We all live and work in buildings. Inspecting these structures for energy losses or gains is an important step to keeping the cost as low as possible while maintaining a comfortable environment within the structure. The inspection conducted the most often is an insulation (conduction) inspection. Often overlooked or misperceived, is the air leakage (convection) inspection. After all, it can contribute to ½ of the cost of heating or cooling a structure. And that is what we are going to discuss in this tip.
A convection (air leakage) inspection should be conducted after the insulation (conduction) inspection is completed. The best method for doing an air leakage inspection is to control the flow direction thru the building. Relying on natural convection can be problematic as the neutral plane in the structure is not readily apparent. We are not saying that it cannot be done but, the natural airflow and construction of the building must be understood. In most structures, air enters at the bottom or ground level and exits at the top of the building at the soffit or roofline sometimes referred to as the stack effect. You have to be on the side that the air is moving to, not coming from. The best method is to create a controlled pressure difference from outside to inside, either using a blower door, the buildings exhaust fans or even a box fan in the window. The goal is to put the inside of the building in a negative or vacuum as compared to the outside, drawing the outside air into the building, making your inspection point from inside of the building. We can do the opposite as well, putting the building into a positive pressure condition. But the flow direction and the temperature differences must be understood. As with the insulation inspection, there is a recommended minimum ΔT (temperature difference) from inside air to the outside air of 9°F (18°F for the insulation inspection). From what I have seen it seems to be easier to detect the effects of cooler air across a surface than warm air. (it may just be me) remember that we cannot “see” the air, just the effects of convection as it moves across the surface. Air is kind of like electricity, always seeking and taking the path of least resistance.
So, what does that mean to us? It means that one convection (air leakage) inspection is not going to “find” all of the leakage points in a structure, the more air we can move may help in finding some of the smaller leaks that the larger leaks mask. We must be careful not to damage the structure by inducing too much air movement. This may take several inspections. There are just some leaks that are not worth the time and effort to seal up. There is another factor to also keep in mind, there is a small amount of leakage that is needed to keep the building “healthy” fresh air is needed for make-up air for such devices as fuel-fired heating (gas, oil, and even wood) unless that make-up air is ducted from the outside. Fresh air is also needed to control moisture, odors, fumes, any number of items that can make the occupants of the building uncomfortable or unhealthy.
This type of inspection can also reveal air movement in the insulated wall rendering the insulation non-effective (the insulation becomes an air filter instead of an insulator) that is why the conduction inspection is done first (to verify the presence of insulation) before we force the air to move thru the building looking for leakage paths that may reduce the performance of the insulation.
We have only scratched the surface here; several specialty courses deal with building inspections. These courses take 2-3 days to cover the conditions and methods needed to successfully conduct these inspections.