Blower Door - 11/22/2004 - Home Condo Doors Windows

Blower Door

Tool measures how airtight a home is.

What do carpets, showers, and cooking have in common? They all can be sources of indoor air pollution. As homes continue to be constructed more tightly and fresh air cannot enter freely through common pathways, new homes may need mechanical ventilation to bring in fresh air to maintain optimum indoor air quality. On the other hand, a very leaky home may feel drafty, be expensive to heat and cool, and suffer from durability problems.

Blower doors are designed to measure how airtight (or leaky) a home is. They are a tool that can be used to determine construction quality and compliance with design or program standards (such as utility home efficiency programs) and to diagnose comfort, indoor air quality, and durability problems.

Blower doors are used to determine how much fresh air can get into a closed building. A leaky house will lose conditioned air (and, hence, energy) to the outdoors or draw outside air indoors. A tight house without ventilation can create health and safety concerns. For example, high indoor moisture levels can cause mold growth, and the potential spillage of combustion gases into a home, from naturally ventilated gas appliances, can create indoor air quality problems. Leaky houses can be drafty, expensive to operate, and have durability problems such as blistering paint or ice dams.

Air leaks are usually found around windows, doors, utility penetrations, and foundation-to-wall connections and through attics, crawlspaces and attached garages. Diagnostics can determine where leaks occur and where air sealing efforts should be concentrated.

Blower doors measure the amount of air that needs to flow into (or out of) a house to maintain a given pressure difference between indoors and outside. By using a calibrated fan and metering equipment, airflow can be measured at a variety of pressure differences up to 50 pascals (Pa). Readings are generally given at 50 Pa.

Blower door testing results are expressed in three ways: air changes per hour under natural conditions or at test pressure (ACHnat and ACH50); airflow at test pressure (CFM50); or equivalent leakage area (usually sq. in.). Tighter homes have lower test scores than leaky homes.

Blower door tests do not measure natural air infiltration rates. They measure air infiltration at various pressure differences between indoor and outdoors. Codes and standards, however, address natural infiltration, not pressure-induced infiltration. There is controversy over the accuracy of the equations used to convert ACH50 to ACHnat. In addition, other mathematical formulas are used to derive size of leakage area based on leakage rates. There are different formulas for this extraction that result in different answers. The bottom line is that building science is an evolving science and there is still a great deal of uncertainty about measurement and acceptable levels of infiltration.

	Contractors are available nationwide that use blower doors and other equipment to measure air leakage and diagnose air quality, comfort and structural problems.
	The blower door test determines how airtight a home is. A leaky house will lose conditioned air (and, hence energy) to the outdoors or draw outside air indoors. A tight house without ventilation can create health and safety concerns.
	Blower doors can determine construction quality and compliance with design or program standards.

Cost to get blower door testing may range from $100 - $200, which will vary by region and contractor. Check with your local utility to see if they offer free or discounted testing.

The cost to purchase new blower door equipment can range from $1,500 to $2,500 depending on the manufacturer and accessories.

There is no ongoing cost associated with blower door testing. If a potential energy problem is diagnosed, steps can be taken to reduce energy bills.

1. ASHRAE Standard 62.2-2004 - Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings (ANSI Approved)

2. ASTM E1827-96 (2002) Standard Test Methods for Determining Airtightness of Buildings Using an Orifice Blower Door

Not Applicable

To perform a blower door test, all windows and exterior doors are sealed except for one door that is fitted with the blower door system. To install a blower door, an adjustable frame is fitted to the doorway and an airtight tarp is fitted around the adjustable frame. The tarp contains a large hole near the bottom into which a calibrated, variable-speed fan is tightly fitted. A long flexible plastic tube fits tightly through an opening in the tarp and is used to measure the pressure difference between the house and outdoors.

Blower door testing is conducted after a house can be pressurized, usually after drywall is installed. Testing can be used to determine the extent of air leakage and, in conjunction with other diagnostic tools, the location of leaks.

To get blower door measurements, an energy professional starts the adjustable fan to pressurize or depressurize the house to incremental test pressures up to 50 Pa. If, for example, the house is depressurized, the fan flow represents the amount of air that is coming into the house through air leakage pathways. At this point, it is easy to walk around the house and feel where air is flowing into the building, especially on a cold day. Alternately, a smoke stick can be used to demonstrate where leakage is occurring. The airflow through the fan in cubic feet per minute (CFM) is recorded from a calibrated meter that measures pressure and flow. Given the airflow through the fan at 50 Pa and the volume of the house, the air changes per hour at 50 Pa (ACH50) can be determined.

One manufacturer has a two year warranty on the blower door testing equipment. Gauges and fans need to be recalibrated every few years based on manufactuers' recommendation.

Blower door testing can identify the need for mechanical ventilation and can be used to diagnose locations of air leaks. A builder can use diagnostics to determine common air infiltration locations in house designs, to troubleshoot customer callbacks from comfort or indoor air quality issues.

Cost to get blower door testing will vary by region and contractor. Check with your local utility to see if they offer free or discounted testing.