Goals When Installing an Efficient Proper Duct System:
Properly engineered ducts should achieve the following
1. Minimal leakage
2. Good insulation (ideally R-8)
3. Sized to provide ideal air flow to each room
4. Minimal air flow resistance
5. Designed to work efficiently with a correctly sized heat pump
Unfortunately, the vast majority of homes fail to meet these goals. Contractors can usually get away with installing poor quality duct systems. Generally, no one comes to test the leakage and few homeowners even look in the attic to examine that work that was done. Also, most heat pumps are too large for their building. Ideally, a new duct system should be installed alongside a new heat pump, and sized accordingly.
Duct Leakage:
The average duct leakage in California is 30-35%. That includes newly installed systems. It sounds high, right? 30-35% of the air you’re paying to heat or cool is lost! It’s a big waste of money and energy. Leakage in your return means air is being pulled into your duct system from a dirty crawlspace or attic. Leakage in a supply duct means the air in the duct is being pushed out and lost. In addition to the energy loss, bad ducts will make your home uncomfortable. Improperly sized ducts will heat the house unevenly, and leaky ducts can vent poor quality air. The only way to test your duct leakage is to have an energy assessment and Duct Blaster Leakage test performed. Following this, ducts should be thoroughly sealed before insulation is installed.
Duct Insulation:
The effectiveness of duct insulation is quantified by its R-value, a measurement of its resistance to heat flow. Presently, the minimum standard for newly installed ducts is R-6. The recommended value is R-8, especially for attics, which have more extreme temperatures.
Foil-faced fiberglass is the best option, whether you’re replacing old insulation or installing a whole new duct system. This stuff does everything you want for a duct. It’s versatile (can be installed on both rectangular and circular ducts), it’s safe, it dampens sound and prevents condensation, and finally, it can get you that R-8 thermal resistance. Also, it pairs up well with flex duct, which is very useful over shorter distances or irregular spaces (handy for connecting the air supply to the rigid ductwork, for example).
In many cases, foil-faced bubble wrap is used, but not effectively. This material forms a radiant barrier, but not a conductive barrier. It has some applications, but it requires an air gap, and the installers would have to put in spacers to prevent it from being in contact with the ducts, which is difficult to do, and basically never done. Pressed tight against the ducts, this stuff provides very poor insulation, as low as R-1.
