Counterpoint: Open a Window for Improved Indoor Air Quality?

RTAblog_2015_0305_babyAn article by Healthy Child Healthy World claims you should open a window, especially when it’s cold outside. Is that true? What’s best for your family?

An article published on, attributed to Healthy Child Healthy World, suggests people should open a window for five minutes a day to reduce levels of common indoor pollutants. While it’s true the solution to pollution is dilution, opening a window on occasion isn’t the most effective solution. In fact, it may introduce problems.

Controlling Humidity
We’re building tighter new houses and air sealing existing homes to cut down on air leaks that allow humidity to flow in and out of our homes. While the Healthy Child Healthy World article calls dust mites “common pollutants”, it doesn’t note that they love humidity. They multiply more quickly when there are higher humidity and temperature levels in the home. High indoor humidity, especially 70+%, also causes significantly faster mold and mildew growth. Most air conditioners don’t do a good job controlling humidity so it’s best not open a window and let humidity in.

Opening a window when it’s cold outside causes humidity to flow out of the house. The ideal indoor humidity level is somewhere in the mid-40% range. When indoor humidity gets lower than 40%, occupants begin get dry sinuses, bloody noses, itchy skin and static electricity shocks.

In cold weather, humidity levels can drop into the teens. That can make indoor environments in leaky houses very unpleasant. It may cause wood trim and other elements to crack or pull apart. You may also notice gaps in wood floors open wider.

Got Allergies?
Opening a window may also allow allergens like pollen to enter your home. In addition to heavy spring pollen concentrations, you may be more irritated by mold and mildew if you allow high humidity levels in your home. If you stay indoors to avoid the pollen and other allergens, you’ve probably already realized you may not want to open a window every day.

Got Asthma?
Humidity issues and allergies are no laughing matter. Asthma can be much more severe than either. Many of our cities experience “ozone days” where ozone levels are elevated to the point that asthma suffers risk attacks due to air-borne pollutants. Even people without asthma should avoid outdoor activity when ozone levels are high. Thankfully, this issue is seasonal though it’s enough to make us think twice about opening a window.

Does Opening a Window Even Work?
Does opening a window for five minutes decrease indoor air pollution? It may have little to no effect if it’s a calm day out.

You need a pressure difference to really move air. If it’s calm outside (no wind), there’s no pressure difference between inside and outside so little to no air movement takes place. That’s why older design solutions include passive design strategies like using the stack effect (temperature difference) or active systems like whole-house fans to move air. Both of those strategies are subject to the three problems mentioned above: bringing in humidity, allergens and other irritants like ozone.

Mechanical Ventilation is Your Best Option
Mechanical ventilation, controlled ventilation driven by a fan, is typically designed to provide ventilation throughout the day to reduce pollution levels and keep them low. Opening a window for five minutes a day allows pollution levels to build up for twenty-three hours and fifty-five minutes.

The housing market has evolved to use energy recovery ventilators (ERVs) [and heat recovery ventilators (HRVs) in colder climates] to provide balanced ventilation and avoid wasting energy. The volume of fresh air they bring in and indoor air they exhaust is the same. ERVs have air filters to avoid bringing pollutants into the home. Some units come with dehumidifiers to remove humidity from incoming, fresh air. They also work the (hopefully) 8+ hours your family is sleeping… and can’t open a window.

Your home may be one of tens of thousands of homes built or retrofitted under recent building codes and standards so you may already have mechanical ventilation. If you don’t, air sealing and mechanical ventilation are great investments in your health, comfort and the efficiency of your home.

High-quality mechanical ventilation from an ERV may cost just one-quarter to one-third the expense of a central heating and cooling system. Mechanical ventilation equipment doesn’t need to be connected to other equipment so you can install it just about anywhere you have room.

Do You Have any Choice?
Recent building codes and standards mandate homes with very little air leakage. Building tight homes gives us control of the indoor environment (temperature, humidity level and indoor air quality). Tight homes also avoid wasting energy. It takes energy to heat or cool your indoor air so you don’t want it escaping through air leaks.

Tight houses don’t have fresh air leaking into the home so recent building codes and standards also mandate mechanical ventilation to protect the occupants. The 2012 International Residential Code1 (IRC) requires “whole-house mechanical ventilation” intermittently. It contains two tables: one to figure how much airflow you need for ventilation and the other to figure the percentage of time the ventilation needs to happen in each four-hour period of the day. [Section 1507.3, page 509]

Building codes are a minimum standard – the lowest acceptable standard. Even so, it doesn’t seem opening a window for five minutes a day would come anywhere close to meeting the 2012 IRC requirements.

You should open your window(s) when the outdoor conditions are favorable: comfortable temperatures, low humidity and low concentrations of irritants like pollen and ozone. There’s nothing like enjoying a beautiful day.

In Conclusion
The Healthy Child Healthy World article referred to in this post has 8,000 Facebook likes. That’s a lot of open windows. It’s frustrating to read what seems to be bad or at least incomplete advice that’s been widely distributed.

Healthy Child Healthy World’s article concludes with, “Manual is the way to go.” Checking your sources and finding educated advocates is the way to go.

If you have questions or want some guidance, you’ll probably be better served by speaking with an architect or HERS Rater knowledgeable about this topic – someone who doesn’t have equipment or products to sell.

It’s taken 1,082 words to point out the issues of just opening a window. If you’ve read this far, thank you and well done! You know better than to take one-size-fits-all advice at face value.

As always, if you’ve got a comment or question, please share it below. Thanks!

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Thinking About Replacing Your Windows? Don’t Believe the Big Energy Savings Promise!

1 Your state, county or local jurisdiction may or may not be using the 2012 International Residential Code (IRC). Not all jurisdictions use the same version of the building code. The 2012 IRC example is offered in this blog post to point out the need for ventilation is a known issue and has been in codes and standards for years.

Image by Tanel Teemusk used under creative commons license.

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