When “Nice” Weather Causes Indoor Air Quality and Humidity Problems

The shoulder season between cooling and heating our homes often means indoor health and comfort problems. Are you ready for them?

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When You Don’t Need Cooling or Heating
A shoulder season between cooling and heating our homes comes in the fall and spring of each year. Many heating/cooling systems have to be switched between heating and cooling – they do one or the other so they don’t both cycle on and off. In the early fall as there are still some warm days, it’s common to have your cooling system switched on and the thermostat set to some point in the 70s. If it’s a cool day and your indoor temperature doesn’t rise to the level set on your thermostat, your cooling system won’t come on.

In the shoulder seasons, there may be multiple days when your heating/cooling system doesn’t run. We often enjoy the change in weather and bundle up with more clothing layers and blankets – which is a novelty we’ve not enjoyed for many months and a welcome change from uncomfortably hot temperatures just weeks before. Still, there can be a couple of consequences from not operating your heating/cooling system.

Higher Humidity
The shoulder season between the warm weather of the early fall and the cooler temperatures can cause your indoor humidity levels to be much higher than usual, certainly much higher than desirable. Even though the temperature may be just below your thermostat setting, the humidity levels can easily reach above 80%. The ideal indoor humidity level for your home is somewhere between 40 and 50%.

As indoor humidity levels climb above 50%, the environment becomes less comfortable. Indoor humidity levels of 70% and above contribute to significantly faster growth of mold, mildew and dust mites. You may also notice high indoor humidity problems manifest in other ways like your towels become musty after one or two uses.

The cooling load in a home includes a sensible load (temperature) and a latent load (humidity). Many thermostats don’t have a setting to control the humidity level in a home though the air conditioning system removes indoor humidity when it runs because the humidity in the air condenses on the cold air conditioning coil in the air handler.

Humidity is generally not a significant issue in the winter months when your heating system is working to keep your home warm because cool air has less capacity to carry water vapor (humidity). Humidity spreads out until it’s uniform. When the outdoor humidity is low, the humidity in your home tries to escape to the outside. In the winter, you may have the opposite issue: you need to add humidity to get up to a comfortable level that prevents static electricity shocks, dry skin and nose bleeds.

Lack of Air Filtration
Many homes only get air filtration when the heating/cooling system circulates air. If the system isn’t flowing air for heating or cooling, you’re not getting any air filtration. You may notice additional allergy problems, lingering odors and other issues. Even if you are getting some air filtration, older systems may not provide any fresh air and the level of air filtration may be very low – in some cases the filter really only keeps dust, hair and other particulate from getting to and clogging the coil in your air conditioner.

It’s common for homes to have leaks around doors, windows, electrical outlets, plumbing and other places that allow air, humidity and particulates like allergens to flow in and out. When your heating/cooling system is running, you get some filtration of the air that enters through those leaks but that filtration may be greatly reduced or stop during shoulder seasons for heating and cooling.

People with pets may also notice that they have some allergy problems in the shoulder seasons between heating and cooling due to less air filtration. This is especially true if you’ve been relying on a high-quality air filter to remove allergens like pet dander that you may not notice during warm or cool weather when your heating/cooling runs more often.

Addressing Indoor Humidity
The first step for addressing indoor humidity is sealing air leaks that allow humidity to leak into your home through openings in your building enclosure. If you have a relatively new system, you may have a setting on your thermostat that allows you to control your indoor humidity level separately from the indoor temperature.

Removing indoor humidity universally involves using a refrigeration cycle to cool a coil that you expose to the air in your home. The humidity in the air condenses onto the coil and drips off into a drain so it can be removed from your home. That coil may be in your central cooling system, a stand-alone dehumidifier or a product like a ventilating dehumidifier.

Ideally, you’ve sealed and insulated your house enough to gain control of the health, comfort and efficiency of your indoor environment. When that’s done, you should have a design that incorporates heating, cooling and ventilation for your home. It should address both indoor air quality (by providing fresh air + filtration) and your indoor humidity level.

In Conclusion
People who are sensitive to indoor environmental issues will likely have more difficulty in the shoulder seasons when their indoor environments can become more polluted from lack of air filtration and the effects of higher indoor humidity levels. Investing in the solutions that will address these issues is investing in your health, comfort and efficiency. After all, most of us spend more than one third of our lives in our homes. Why not take care of our homes so they can take care of us?

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