Building Science: Thermal Testing

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Thermographic Scanning

Thermography or infrared scanning uses specially designed infrared video or still cameras to produce images called thermograms, which show surface temperature variations. Thermographic equipment can be used as a tool to help detect heat losses and air leakage in building envelopes. Infrared scanning allows energy auditors to check the effectiveness of insulation in a building's construction. The resulting thermograms help auditors determine whether a building needs insulation, and where in the building it should go. Because wet insulation conducts heat faster than dry insulation, thermographic scans of roofs can often detect roof leaks.

Several types of infrared sensing devices can be used on an on-site inspection. A spot radiometer (also called a point radiometer) measures radiation one spot at a time, with a simple meter reading showing the temperature of a given spot. The auditor pans the area with the device and notes the differences in temperature. A thermal line scanner shows radiant temperature viewed along a line. A thermogram shows the line scan superimposed over a picture of the panned area. This process shows temperature variations along the line.

Thermographic scans can be done inside or outside a structure. Exterior scans, while more convenient for the home owner, have a number of drawbacks. Warm air escaping from a building does not always move through the walls in a straight line. Heat loss detected in one area of an outside wall might originate at some other hard-to-find location inside the wall.

Air movement also affects the thermal image. On windy days, it's harder to detect temperature differences on the outside surface of the building. The reduced air movement and ease of locating air leaks often make interior thermographic scans more effective.

The most accurate thermographic images usually occur when there is a large temperature difference [at least 20°F (14°C)] between inside and outside air temperatures. In northern states, thermographic scans are generally done in the win ter. In southern states, however, scans are usually conducted during warm weather with the air conditioner running.

Blower Door Testing

Infiltration is airflow through unintentional openings in the building envelope that is caused by wind and stack pressure differences between indoor and outdoor air. Energy is required to heat or cool unconditioned air that has leaked into the structure.

The rate of infiltration in a building depends on weather conditions, equipment operation, and occupant activities. The characteristics of infiltration airflow may be determined by measuring the air leakage of the building envelope, which de scribes the relative tightness.

A blower door is a large assembly with a powerful fan that is placed in an exterior door of an existing building to measure infiltration and determine the location of air leaks in the building envelope. It creates a strong draft that depressurizes the building interior by drawing indoor air out of the building. Energy contractors use a blower door to determine how much air leaks through the building envelope. In a residence, it takes about 15 min to set up the blower door, and about 5 min to run a simple blower door test.

The typical test pressure is about 0.20 in of water (50 Pa), about equivalent to a 20 mile/hr (32 km/hr) wind. Internal and external temperatures and the external barometric pressure are measured during the test to make corrections for changes in the air volume flow rate. All exterior doors and windows must be closed and ventilation openings sealed during the test to pro vide a realistic measurement of the actual building envelope leakage. Once the building is depressurized, the contractor can identify leakage through the external envelope. Identification of air leakage paths is pinpointed by feeling with the hand or with a hand-held smoke generator.

In blower door testing, airflow measured in ft^3/min (or cfm) that creates a change in building pressure of 50 Pa (CFM50) is a commonly used estimate of building airtightness.

This measure will vary by building size. A range for a small residence is 1200 CFM50 (tight) to 3000 CFM50 (very leaky).

Another commonly used measure is air changes per hour at 50 Pa (ACH50), which is the number of complete air changes that will occur in one hour with a 50 Pa pressure being applied uniformly across the building envelope. Typical ACH50 rates are from 5 to 10 air changes per hour. The ACH50 rate method is useful in comparing leakage rate by size (volume) of building.

The natural infiltration rate can be approximated by dividing the measured ACH50 by 20; for example, for a 10 ACH50: 10 ACH50 20 _ 0.5 ACH natural infiltration rate. In another method, infiltration is expressed as the effective leakage area (ELA), the area of a special nozzle-shaped hole (in a blower door) that would leak the same amount of air as the building does at a pressure of 10 Pa. The LEED Green Building Rating System has set an airtightness standard for multifamily dwelling units of 1.25 square inches of leakage area per 100 ft^2 (0.868 cm^2/m^2) of enclosure area.

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