With the advent of the 2012 IECC (International Energy Conservation Code), builders are beginning to raise questions about section R402.4.1.2, which states that, “the building or dwelling unit shall be tested and verified as having air leakage rates not exceeding 3 air changes per hour in Climate Zones 3 through 8.” It further states that, “testing shall be conducted with a blower door at a pressure of 50 Pascals.”
So what is a blower door? (Don’t feel bad if you don’t know, because you’re not alone; but this is probably something you should become aware of with such a requirement on the way!)
The equipment portion of a blower door utilizes a canvas with a metal frame around it, fan, and manometer (small computer-like device). The metal frame and canvas fit snug into most exterior door openings which adjoin the outside. The fan is then inserted into an opening in the canvas. The fan is then turned on and the air in the house is sucked out (the home is depressurized). The leakier the house, the faster and longer the fan has to operate to suck the air out and for depressurization to occur. Sometimes the house is so leaky the fan ends up pulling out as much air as what’s coming in around the many cracks and openings in the building envelope. In such a case, the house cannot be de-pressurized. This obviously is not a good situation. The manometer (little computer) that’s attached to the fan unit measures the pressure inside and outside the house while the fan’s running. As it measures the pressure difference between the two locations, based on the fan speed, this device can tell how leaky a house is. There are several ways it provides such information: CFM (cubic feet per minute), ACHnat (air changes per hour under natural conditions, or when the house is not under artificial negative pressure like what the blower door produces), ACH50 (air changes per hour when the blower door is operating and a pressure of 50 pascals is achieved, which is like a 20 mph wind blowing outside), etc.
To give you an idea of how leaky or air-tight a house should be, let’s look at the energy code’s requirement: Based on current code (2009 IECC), air changes per hour when the blower door is running (ACH50) should be 7 or less. This means that all the air in the home should be completely replaced with new fresh air 7 or less times in one hour. Requirements are getting tougher upon the release of the 2012 IECC. Once the new code is enacted, this changes to 3 or less air changes per hour in Zone 5, which is generally from mid-Michigan through Southern Michigan (if you are only looking at Michigan). Such requirements mean that the building envelope and insulation will have to be pretty tight and well installed!
Determining if a home has enough ventilation is very important. A home that is not getting enough air changes per hour can suffer from poor indoor air quality. These days, new home are being built much tighter then they used to be, which means proper ventilation is very important. Most homes have carpet and humans spend on average 90% of their time indoors. Add these two scenarios together and we wonder why we suffer from allergies, asthma and in some cases much worse. Ventilation needs to be a high priority for builders. This also needs to be considered when a home is remodeled. Many older homes are so leaky that air quality is not usually an issue. If the home is renovated by adding insulation, air sealing, replacing windows and doors, this could lead to the home being to air tight (if no mechanical ventilation is present). Blower door numbers alone are not a great way of telling if a home is too air tight. These numbers have to be used in conjunction with building science knowledge, which is out of the scope of this article (this is a very controversial subject in this industry). Look for a future blog post on this topic. The ideal situation is to find the perfect balance between efficiency and indoor air quality. The goal is to have an air tight well insulated home with planned mechanical ventilation.
Now you know what a blower door is and how important this test can be… good luck and hold on to your hat!
GreenEdge of Michigan offers a Blower Door Test with a few of our services. These services include: Home Energy Audit, HERS Index Rating, ENERGY STAR Certified Home Rating and as an A la Carte Service.