Thoughts of Home: Explaining Air Sealing and Ventilation to Homeowners

This is the second edition of Efficiency Vermont’s “Thoughts of Home” series as written by Dave Keefe. It is intended for building science professionals in Vermont and not intended to be used as guidance or assistance directly for homeowners. Comments should be directed to

Your clients are likely to have misconceptions about tight houses. Some folks think that houses shouldn’t be tight, that they “need to breathe.” Certainly there have been cases where air-sealing a house has done more harm than good. But just because something has been done incorrectly in the past doesn’t mean that it shouldn’t be done or that it can’t be done correctly. When air sealing produces undesirable results, it’s usually because we have either failed to identify and treat pollutant sources or failed to provide adequate ventilation.

Those of us in the home performance business sometimes have to explain to customers why we simultaneously recommend air sealing and ventilation. People are confused by this. It seems as though these two things conflict—that each undermines the other. After all, if someone told you that you should seal the holes in the ceiling and then cut holes in the ceiling to make up for the holes you sealed, wouldn’t you think that was crazy? If you have to cut holes to replace the holes you sealed, why would you seal the holes in the first place? And since you want fresh air to breathe, why would you even want the holes sealed?

This document is intended to help industry professionals answer questions like those from homeowners. It has three sections: The first is a series of points you might want to make. The second is a chart that summarizes the advantages and disadvantages of loose houses versus tight houses with ventilation. The third is a useful analogy you can present to your customers.

This is not intended as a handout for homeowners—it is way too wordy for that. It’s intended to help clarify your thoughts so that you can craft your own way of explaining, and to provide talking points you may not have thought of.

Points to Make with Homeowners

Why should we spend money to seal up air leaks in the house and then spend more money to install a fan to move air in and out? It seems counterintuitive. Why not just leave the holes? Don’t they accomplish pretty much the same thing?

A tight building with mechanical ventilation works better than a leaky building, for several reasons. Mostly, it affords control—you can have the air come and go when and where you want it, rather than when and where it happens to come and go.

With mechanical ventilation, you can choose how much air comes in and goes out of the building. Random uncontrolled holes don’t offer this choice. The air moves according to the weather. When it’s windy, you get a lot. When it’s really cold, you get a lot. When it’s not windy or cold, you get little or none. Note that this is just about the opposite of what you want. And Mother Nature doesn’t know or care about what is going on in the house. We mostly need ventilation for human activity. Ideally, we would have little or no ventilation when the house is empty and lots when our activity warrants it. The only way you can create this situation is to have control of the flow.

Mechanical ventilation allows you to choose where the air comes and goes. We mostly need ventilation where we generate moisture and odors, but that’s not where most of the random holes tend to be. One reason this is important is that it is difficult to get rid of moisture and odors once they have spread throughout the house. It’s much more effective to remove them from the house when and where they are more concentrated. That way you don’t need to remove as much air in order to remove the pollutants. So we can get the desired effect without as much heat loss (or, in the case of air conditioning, heat gain).

Fans move air in the desired direction. Air going through random uncontrolled holes is going the wrong direction about half the time. To carry moisture, odors, or other pollutants out, air needs to be going out. To deliver fresh air, air needs to be coming in. Fans can accomplish both.

With fans, you can turn the flow up. If you have just taken a shower and want to dry out the bathroom, or if there is an odor you wish were elsewhere, a fan can respond to your command. Random holes can’t; they just leak according to the weather.

With fans, you can turn the flow down or off. If the weather is severe or you are going on vacation, you might not want as much outside air coming in. Just turn the fan off. That’s easy. You can’t seal and unseal random holes whenever you want. That’s hard. It’s also worth noting that if you want options and flexibility, then you want a tight house, because you can always easily make a house temporarily leakier by opening the windows. You can never easily make a house temporarily tighter.

Wind and cold is not as much of a problem in tight houses. Tight houses are largely unaffected by wind, but loose houses are uncomfortable and expensive to heat when it’s windy. Tight houses don’t leak much more when it’s cold than they do when it’s warm. But leaky houses leak a lot more when it’s cold than they do when it’s warm. That’s because of the “stack” or “chimney” effect. Warm air wants to go out the top, and cold air wants to come in the bottom. The bigger the temperature difference between inside and outside, the stronger that stack effect is. That’s why we have more air leaking through the building when it’s cold outside (just when we would prefer to have less air leaking through). Fans aren’t affected much by the stack effect. They move just about the same amount of air regardless of outside temperature.

Tight houses are more comfortable than leaky ones. The main purpose of a house is to keep us comfortable inside when it’s cold and miserable outside. The house can’t do that if the wind blows through it.

Mechanical ventilation allows control of the airflow path. Air leaking out can cause problems. That air contains moisture from our showering, cooking, and breathing. In cold climates, that moisture can condense on cold surfaces in the walls or attic and damage the building. With fans, we can control the pathway (a duct) and make sure the air doesn’t cause moisture problems on its way out. Air going out through random holes is also a very common cause of ice dams. If we don’t allow the warm air to touch the roof, then we won’t melt the snow and make ice on the roof.

Air leaking in can also cause problems. If you don’t know how and where the air is coming in, then you don’t know what it’s picking up on the way. If your fresh air comes in through a damp, moldy crawl space, is it still fresh air? What if it comes in through the attached garage, or the soil?

A tight house with mechanical ventilation allows humidity control. The outside air in the winter is very dry. That’s why the owners of many of our leakier houses have problems with static electricity, chapped lips, and cracking furniture. If you reduce the amount of outside air coming in, you reduce that drying effect. If you increase the amount of outside air coming in during the winter, you dry things out more. If you can’t control the amount of air moving in and out, then you can’t control the amount of water vapor moving in and out.

Similarly, if you air condition in the summer, your air conditioner is trying to reduce both the temperature and the humidity of the inside air. Any warm, humid air sneaking in makes that job harder (meaning more expensive).

Typically, a tight house with mechanical ventilation either has better air quality or is cheaper to heat and cool, or both, as compared with a leaky house. For every house and the activity it contains, there is an optimum ventilation rate. With mechanical ventilation, we can match that flow to the task and get the air quality we want without the cost of excess airflow. With random uncontrolled holes, we either get too little flow (worse air quality) or too much flow (expensive to heat and cool). We almost never get the optimum rate, because the flow is determined by the weather. We get the least when we want the most (when it’s mild outside and the effect on comfort and cost is low) and we get the most when we want the least (when it’s cold and windy and the effect on comfort and cost is high).

A Comparison of Ventilation Strategies
Leaky House without FansTight House with Fans
Ventilation rate Unknown. Erratic. Varies with weather. Mostly more when you want less, less when you want more. Measurable. Controllable. Whatever amount you want at the time.
Locations of incoming and outgoing air Unknown. Uncontrolled. Mostly not where you want. Wherever you choose.
Direction of airflow at specific locations Wrong about half the time. Correct all the time.
Ability to increase ventilation temporarily If it’s windy, or cold, yes. Otherwise, no. Yes, at any time.
Ability to decrease ventilation temporarily No, never. Yes, at any time.
Ability to control quality of incoming air Poor. Good.
Ability to manage indoor humidity Poor. Good.
Ability to prevent moisture problems in building cavities Poor. Good.
Cold drafts in winter More. Fewer.
Bathroom conditions Damper. More odor. Drier. Less odor.
General comfort Not as good. Better.
Heating and cooling costs Higher. Lower.

An Analogy That May Help Homeowners Understand Ventilation Principles

Everyone knows we want fresh air in our houses. So why shouldn’t we want the house to leak? Because air sealing and ventilation are really all about control. Here’s an analogy.

There’s another essential compound that we also want in our homes: water.

We use it every day. We can’t live without it. We need a reliable source, and we want it to be clean. And when we’re done with it and it’s dirty, we want to get rid of it without problems.

But that doesn’t mean we want the roof to leak.

If it did, we’d get water only when it rained. We wouldn’t be able to choose how much we got. Sometimes, we’d get none at all. Sometimes, we’d get way too much. It would damage the building. Bad things might grow.

So we control water’s passage into and out of the house. We bring it into the building when we want. We can shut it off. We can get the amount we want where we need it and when we need it. We can prevent it from damaging the building. If we want, we can filter it, or heat it. When it’s dirty, we can get rid of it effectively.

We should think about our fresh air the same way. Air-sealing our homes and adding mechanical ventilation may seem odd at first, but it’s really the same strategy we already use for water. Air sealing stops the airflow we don’t want. The fans create the airflow we do want. Both elements working together allow us to take control of our indoor air.

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