Energy Audit-Homemade Blower Door

A blower door is an expensive tool.  A full kit including frame, panel, fan and manometer will cost more than $3500.  Add in the other tools for finding air leaks and you could easily drop over $5000.  And then you still need to have some training to understand how to operate and interpret the results.  Most of the time, it’s more cost effective to hire the service out.  I charge between $200 and $500 for the test depending on the time that needs to be spent finding air leaks.  When I conduct an energy audit, part of the report usually will contain thermal images identifying the locations of the air leaks.  I say usually because there has to be a temperature difference for thermal imaging to work.  Once the air leaks are identified, air sealing is something a handy homeowner can accomplish on their own.

Thermal image of a large air leak during a blower door test.

Let’s say, as a homeowner you’re intending on doing the work to air seal as much of your home as you can, but want to know if you’re doing a good job.  You can hire me to come back, or you could build a blower door to perform blower door assisted air sealing yourself.  This homemade tool couldn’t tell you what pressure you’re creating on the home, it won’t give you the information to calculated an air exchange rate.  But it may help identify the locations of and to confirm if air leaks have been sealed.

To make sure a homemade blower door will be effective on a home, especially a home with lots of air leaks, I built and used one on my home.  Before I get into how to build one, lets go over what I did to confirm I could create enough pressure with a homemade unit to identify air leaks.  To do this, I first ran a real blower door test using a multipoint testing standard with my Retrotec 5100 and DM32 manometer.  This test uses several different test pressures to accurately calculate CFM rates.  The screen shot below shows the test results for my home.

Retrotec’s 5100 blower door kit with the DM32.

What the information is telling me, my home leaks 2154 cubic feet per minute of air at the test pressure of 50 Pascals, this equates to an air change rate of over 8 or 8ACH50.  The multipoint test also gives me the leakage rate at other pressures, for instance, at a pressure of 10 Pascals, my home was leaking 815 cubic feet per minute.  This information can be used to estimate the house pressures at different CFM rates.  Depending on the output of the fan or fans used in a homemade blower door, an estimate can be made as to the pressure of the home.  Now that I have this baseline information, we can build a blower door.

The design is very simple, a piece of rigid foam, some painters tape and a fan capable of moving a fairly large amount of air.  The fan I used is one I own, a unit designed for drying floors.

This fan does not list how much air it’s capable of moving, but in the model number, the number 400 is included so I’m guessing it’s somewhere around 400CFM.  If that’s correct, the home should be around 5 Pascals of pressure.  A little light, but I should be able to detect larger air leaks.

I need to create an opening in the home to place the fan.  This opening could be a window or door.  Because my fan is designed to sit on a floor and is fairly heavy, I choose to use a door.  I used a $10 sheet of 1/2 XPS foam cut the size of the door.  I cut a hole the size of the fan at the bottom of the foam and then taped the foam into the door opening using blue painters tape.  Because I own several manometers used to test pressures, I ran a reference tube through a small hole in the foam panel so I could determine what the house pressure was while the fan was operating.

With the panel in place and sealed with the blue painters tape, I started the fan and watched the manometer to see what pressure I was able to achieve.  The fan’s rating must be around 400 CFM because I was achieving around 5 Pascals, about 10% of the test pressure of the blower door test.  Not great but still a negative pressure.  This was with all the interior doors open in the home.  To see if the pressures could be increased, I closed all the interior doors.  This would isolated some of the interior spaces from the main area of the home where the homemade blower door was operating.  The pressure increased to almost 7 Pascals.  To further increase the pressure, I started every exhausting device in the home, which included the dryer and bath fan.  At that point I was able to achieve a -10 Pascals, or around 800 CFM of air being moved out of the home.

We have a negative pressure inside the home, is it enough to detect?  There are a couple very accessible areas in my home that have air leaks.  The first is an old laundry chute that goes from the upper level to the basement.  The chase, which was built during the construction of the home in 1952 has some connection to the attic.

The other area is a small hole in the ceiling where an interior renovation is partially complete.

Air could easily be detect moving through both these areas by both feel and by using smoke.  Had there been a temperature differential between inside and out, I would have used a thermal imaging camera to visualize the leaks.

I would say the homemade blower door worked on my fairly leaky home, with some help.  I have several recommendations if you want to try building your own blower door:

  1. Before you get started, hire a professional to have a blower door test completed on your home.  They will find many of the air leaks and provide additional information.  Ask for a multi-point test with the graph showing the CFM rates at the different pressures during the test.  You can use this information to roughly calculated the pressure created by your fan and exhausting equipment.
  2. You’re going to need a fan capable of moving a fairly large amount of air.  The more air leaks a home has, the bigger the fan.  Another option is to set up several fans.  Try using fans with listed CFM rates.  You should have a goal of around 10 Pascals, more would be better.  In my home, that required 800 CFM of exhausting air.
  3. Isolate as much of the home as possible by closing doors while operating the homemade blower door.  Only open these doors to the space containing the blower door when looking for and air sealing the air leaks.  Pro tip: if you slightly open a door and put your nose into the opening while a blower door is running, you can often feel wind moving through the cracked door.  The more wind, the more air is leaking into the room, the more air sealing is needed.
  4. Use other exhausting fans to help create the negative pressures.  Bath fans can move 50-150 CFM, dryers, 100-250 CFM, and kitchen exhaust fans, 100-1000 CFM.
  5. Smoke generators can help identify air leaks, incense is a cheap way to create smoke, just be careful not to start a fire.
  6. Thermal imaging is even better.  For a couple hundred dollars, you can purchase a Flir thermal imaging camera that attaches to your smart phone.  A temperature differential between inside and out will be needed to “see” the air leaks.

When producing a pressure on a home, there are a couple safety concerns to be aware of.  First, when I conduct blower door tests producing either positive or negative tests, I turn off any combustion appliances, including space and water heating equipment.  If enough negative pressure is produced inside a home, even sealed combustion appliances can backdraft bringing carbon monoxide into the home.  Second, don’t induce an intentional negative pressure with a fire burning in a fireplace or woodstove.  I could not perform a blower door test with a fire burning, I don’t recommend you try either, even if it may not have a harmful outcome, it is a risk.  Make sure all ash is cool and cover or better yet, remove any that’s present in a fireplace or woodstove.   Lastly, make sure there is a working carbon monoxide detector in the home with any combustion appliance. This should go without saying.

One additional concern, I don’t recommend testing a home, either as a professional or using a homemade blower door with vermiculite insulation present. Vermiculite can contain asbestos, it’s best not to disturb this insulation.

Next up, we need to have a discussion on what can happen when a home is tightened. Less natural air leaks, which is good for increasing comfort and reducing energy costs, can lead to not so good indoor air quality. It can also increase humidity levels inside the home during the winter months. A second blower door test after air sealing is complete will show if air sealing has been successful. Monitoring indoor humidity levels is something I suggest regardless of air sealing measures. You don’t know unless you monitor, so purchase a $15 monitor. If air sealing is successful, there’s the possibility of needing some sort of mechanical ventilation, hopefully balanced. This is a goal for my home, tight enough that I need to install an air exchanger where I control the ventilation rate, the incoming air is filtered and is tempered by transferring some of the heat of the outgoing air with the incoming.

Building your own blower door isn’t for everyone and is really only useful for blower door directed air sealing. But if you’re handy, this homemade tool can be effective.

Leave a Reply

Your email address will not be published. Required fields are marked *