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Will my home be too airtight?

By Richard Rue - Taken from ICF Builder Magazine

You may have heard people say that a house has to breathe in order to be healthy. So when we talk about creating an airtight home, or structure, that may sound like a bad thing, but in reality it is critical to achieving ultra energy-efficiency. So, you ask, how do I overcome my fears – especially when my contractor is telling me that I need to bring in “fresh air,” or I am reading about Energy Recovery Ventilators (ERV) or Heat Recovery Ventilators (HRV)’s?

One of the true secrets to energy efficiency is how airtight you can make the structure. This is also one of the areas that is most misunderstood about energy efficient construction.

Basically, you want to tighten the structure down as tight as you can possibly get it, but your still going to have a certain amount of air leakage through the windows and the doors.

At Energy Wise, our rule of thumb is that if you have at least 10% glass to wall ratio. In other words, if you have 4,000 sq. ft. of wall, you have to have at least 400 sq. ft. of glass—and it’s operable glass not fixed glass—that you have enough natural leakage in the house. Assuming, of course, that you don’t have anything excessive in there, like a commercial dryer or something of the sort. In nearly every case, if you meet the 10% glass-to-wall ratio, you will have enough natural leakage into the house that you don’t need to use an any sort of air-to-air heat exchanger.

I talk to people every day that are concerned about the airtightness of the houses they’re fixin’ to build, and whether or not they’re going to suffocate themselves, and they want to get my advice on whether or not to put in an ERV or HRV system. I have to explain to them that with the way that houses are constructed today, even if we did everything perfect, and got it as tight as humanly possible, there would still be enough leakage through the windows and doors that they would not need, in most cases, an HRV or ERV system.

The reason that happens is because the average house that we see will have somewhere between a 20% to 30% glass-to-wall ratio. And as long as you have that 10% minimum, there’s enough natural leakage in the house that you will not need any kind of additional mechanical ventilation system.

If you install an air-to-air heat exchanger, especially in hot and humid or polluted climates, you’re just asking for trouble. In fact, mechanical ventilation is a real problem if you put it into an Energy-Wise engineered house. I get real tickled when they call them fresh-air-to-air heat exchangers, because the air in Dallas, Texas certainly isn’t fresh—and in most parts of the country, it’s the same situation.

It’s not fresh air. And not only is it polluted air, but it’s very humid air in most areas of the country. Why bring in additional outside moisture or polluted air? There’s no need. There’s enough leakage through windows alone to make up for any air exhausted through bathroom or kitchen venting. Don’t bring polluted, hot, humid outside air into a structure just because somebody thinks that you might be building this structure too tight.

If there is a situation where it’s too tight, we will come back and will alert the homeowner that they have too much fixed glass, and that they should look at some kind of air exchanger. What we like to recommend in those situations is “ventilation on demand.” What ventilation on demand is, you’re going to monitor the air quality with a CO2 sensor, and that CO2 sensor is designed so that if it detects a buildup of CO2 to unacceptable levels, then it will send a signal to a mechanical damper that is tied to the return of the unit and open that damper up to allow air to come in and “freshen up” the air. Then the mechanical system cuts off. You never have air just automatically coming in, which most systems are designed to do.




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