There is a hidden irony in a standard North American construction practice. Virtually all homes and many commercial buildings are equipped with forced-air heating and cooling systems that use ducts to distribute conditioned air. This air is intended to warm or cool the building. And yet, the ducts almost always run through unconditioned spaces. Conditioned air runs through unconditioned space through poorly insulated ducts resulting in lots of energy loss.
You’ve probably seen ducts in attics and crawlspaces, but those areas are really outside the building’s thermal boundary. In areas of the country where basements are not common, the furnace itself usually sits in an unconditioned garage. It wouldn’t be much worse to run the duct out an open window, down the outside wall and back into the house. Standard practice for duct insulation is R-8. Compare that to the R-21 required for walls by many building codes and R-30 often targeted for high-performance buildings. That’s not much insulation for the conditioned air that is supposed to maintain your comfort. Plus, ducts are leaky. Standard practice can lead to ducts that lose 15% to 20% of the air that flows through them. Even with careful duct sealing, leaks may be around 5%. So the air that you just paid to warm up, is now traveling outside through cold, leaky tubes.
The situation is worse in hot climates where many homes have central air conditioners with ducts in attics where temperatures can reach 150°F. Hot attics can increase the cooling load from 0.5 to 1 ton, increase cooling costs 15%, requiring a bigger air-conditioner and more energy use. And that’s not all. Climates that are both hot and humid create a situation where water collects on ductwork similar to the condensation on a glass of iced tea on a muggy day.
It’s unfortunate that most designers fail to consider the entire heating and cooling system in their plans leaving builders with little choice but to install ducts in attics and crawlspaces. With a little forethought, planning and a fair amount of coordination within the construction team during the design phase, homes can avoid the worst of these problems.
The first step in creating an energy-efficient building is to make the shell as efficient as possible with high levels of insulation, careful air sealing and energy-efficient windows. In existing homes it may be difficult to maximize these shell measures, but it can be done. In new construction, it’s easier to optimize the shell with the help of an energy model. These shell improvements will dramatically reduce the heating and cooling load which means smaller, less expensive equipment. It’s one of the main cost-saving strategies for high-performance homes.
Once the building envelope is well insulated and air-tight, next look for ways to reduce duct losses. Here are 6 options.
Center the Furnace
In most designs, the furnace and water heater are afterthoughts, relegated to basements or garages,both of which are often outside the thermal boundary. Instead, designers can identify a spot near the center of the plan to install this mechanical equipment. This reduces the length of the ducts. (If the water heater is located there too, it will also reduce hot water pipe length, wait time for hot water at the faucets and the amount of water wasted.) Furnaces can be loud, so this room (or closet) should be insulated with a solid wood door and other sound-absorbing measures.
Drop the Soffits
Ducts can be placed in soffits framed down from ceilings. Hallways, utility rooms and bathrooms are perfect candidates for a slightly lower ceiling height. Since nine-foot ceilings are common, a one-foot soffit would fit nicely and could add a bit of architectural interest.
Insulate the Roof
Some builders, especially in hot climates, like to move the thermal boundary from the ceiling below the attic to the roof above the attic. With the attic now inside the thermal boundary, ducts can easily reach most rooms in a more or less conventional fashion.
Switch to Open Web Floor Trusses
Two story designs have the opportunity to replace standard i-joist floor framing with open web floor trusses. This allows ducts to reach all the rooms from the central furnace location, while remaining inside the thermal boundary.
Condition the Crawl Space
Similar to bringing the attic inside the thermal boundary, it’s possible to insulate and seal the crawlspace and then run ducts through that space. But this is not as simple as it sounds. To do this right, you must use rigid insulation for the walls and the entire floor of the crawlspace. The area must be carefully sealed and then ventilated to the outside. Of course, drainage and waterproofing are essential. The measures described here for a crawl space would be the same if you were building a below-grade living space. That’s why some experts refer to this as a “short basement.”
Ban the Ducts
The best way to reduce duct losses may be to eliminate the ducts altogether by installing a non-ducted heating system. Ductless heat pumps (aka mini-split heat pumps) offer very high efficiency and distribute the heat (or cool) through long, thin tubes instead of ducts. These are very easy to install in retrofits as well as in new homes. There are several styles of indoor units; some hang on the wall, while others are hidden from view. If you don’t need cooling, a well-vented small gas unit heater can provide ample comfort for a small, tight house.
All these ideas require careful design, planning and coordination. It’s best to call a meeting of everyone involved, including the designer, energy consultant, mechanical contractor, insulator, framer and general contractor. Discuss the various options and choose an approach or a combination of ideas appropriate for the project.
This post only scratches the surface of the many great ideas for bringing ducts in from the cold. To learn more, visit a comprehensive website called Ducts Inside for implementation details, examples and downloadable fact sheets.
Want to dig deeper? Sign up for the Zero Energy Project Forum and post your question on ducts inside or share your experience and expertise with other builders and designers.