![]() ![]() Two 50 cfm bath fans would induce a 50 Pa pressure difference across such a house. It takes 5,200 cfm to induce a 50 Pa pressure difference, so 50 cfm will be lost in the noise.Īt the other extreme, some Passive House projects have air leakage rates of 100 cfm50 or even lower. With an air leakage rate of 5,200 cfm50, my house won’t show much of a negative pressure if I turn on a 50 cfm bath fan. That’s pretty high, but it’s a 3,800 square foot house built in 1961, and it was close to 7,000 when I bought it last May. My house currently has a blower door test result of 5,200 cubic feet per minute at 50 Pascals of pressure difference (5,200 cfm50). The more airtight the house, the bigger that negative pressure will be. Exhaust fans in your bathrooms and kitchen, of course, will induce some negative pressure. ![]() That’s why it’s so strong in that video taken in a 44 story building on a cold day in Denver.Īnd then there’s the wild card of mechanical systems. The two factors that affect the magnitude of stack effect are height and temperature difference. That can create some pretty big pressure differences, as you can see in the video below. Warm air is less dense and thus rises when surrounded by cold air, like an air bubble in water. Stack effect is a density-of-air phenomenon. That’s why building codes require hurricane straps in high-wind areas. Hurricanes and tornadoes can induce pressure differences big enough to blow the roof off of a house. Wind, of course, can create pressure differences between inside and outside. Three factors are responsible for pressure differences in buildings: The 3 causes of pressure differences in buildings If you have positive pressure, the air will be moving toward the outdoors. If you still can’t feel any air movement through that opening after you lick your fingers, you don’t have enough negative pressure to worry about, at least not right there. You can feel it with your fingers, too, and it works even better if you lick them (part of what Joe Lstiburek calls the “look, lick, and squirt test”). If you stand inside and put your face near the crack in the door, you’ll feel the air blowing on you if there’s a negative pressure. With the house operating at what you suspect is a negative pressure, go to a door and open it just a bit. You also could use a magnehelic gauge for less than $100. ![]() You could get one with lower precision and spend less than $100. All you need is a manometer, and the nice ones cost over $1,000. It’s quite easy to find out if a house is indeed running a negative pressure. For air to move across the building enclosure, you need two things: a pressure difference (positive or negative) and a pathway. A negative pressure in the house means the indoor pressure is lower than the outdoor pressure, which can cause air to move from outdoors to indoors.īut that infiltrating air needs a pathway to do so. First, when we talk about negative or positive pressure, we’re talking about the pressure difference between two places, usually indoors and outdoors. Rob suspects negative pressure is behind the fireplace smells in his home. What’s going on here? Do you have negative pressure? I assume somehow negative pressure is forcing air back down the chimney especially when we run bathroom fans or the dryer I suspect. Then we just built our first fire since cleaning this weekend and the smokey smell is back. I had the chimney cleaned really well and the damper closed and it was fine. We always have a smoke smell from our fireplace, more on some days than others. Here’s a question I got from a reader named Rob last month: Ever have the smell of your fireplace inside your home? It’s not uncommon. ![]()
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