All About Infiltration Part 2: Blower Door Testing

Blower Door Testing to Measure Air Leaks

Every home has air leaks, but the cumulative amount of leaks can vary widely based on the air sealing efforts. Infiltration and air sealing basics are covered in part 1 of this post.

To measure the amount of leakage in a home we use a tool called a blower door, which is comprised of a calibrated fan, a mounting system to attach the fan to an exterior door, and a manometer which measures pressure.

To understand the principle behind the blower door test imagine a large parade balloon like Kermit here. If the balloon is completely air tight we can pressurize it, shut off the valve, and the balloon will remain inflated indefinitely.

Now imagine the balloon has some small leaks at the seams. To keep it inflated we need to continuously blow in air to replace the air leaking through the seams. The larger the leaks are, the more air is required. Thus, if we can measure the amount of air we are blowing into the balloon to keep it fully inflated, we can infer how leaky the balloon is.

That’s exactly what a blower door test does: it measures the amount of air needed to keep a house at an elevated pressure of 50 Pascal (i.e. “inflated”), and we use that measurement to infer how many leaks are present.

Blower Door Test Metrics

The blower door results can be expressed in a few different metrics. The most common one is air changes per hour (ACH), or how many times a house’s air completely replaced in a given hour. Since we take our blower door measurement at 50 Pascal most codes and standards reference the air changes at that elevated pressure (ACH50), but we can also calculate the air changes under natural conditions (ACHn).

For example, a code-built new home with decent air sealing might have 7 air changes per hour at 50 Pascal (ACH50), meaning if we kept the blower door running for an hour it would pump in enough air to completely replace the home’s air 7 times. This would translate to about 0.35 natural air changes per hour (ACHn), or about one complete air replacement every 3 hours.

What’s A Good Blower Door Test Number?

The metrics and math can get a little technical so let’s put them in context. Here’s a rough scale to compare your blower door test number to other standards:

10-20 ACH50 – Older homes, like living in a “barn”

7-10 ACH50 – Average new home with some air sealing but no verification and little attention to detail

7 ACH50 – OK infiltration level and the 2009 IECC energy code requirement

3-5 ACH50 – Good and achievable target for most new homes. The ENERGY STAR reference home is 5 ACH50 for climate zone 4 which covers DC, MD, VA and part of PA. The majority of PA is 4 ACH50 for the ENERGY STAR reference home.

3 ACH50 and lower – Tight home with great air sealing, and required by the 2012 energy code adopted in MD and coming to other jurisdictions soon.

.6 ACH50 – Super tight home and the Passive House standard.

Using a Blower Door Test to Reveal Defects

In addition to quantifying air sealing effectiveness, a blower door test can also help find defects, especially in conjunction with an infrared camera. The blower door will exacerbate the natural infiltration occurring in a house making air leaks easier to find because the air outside forcing its way in shows up as a different color on the IR camera. For example the image below shows a bathroom soffit built below an attic without a proper air barrier.

The photos below were taken in the summer during an existing home energy audit. The infrared photo on the right shows warmer colors in yellow and is the hot summer air coming in through the can lights and walls next to the soffit.

The problem is the air barrier doesn’t align leaving pathway for air to infiltrate. Everyday Green reviews plans for inclusion of proper air barriers and then we inspect them onsite before drywall is installed to prevent bypasses like the ones in the IR image above.

Stop Those Air Leaks – All About Infiltration

What is Infiltration?

Infiltration is the uncontrolled or accidental introduction of air, often called air leakage.

A lot of people assume air leaks happen predominately around windows and doors. In actuality air is driven through our homes and buildings by the stack effect – warm air rising. This means the attic or the roof, and the basement, are most critical for preventing air leaks and infiltration. Infiltration is a bad thing: not only is it a huge energy waste, it brings in air from the dirtiest places like attics and crawlspaces, and spreads that contaminated air through the living space.

The key to stopping infiltration is creating a good air barrier.

Think of a building’s insulation like a wool sweater. On a calm fall day the sweater is enough to keep you warm. If a breeze picks up, though, the cold wind will blow right through the wool and you will probably reach for your windbreaker. In a home we call the windbreaker layer the air barrier, and it is just as important as the insulation. Insulation limits heat transfer through the walls and roof, but only when paired with an effective air barrier.

Stop Infiltration – Air Barrier Rules

  1. air sealing detailsThe air barrier needs to be totally continuous. If you take a cross-section plan of the building, you should be able to draw the air barrier all the way around without lifting your pen.
  2. The air barriers, such as drywall, should be in direct contact with the insulation. This often breaks down in locations like walls under staircases, behind fireplaces, and under tubs where there is (hopefully) insulation but no drywall air barrier.

Where Does Most Infiltration Occur?

There are three critical types of air leaks to watch out for:

  1. Big holes.  Some common design elements can result in big holes in the air barrier. For instance, a dropped soffit is a great pathway for air leakage. Tubs and fireplaces on exterior walls can create similar holes if a solid piece of rigid insulation isn’t installed behind them. Floor joists that extend from conditioned space to a garage or balcony are another way to blow open the air barrier. While these locations can be air-sealed and insulated, good design would eliminate the potential for big holes altogether.
  2. Cracks.  Every building has a number of cracks that seem minor when taken on their own, but add up to a big air leak. These cracks occur between the sill plate and foundation, at exterior wall bottom plates, between adjacent studs, and around window and door frames.
  3. infiltration at can lightPenetrations.  Every hole cut in the exterior envelope (ceiling drywall, exterior sheathing, top plates below attic) creates a potential air leak. Penetrations include plumbing pipes, duct registers, can lights, exhaust fans and exhaust ducts, and electrical wiring.

Air is relentless: it will find any and every pathway into a building. Sealing 50% of the apparent leaks will not cut 50% of the infiltration because air will find another way in. Good air sealing aims to seal 90% of the leaks. It requires patience, attention to detail and the expertise to recognize tricky air bypasses. It also requires a clear understanding of the thermal envelope, especially at complicated architectural details.

Tips for Successful Air Sealing:

  • Good air sealing requires a plan, and should be a priority during the design phase. Ask yourself where is the air barrier? Can you draw it without lifting your pen? Check out our tips for multifamily compartmentalization.
  • During construction, air sealing should be the responsibility of all the trades. Air is persistent, and the whole project team needs to be just as thorough in fighting it.
  • A good rule for a job site is if you cut a hole, you seal it. It is easier for each trade to seal their own holes, rather than relying on one person to find everyone else’s holes.
  • Fire-stopping is not necessarily air sealing. Fire-stopping material like rock wool does virtually nothing to stop air infiltration. Use caulk or foam to air seal.

In our follow-up post we cover how air leakage is measured with a blower door test and what a good target is.