing, runny nose, coughing, irritated eyes, etc. Severe mould allergies can produce symptoms even at very low exposure levels.
– Asthma – People with asthma are especially vulnerable to mould spores irritating their airways and triggering asthma attacks. Damp indoor environments with mould often worsen asthma symptoms.
– Respiratory illness – Mold spores inhaled deep into the lungs can cause more serious respiratory illnesses and infections, especially in people with weakened immune systems. Long term exposure may result in chronic lung disease.
– Headaches, fatigue, and nausea – Volatile organic compounds (VOCs) emitted by some mould species can cause headaches, fatigue, nausea, dizziness and other symptoms in those exposed. Even relatively low levels of mould VOCs may trigger reactions in sensitive individuals.
– Skin irritation – Contacting mould directly can lead to skin irritation and reactions such as rashes, hives, or more severe fungal infections of skin tissue in those with weakened immunity.
Mould exposure does not affect everyone the same way. Children, the elderly, and those with preexisting conditions like allergies or lung disease are most vulnerable to mould-related health issues. But mould can degrade air quality and impact the well-being of any building occupants. Controlling humidity and mould is well worth the effort to keep homes healthy.
Professional Mould Remediation
If a significant mould problem already exists within a home, professional remediation is required to remove mould and correct underlying moisture sources supporting its growth. DIY mould clean up is not recommended. Key aspects of professional mould remediation include:
– Inspection – A trained professional will evaluate the extent of visible mould as well as check for hidden mould behind walls, under materials, and elsewhere. They will identify areas with moisture issues supporting mould growth.
– Containment – The remediation area will be isolated and contained with plastic sheeting to prevent spores spreading. Negative air machines may be used.
– Removal – Visible mould is removed from affected materials. This may involve cleaning surfaces or replacing building materials that are too contaminated.
– Drying – Equipment like dehumidifiers and fans will dry and remove residual moisture from the remediated areas. Moisture sources are corrected.
– Sampling – Surface, air and HVAC system samples may be taken to verify mould has been remediated before containment is removed.
– Prevention – Recommendations are made for proper humidity levels, air circulation and other measures to prevent future mould issues.
Mould remediation can take anywhere from a few days for a small problem up to several weeks for major remediation projects. The goal is to completely remove existing mould and correct building conditions that allowed it to grow in the first place.
Conclusion
Opening windows can help lower indoor humidity when done carefully and strategically. However, the practice is often not sufficient on its own, especially given the potential downsides. Preventing excess moisture buildup requires an integrated plan including proper ventilation, dehumidification, air circulation, maintaining optimal temperature and humidity levels, and prompt cleanup of any moisture. With diligent proactive moisture control, opening windows periodically may provide some supplemental humidity management. But used alone, leaving windows open in winter is generally not the most effective way to prevent mould growth long term.