There may not be a bigger, more severe energy loser in a home than a leaky and poorly insulated duct system that is located in an attic or crawl space. Leaky duct systems are a massive problem that many homeowners often don't even realize they have. When duct systems leak inside a home, they bleed hot and cold air to wall bays, floors, interstitial spaces, etc. This is bad because instead of the hot or cold air going to the rooms that need it, it gets lost along the way, sometimes creating a hot or cold spot on the wall or floor. This requires your furnace or air conditioner to run longer to satisfy whatever temperature your thermostat is calling for, which in turn drives up the home's energy bills and shortens the lifespan of the HVAC equipment.
But what is far worse, is when you have a leaky, poorly insulated duct system that is located in an attic or crawl space. In this situation not only is your home being robbed of hot and cold air that your HVAC system is burning gas or using electricity to generate, but that conditioned air is getting vented literally to the neighborhood! And added to this, when you have a duct system in an attic or crawl space that is also poorly insulated -- on top of being leaky (as is the one in this video) -- the air conditioned air flowing through those ducts during the summer is picking up all that heat from a hot attic. The same thing happens during the winter, but the other way around. When you attempt to push hot air through a poorly insulated duct that is located in a cold attic or crawl space, that warm air gets colder along the way. Both of these issues not only cause comfort problems in the home, but also massive losses of energy.
These are the some of the reasons why building scientists hate to see duct systems located in unconditioned spaces like attics and crawl spaces. Most of the time the HVAC technicians that install this ductwork do apply some insulation to the duct, as you can see at the 30 second mark in this video. The problem is that the ductwork beneath the insulation wrap is almost always improperly sealed and full of leaks. Added to that is that the insulation wrap is usually a poor insulator in itself! So you really often have a double whammy with ducts that are located in unconditioned spaces.
The best way to address this issue is by:
1) Removing the existing duct wrap
2) Sealing the larger duct leaks with HVAC mastic tape
3) Blocking off the underside of the duct / trunk line with either foam board, drywall, or plywood and seal off all 4 sides with caulking or foam sealant ( to isolate the underside of the duct / trunkline and make sure that it cannot leak after the top 3 sides are spray foamed)
3) Encapsulate the duct system with a 1.5 - 2" inch thick layer of spray foam to seal all of the remaining duct leaks and also insulate it to an R-9 to R-12 insulation level.
This process dramatically improves the overall energy efficiency of the entire home, as well as improves the home's comfort and interior air quality. This is true because less conditioned air is being lost to the outside, and the HVAC system has to run less in order to satisfy whatever temperature the thermostat is calling for. And because the return ducts are also sealed -- as well as the supply ducts -- there is less attic dust and air contaminants getting sucked into the home -- contributing to better interior air quality. In short, upgrading an attic duct system by encapsulating it with spray foam is one of the best and top performing energy efficiency and comfort upgrades that a home can have.
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