What is the blower door test?
The blower door test measures air leakage in a house.
With the fan on and the pressure gauge steady, the volume of air blowing out equals the amount leaking in.
The blower door apparatus is composed of a wooden frame covered with an airtight nylon fabric, which is fit snugly into the frame of the open back door. Then the technician attaches a large cylinder, which enclosed a fan, into an opening in the fabric.
What does the thermogram do during a blower door test?
A thermogram detects the temperature of the inside wall. Cool surfaces appear dark, and warm surfaces are light.
On a cool day, wood framing is cooler than insulation. Windows are cold and so appear dark. Notice the cool area around the light fixture where air leaks in. Cool attic air also can leak down a stud cavity to the electric outlet, probably due to a hole drilled for the wire.
How does a blower door test work?
The fan takes air from the house and blows it out the door, reducing the air pressure inside. One the technician switches on the fan, immediately the nylon fabric blocking the doorway bellied inward, as air tries to get back inside.
Of course, with the pressure reduced inside, outside air will leak in through whatever cracks it can find — around windows, through cracks in the sheathing, under weather stripping, and so on.
That’s why you close all the doors and windows. Also turn off the furnace and water heater and seal off their exhaust flues, so there is no air being drawn back down the chimney (called back-drafting).
Even with the house closed up tightly, the fan continues to blow air out. Obviously, air has to be leaking in from somewhere — probably from many small gaps. The goal isn’t to close up every leak, just to bring the leakage down to 15 hundredths (.15) of an air change per hour (acph).
To get an idea of what one air change is, imagine all the air in your home suddenly pumped away and then replaced by fresh outside air. In the average American home, that much air leaks in and out every hour. It’s about the same as having a large window wide open all the time!
If you go below .35 acph, your home can get too tight and the inside air can become stale and loaded with pollutants. Though energy efficient homes can and do go much lower, they all must run a ventilating fan continuously to keep the air inside fresh, just as commercial buildings do.
The blower door test equipment has two types of gauges. One type measures the air pressure in the house, and the other tells how much air the fan draws out – in other words, how fast air is leaking back in. By changing the fan speed, the technician increases or decreases the pressure and compares the leakage rate at each pressure. Then a special computer software tells how tight the house is.
How does the thermogram work?
The thermogram is a heavy black instrument that looks like a fat, old-time home movie camera. It allows the technician see into your home’s walls and ceilings. Only that instead of seeing see light, it sees heat.
Every object radiates heat waves, a different wavelength for each temperature. The camera senses those wavelengths and outlines the objects on a small screen according to their temperature, sort of a video thermometer.
If you put your hand on a wall for five seconds and heat the wallboard a few degrees, it will show through the camera from the opposite side of the room – a bright “heat print” shaped like a hand on the cooler wall. Like magic. You can see the wall studs clearly behind the wallboard. too. The camera renders the cooler areas dark and the warmer areas light. Wood doesn’t insulate as well as fiberglass or other insulation, so the studs draw heat from the room faster, cooling the drywall where it touches them.
For this camera to work well, the temperature outside must be at least 10 degrees cooler or warmer than inside. Otherwise, the temperature differences are so small that you can’t find the problem spots.
The picture above was taken on a cool day, so as cool air leaks in, it cools nearby surfaces and shows up as dark areas on the camera screen. Air leakage is easy to spot.
About home air tightness
When the technician snapped open a suitcase he had set on the kitchen floor near the back door, you had to be impressed. In it were four large, round, black and white gauges that looked like clocks with red hands. It also contained a small computer, a variety of switches, plus coils of clear tubing. From his hardware you might suspect that he was a government agent sent to electronically eavesdrop on some household secrets. While he isn’t a government agent, his blower door test instruments are calibrated to reveal a mystery about this house — how much air leaks in and out through its walls and ceilings.
Frankly. few people cared how tightly their house was built in the 1950s or 1960s, unless winter drafts froze their socks. You could always crank up the heat a little higher. But fuel bills have gone up, and homeowners don’t want to waste a penny of their budget. New homes are the result of several years of dreaming and careful planning. People might skip the hardwood floors and several other features at a first stage, opting to put their bucks into a special energy-efficiency package offered by a savvy building contractor. These packages guarantee good insulation and a minimum leakage rate (air-tightness). The reasoning is simple: save on fuel bills and buy other features later with the money saved.
The deal also includes a special ventilation system, to assure a good supply of fresh air. More on this later. Though the energy package promised to be a good deal, new homeowners face a perplexing problem. How can they know if it worked? They could measure the insulation thickness, but there’s no easy way to gauge air leakage. And that’s not a small item: in the average new home. leakage accounts for a whopping 25-50 percent of the fuel bill.
That’s where a professional thermographer comes in. His job is to map heat movement in homes. The instruments pulled from that suitcase are for a blower door test. which measures the air leakage rate of the house and will determine if the contractor has met the terms promised in the contract.
These professionals are usually passionate about energy conservation and enjoy working with clients interested in energy-efficient homes. Heating bills over the most frigid of winters can be greatly reduced, and I’m not talking “superinsulation” or unusual design either. Savvy contractors build homes to meet the local energy code, and also make a special effort to make them more airtight.
It is common for a house to fail its first test. Even homes done by the best contractors often fail at first, because the airtightness standards are tough to meet. In fact, contractors depend on pros to pin-point the worst leaks so they can plug them.
Usually people don’t know or care much about what goes on behind the walls of their homes before deciding to buy one of these energy-efficient home packages. The payoff they want was those low heating bills in the middle of the cold winters.
The thermogram easily shows gaps in the insulation (which also would show up as dark areas), a good sign that the insulating crew worked carefully.
It also allows to find several major leaks. One type of common air leak involves recessed lights in the ceiling below the attic. That’s called an “attic by-pass” (an air gap that leads through the ceiling into the attic). All you need is to have electricians cutting through the ceiling to install lights, insulators that failed to seal the fixtures, and a distracted contractor who didn’t check these compounded mistakes.
(By the way, sealing recessed light fixtures is both a common problem and a tricky job. Insulating and sealing the wrong type can cause the fixture to overheat and start a fire. Always ask an electrical inspector or an electrician to check your work to make sure you followed a safe insulating procedure.)
Another problematic area is the exterior framing, which allows air to leak around the lights into the ceiling area.
Keep in mind that even with ceiling leaks, a home can pass the airtightness test quite easily.
Energy-efficient homes are already a present necessity. Finding a trained and certified thermographer can be easy – use the internet or ask your local utility, insulation contractors, or your state and federal energy agencies for information.
The training element is vital; it’s much more than reading a few dials and plugging numbers into a computer. Home safety is the key ingredient. The first thing a good technician does when entering a home is to set up a carbon monoxide (CO) monitor near the furnace and water heater. Poorly adjusted or worn-out combustion appliances can leak C0, a deadly gas, into the living space Serious professionals check for this danger right away.
And before leaving a job, responsible technicians check both the furnace and water heater for backdrafting.
What is backdrafting?
Backdrafting is caused when a home doesn’t have enough incoming air to allow exhaust fumes to rise naturally up and out the chimney. Rather, your house can draw air down the chimney instead, causing the deadly fumes to spill inside. That is a big concern. While there are no official statistics about this issue, it is estimated that backdrafting can be found in 1 house out of every 5.
Backdrafting can be a problem in tight homes because they often have unbalanced ventilation. Kitchen and bathroom fans can blow air out faster than replacement air leaks in. That can cause the natural upward draft in the chimney to reverse.
A balanced ventilation system happens when your home has a well dimensioned, continuously running blower. Not only does it expel stale inside air, but it also draws in exactly the same amount of air, so the pressure inside remains the same. In other words, can’t cause backdrafting.
Modern ventilation systems have an additional energy-saving feature – an air-to-air heat exchanger, which uses the warm outgoing air to heat the cooler incoming air. It wastes less heat while ensuring good ventilation.