5/25/2023 0 Comments Tight building envelope![]() With an extremely airtight envelope and a mechanical ventilation system that controls exactly where and how much air is brought in and exhausted, you get the quantity of fresh air you need, you deliver that fresh air where it’s needed, and you get it consistently, whether it’s windy or not and no matter the outside temperature. With a ventilation system-which can be as simple as the continuous or intermittent operation of quiet bathroom fans with intentional air inlets, to a whole-house ventilation system-you will be sure of getting the fresh air you need. My answer to the question of how tight we should make our houses is “really, really tight.” But we also need to provide mechanical ventilation. On a day without much wind during the spring and fall months, when it’s not that much colder outside than in, the differences in pressure won’t be enough to cause much air exchange-even with a quite leaky envelope, so you won’t be ensuring fresh air. The problem is that there isn’t always one of these situations to create that pressure differential we need for fresh air. And when it’s really cold outside, the “stack effect” pushes warm air out through the envelope high in the house and sucks in outside air near ground level. When it’s windy, there’s a pressure differential-on the upwind side fresh air is pushed in through those gaps in the house, and on the downwind side stale house air is sucked out. Air movement through a building envelope depends not only on the envelope leakiness, but also on the “pressure differential” across the envelope. ![]() The strategy of keeping your house intentionally leaky can’t even be relied on to provide fresh air. You can’t rely on air leaks to be reliable Mold can grow-potentially making you sick-and cellulosic materials like wood can rot. When condensation occurs within your walls or ceiling, stuff gets wet. The dew point depends on the temperature as well as the relative humidity-the higher the relative humidity the higher the temperature at which the dew point will be reached. When warm air leaks out through cracks and gaps in your building envelope during the winter, that air cools off and may reach the “dew point.” This is the temperature at which water vapor (a constituent of all air) can condense into liquid water. To make matters worse, the rate of air leakage is highest when the energy impact of that leakage is the greatest-when it’s very cold or very windy. Cold air leaking in means dollars leaking out. If we increase insulation levels and put in better windows but leave the house leaky, the fraction of total heat loss coming from air leakage increases. In a typical house, air leakage can account for 25-40% of the total heat loss of the house. Leaky houses costs you money and waste energy There are several problems with the idea of relying on a leaky building envelope to ensure adequate fresh air in a house. But the solution-to keep the house leaky-is wrong. The concern is right on-that a tight house without enough fresh air is a bad thing. Without as much fresh air getting in to dilute those pollutants and replenish the oxygen we need, aren’t we going to suffocate? Shouldn’t the house be left leaky? These pollutants can include carbon dioxide from our breathing, smoke from burning our toast, volatile organic compounds (VOCs) from cleaning materials and furnishings, moisture (which isn’t a pollutant itself, but causes mold and other problems), and, yes, even bathroom deodorizers that often contain harmful chemicals. With less circulation, pollutants in the house can build up. The first question I usually get when I start talking about insulating and buttoning-up houses is, “Won’t my house be too tight?” It’s a very logical question.Īs we make houses tighter, less air flows through them.
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