For those of you who don’t know, an air exchanger (also known as a “Venmar”, which is a major manufacturer of air exchanger and HRV systems) is basically a fancy fan which extracts stale air from within your home, and pumps it outside while replacing that air with fresh air from the outdoors. An HRV system is similar but also has the added function of transferring the warmth (or coolness) from that indoor air to the fresh incoming air, so you don’t end up pumping out all your heat (or air conditioning) at the same time. In case you’re curious, HRV stands for “Heat-Recovery Ventilator”.
Now, the idea behind the bucket stuck to the attic vent is, the bucket creates a somewhat air-tight chamber directly underneath the active attic vent. Therefore, when the vent “does its thing”, which is create vacuum pressure to vent out the attic, it ends up pulling air through the flexible duct lines instead, which, in turn, pull stale air out of the house, and vent it off to the outdoors through that roof vent. Now, in theory, this is not a bad idea, as it basically performs the same function as an air exchanger… well, maybe half of its function, as it does not pump in fresh air in return (hence the term “air exchanger”). However, in practice, this can cause more harm than good.
Active roof vents are there for a reason. Their purpose is to create vacuum pressure (ie: “suction”), which would pull stale and possibly warm air out of the attic, and vent it off to the outdoors. This thus creates a pressure differential between the attic and the exterior. As air always moves from a high pressure area to a lower pressure area, fresh cool air from the exterior vents into the attic through the soffits, which are those perforated metal panels beneath the roof, along the eaves of the house. In the end, you have active ventilation (enhanced airflow) within the attic space, which is crucial, especially if your insulation levels are a little on the low side. A lack of ventilation (and/or insulation), can lead to a buildup of heat and humidity within the attic space. This can lead to all sorts of problems, such as ice damming at the eaves (which can cause roof leaks), the formation of icicles, mould within the attic space, deformation of plywood roofing panels (due to the adhesive within the plywood basically diluting), etc. You always want your attic space to have the same air temperature and humidity level as the outside air, which is why I always tell my clients to remember the fact that the attic space is outside of the building envelope. If your attic is warmer than the outdoors, and that warm air is not ventilated out, than condensation will form on the underside of the roofing panels, due to the roof being cold from the outdoors. This condensation will lead to mould and can cause warping of your roofing panels, particularly if you have plywood instead of OSB (ever see a roof that appears to be wavy?) Furthermore, the warming of the roof panels can lead to melting of the underside of the snow, which is sitting on your roof. This melted snow can then re-freeze when it reaches the colder eaves, directly above the soffits, which would lead to an ice dam. This dam can then prevent further water from draining off the eave, and redirect it underneath the shingles, which will lead to infiltration within the attic space, and leakage within your home. Finally, a “hot roof” (as the phenomenon is called) will cause a substantial reduction in the lifespan of the asphalt shingles. This is due primarily because of two factors. One, the temperature differential leads to an increase in expansion and contraction of the asphalt bitumen within the shingles, which will cause the bitumen to degrade quicker, thus causing the shingles to curl and degrade more quickly. Also, a hot roof will cause condensation to form between the shingles and the roof sheathing, which also contributes to degradation of the asphalt bitumen, and thus causing your shingles to degrade even quicker. I’ve seen cases of century homes having thirty-year shingles, needing a complete roof replacement after only ten years.
So, if you happen to have one of these “homemade air exchangers”, my advice would be to remove it (and seal off those duct lines). Especially if you notice that your attic already has ventilation problems such as speckle mould (small black spots) on the roof panels, darkening of the roof panels (condensation/humidity), etc. If you’re unsure (about anything at all), call me up or send me an email. I’m always happy to give free advice.