How Ductwork in Attics Costs you More

Are you building in a warm climate? If your residential build occurs in building science climate zones one, two or even three, you may want to reconsider placing the ductwork for your air conditioning units in the attic, even if the attic is ventilated. Studies show you could be adding 15% or more to your energy bill with poor ductwork placement.

Effect on Cooling Systems

Dave Roberts and Jon Winkler from the National Renewable Energy Laboratory did an extensive study on the effects of placing the ductwork in the attic and their results were fairly damning for this construction method: “For a typical new home in these climates, we estimate that locating ducts in attics rather than inside conditioned space increases the cooling load 0.5 to 1 ton, increases cooling costs 15% and increases demand by 0.75 kW.”

Building codes stipulate that ductwork should be free of leaks to prevent cool, conditioned air from escaping, but that doesn’t negate the effect of running your newly conditioned cool air through the hottest part of the home. Roberts and Winkler estimate that, even when ductwork is installed professionally, running it through the attic can reduce system efficiency by about 20% in a ‘best case’ scenario.

What this means is that opting to put your air conditioning ductwork in a conditioned space is a far greater energy saving move than many conventional ‘green’ building staples.

Where to place your Ductwork

  • You can still place ducting in the attic if you raise the thermal and air barrier from the floor to the roof deck.
  • Move the ducting into the home itself, utilizing drop ceilings and soffits under the floor of the attic.
  • If you have a two-storey home, utilize the space between the floors to install ductwork.
  • Create a plenum under the attic and above the living space using a scissor truss.

When considering energy-saving building components, designers, homeowners and builders must take into account not only the potential savings of the components, but also their cost and the cost of replacing them over the lifetime of the home. In this regard, proper placement of ductwork outperforms many conventional energy-saving building components.

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