The Building Envelope & Your Bottom Line

The "building envelope" is the area that separates conditioned space from unconditioned space. In this picture, the building envelope is the area surrounded by the insulation. We are concerned with creating the highest quality building envelope for the most efficient cost. The dimensions, performance and compatibility of materials, fabrication process and details, their connections and interactions are the main factors that determine the effectiveness and durability of the building enclosure system. Common measures of the effectiveness of a building envelope include physical protection from weather and climate (comfort), indoor air quality (health), durability and energy efficiency.The four building systems to address when discussing the quality of your building envelope is: foundation, walls, doors and windows, and roof. The factors to address in those systems are the R-value, air infiltration and solar heat gain coefficient (SHGC) and U-factor of your windows. R-value is a measurement of a material's resistance to heat flow. The solar heat gain coefficient (SHGC) is the glazing's effectiveness in blocking heat caused by sunlight expressed as a number between 0 and 1. The lower the glazing's SHGC, the less solar heat it transmits. The U-factor measures how well a product prevents heat from escaping.



Infiltration (shown in the graphic above) is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage. The leakage of room air out of a building, intentionally or not, is called exfiltration. Infiltration is caused by wind, building pressurization and by air buoyancy forces.

Building Envelope Thermography also helps to identify the weak points of the building envelope. It involves using an infrared camera to view temperature anomalies on the interior and exterior surfaces of the structure. You can see that the greatest amount of heat loss is occurring the eaves, ridge and windows where there is air infiltration is occurring.

In typical modern U.S. residences, about one-third of the HVAC energy consumption is due to infiltration. Another third is to ground-contact, and the remainder is to heat losses and gains through windows, walls, and other thermal loads. As such, reducing infiltration can yield significant energy savings, with rapid payback. In cold climates, with a 15 MPH wind, residences often have air exchange rates of 1.0 to 1.5 air changes per hours (ACHs), far in excess of the ventilation air needs. It is very easy to reduce infiltration rates to less than 1.0 ACH. Smoke candles and blower-door tests can help identify less-than-obvious leaks in existing homes. Therefore by decreasing heat losses and gains by building better envelopes we can significantly improve our energy efficiency.