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Field Test Best Practices: A Resource for Practical Residential Building Science

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Weather

Weather plays a large role in how a house (and its components) performs and how much energy it consumes. Outdoor temperature and humidity affect the cooling or heating needs of the house. Solar radiation also can affect HVAC loads, both negatively and positively.  If photovoltaic panels or solar thermal system is installed, the local solar radiation needs to be measured to determine their expected performance. Wind can play a large role on the infiltration in the house and would affect the performance a small wind turbine, if present. In short, weather affects all systems in the house and without knowing the outdoor weather conditions, it would be nearly impossible to determine if the systems in the house are behaving as expected. Therefore, having a small, on-site weather station is recommended for all field tests. If there are a number of homes in the same neighborhood or development involved in the same project, a single weather station may be sufficient.

Weather measurements for residential building science include:

Temperature and Relative Humidity Sensor

A temperature and relative humidity (T&RH) sensor is a simple, durable and low-cost type of electronic hygrometer used in building science. Commonly available models use thin-film polymer technology. The dielectric properties of the polymer film changes with the change with ambient air relative humidity, and the capacitance of the sensor changes correspondingly.

Wind Speed and Direction

Wind primarily affects the infiltration in a house, so measuring wind speed and direction in a field test is particularly important if natural infiltration is being measured. Tracer gas tests, for instance, are very sensitive to outdoor conditions and wind speed should always be measured when tracer gas tests are performed. Wind speed is measured using an anemometer, pictured below. Wind direction is typically measured separately by a wind vane or weather vane. Wind direction is not always measured in past field tests, even if wind speed is measured.

Solar Irradiance Measurements

Solar radiation affects many systems in the house and can vary considerably within the same town. On-site, solar irradiance is a particularly useful measurement if there is a photovoltaic (PV) system or solar thermal system installed at the field test location. Also, if space conditioning is a focus of the field test, solar irradiance is an important measurement as solar radiation has a large effect on heating and cooling load requirements, electrical lighting demand (daylighting), and envelope performance.

Precipitation

Precipitation can affect the performance of a house and some specific systems. For example, if a large amount of rain falls, the soil conductivity will change, which will affect the performance of a ground source heat pump. Rain wets the house envelope assembly, and the assembly needs to be properly designed to withstand frequent wetting and drying cycles. A layer of snow can insulate the roof and increase the reflectance of the ground. While precipitation can affect a number of systems in or around the outside of a home, it is not a commonly monitored quantity for field tests. However, if the effect of precipitation on specific system (for example: rain water harvesting systems) was a focus of a particular field test, it would be appropriate to install precipitation sensors.

Barometric Pressure

Barometric Pressure is used in detailed psychrometric calculations, and in calculation of the density of air, which can be important in accurate determination of heat transport via airflow. Barometric pressure is by definition an absolute pressure measurement.

Collecting Weather Data from Nearby Weather Stations

While it is preferable to install the necessary components of a weather station on site for field tests, that is not always feasible, for reasons of time or cost. In place of site collected weather data, weather stations maintained by airports, military bases and municipal governments (to name a few sources) can be retrieved through the National Oceanic and Atmospheric Administration (NOAA). A complete set of hourly climatological data is available for hundreds of sites around the country.