If your ducts are located outside the living space, as in a garage, basement or attic, leaks in them are a very serious problem that you should fix quickly. Duct leakage is responsible for a significant part of heating and cooling costs.
Fortunately, discovering duct leaks is fairly simple and sealing them typically easy and inexpensive.
Discovering the duct leaks
Duct leaks only occurs at seams, joints and ruptures, and you may carry out yourself some basic tests to identify them.
You may switch on the blower of your HVAC system, and use your hands to feel air coming from the supply ducts, namely close to joints and the air handler. Or you may use a smoke generator in returns ducts; leaks will pull the smoke in…
That’s a good starting point, or a reasonable method for more simple cases. For other more complex projects, consider a professional audit. Contractors use special fans (duct blowers) to pressurize the ducts and to measure and identify accurately all the leakage points. Causes
Things as simple as a protruding nail can tear a flexible duct during its installation, and make it leaky; joints and junctions may separate over time, namely if the materials used in their installation aren't the most appropriate. Flex ducts are sometimes twisted and bent unnecessarily, which is also a cause of gaps and breaks.
Materials and application
Before beginning any duct sealing job consider the type of materials to use. Mastic is a great material.
Prefer mastic to duct tapes, which only should be used to seal holes in the air handler and cabinets, and whenever you want to maintain access for future service.
A bucket of mastic - that you can buy online or in a specialty store - is usually enough to seal all the leaks in ducts, and it is not difficult to apply it. Just take basic cares: mastics stick to everything and do not expect to wash it out.
Take also into consideration that mastic doesn't hold ducts together; you have to use mechanical fasteners to get it; and do not forget that to seal wider joints and gaps you should also use mesh tape (see: Duct Mastic).
Also do not forget the messiness of mastic application (protect your hands, and use disposable cloth… ) and the possible hazards involved of the sealing job. Ducts run on crawl spaces or attics that include electrical wires and sharp metal hedges, or other risks that you should be motivated to face. You should not advance if you are not willing to face them.
As to the duct-leak locations, consider the following:
- Plenum joints and cabinet: leaks in cabinets and the air handlers and their joints should be a top priority; it’s in them that the waste of energy is higher. Use mastic and fabric mesh tape in the plenum joints. See: Sealing Duct Air Handlers, Boots and Elbows.
- Joints between the room ducts and the main ducts: use a thick layer of mastic.
- Joints in elbows: use dust mastic.
- Joints in rectangular main ducts: use mastic, mesh tape and screws.
- Joints between flex ducts and plenums: see the image below for details on their sealing;
- Sleeves: use mastic, but also some screws to attach the sleeve to the duct.
- Cavity returns: contractors used them abundantly, some years ago; they should be carefully sealed along their edges or – if the results are expected to be too poor - replaced with true metal ducts.