Energy Audit-Advanced Blower Door Testing-Zonal Pressures

Most of us by now know what a blower door is.  A tool to measure the air tightness of a house.  I’ve owned my Minneapolis Blower Door since 2009.  Many states have adopted mandatory tightness testing on all new construction.  In my area, the minimum is three air changes per hour at 50 Pascals (3 ACH50), most new homes I test are around 2 ACH50.  Test results for existing homes vary widely.  Some would pass today’s requirements while others I have tested were as high as 20 ACH50.  Finding air leaks in these older homes can take some work.  Combine a blower door with a thermal imaging camera along with a few degrees’ temperature difference between inside and outside and most air leak becomes visible.  “Seeing” air leaks isn’t always possible and knowing where to start looking is helpful.  Another test I often conduct using a blower door is called zonal pressure testing.

A visible air leak using a blower door and thermal imaging camera.

Zonal testing is the process of testing an area in the home to see if the space has a connection to the outside.  Instead of looking for an individual air leak, the test checks a space or room for all air leakage in this area.  This speeds up the process of finding air leaks by pinpointing the rooms or areas of a house that have the biggest issues.  To conduct a zonal pressure test, the space being tested must be able to be isolated from the rest of the home, such as by closing a door.  Other areas that can be tested are attics or crawl spaces that are supposed to be outside the building envelope.

I recently conducted a blower door test on my own home (not the first one).   I knew there was substantial air leakage in the upper two bedrooms and bathroom in my 1952 Cape.  My plan is to gut each room to improve the air sealing and insulation levels.  By conducting the zonal pressure test, I found one of the bedrooms was much worse than the other rooms.  That bedroom will be the first to be remodeled.  If I had found little or no air leakage in any of the upper level rooms, I might have decided not to renovate the space.

My Minneapolis Blower Door being controlled by a laptop computer and Tectite software.

Zonal pressure testing does not give us an actual air leakage rate, the information is instead a relationship between the area where the blower door is operating, and the space being tested.  I’ll get into this relationship a little later, first let’s go over how I perform the test.

The house is set up for the typical blower door test, all interior doors open, windows closed, HVAC and other exhausting equipment off.  Another important check is to make sure any wood burning appliances are completely out and the ash is removed or covered.  Ask me how I know this.  The test is conducted by setting the blower door to operate at a steady state, typically -50 Pascals, often called cruise control.  I use a DG700, laptop and Tectite software to control my Minneapolis Blower Door.  Once the pressure is set, I use a second monometer, a DG500 to measure zonal pressures.  The DG500 has four ports to attach hoses.  I attach one hose to the “B” input channel.  The “A” channel is used as the reference for the room where the monometer is located, typically outside the area being tested.  The hose connected to the DG500 is then placed in the room being tested.  We are comparing the pressure of channel “A”, the area with the blower door in operation, with the pressure of channel “B”, the room we are testing, which has been isolated from the blower door.  (Illustration 1)

The blower door is operating at a steady -50 Pascals.  This pressure difference is between inside and outside the home.  Because I am inside the home, I expect every room located inside the air control layer to be at -50 Pascals with relationship to the outside.  Understanding that this test is conducted measuring pressures between spaces inside the building’s air control layer, the pressure difference between these inside spaces should be equal.  The readings on both channels of my DG500 should be 0.  If I test a room by placing the hose inside and closing the door, I expect a reading of 0 on both channels of the monometer, meaning I have no connection with the outside.  Most of the time, I have some difference.  This difference can range from .1 to 50.  .1 means the room is at nearly the same pressure as the space outside the room, or very well air sealed.  A reading of 50 means the room is completely outside the air control layer, lots of air leaks to identify (or an open window).  A reading of 25, the room is half connected to the home and half connected to the outside, or a 50/50 split.  With the blower door operating at 50 Pascals, this relationship between spaces inside the home can easily be converted to a percentage.  A 50 Pascal reading on my DG500 would make the room 100% outside the air control layer, 25 Pascals is 50% outside the air control layer, 5 Pascals is 10% outside the air control layer, a 2 to 1 ratio.  The photo shows the results from an actual test.  This is the bedroom in my home I mentioned earlier.  There are attic knee walls where there is no definite air control layer present, and it shows.  The DG500 is reading 27.4 Pascals.  This room was 55% connected to the outside.

The DG500 pressure gauge measuring the zonal pressure difference in an upper level bedroom of a story and a half house.

Another way I can conduct this test is to compare spaces that should be completely outside the air control layer with the inside.  Spaces like unconditioned attics, unconditioned crawl spaces or attached garages should all be 100% outside the building envelope.  Trying to get a floppy hose into these spaces doesn’t work well.  In my blower door kit, I have a 12 Inch long piece of 1/8th inch metal tubing that I use to test spaces that are hard to access, pretty much any space that doesn’t have a door to close.  This metal tube can be attached to the plastic hose.  A small hole can be drilled, and the rigid metal tube inserted into the hole to measure pressure differences.  We are now measuring pressures between inside and outside the air control layer, because the blower door is operating at 50 Pascals, I expect a difference of 50 to be measured on my DG500.  If I get a reading less than 50, I know I have some connection with the inside of the home.

My DG500 pressure gauge. Notice the metal tube attached to the clear plastic tube.

When conducting zonal pressure testing, I almost always read some difference between the space being tested and the area outside that space.  I look for readings on my DG500 of at least 5 (or 45 when testing spaces outside the air control layer) before I investigate the room further.  This means I have up to 10% connection with the outside with the house at a pressure of 50 Pascals, a pressure much higher than the house will see under normal conditions.  The natural air leakage will be something less.

Zonal pressure testing an area outside the building envelope. Test shows the attic space has no connection to the inside of the home, the attic is well air sealed with the air control layer at the ceiling.

There is a low-tech zonal pressure test that can also be conducted if you do not have a second monometer.  Partially close the door to the room you wish to test, leaving it open about an inch.  Place your face close to the door and feel for a breeze.  A room with substantial air leakage will almost blow your hair back.  If you don’t feel any breeze, the room has little or no air leakage.  I often use this method to show a homeowner how much leakage a room has.

Zonal pressure testing is one of the tools I often use when trying to identify air leakage.  Knowing where I should start looking makes me more efficient.  The key is to understand the relationships of the rooms and spaces when the blower door is operating.  Testing zonal pressures in areas inside the building’s envelope should always be close to 0, testing pressures in areas that should be outside the air control layer should be close to 50.

Leave a Reply

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