Friday, May 6, 2011

Energy Efficiency in Remodeling: Ducts


Opportunities

Many remodeling projects involve some addition to, or modification of, the HVAC system, which in many cases includes ductwork. Leaky and poorly insulated ductwork located outside of the sealed and insulated building envelope (i.e., in exterior walls, garages, crawlspaces, and attics) is very common. As many as 1 in 12 homes have major ductwork problems such as disconnected ducts, pinched or crushed ductwork, missing or badly torn duct insulation, or poor duct layouts. Remodeling is an excellent time to consider ductwork improvements, when existing ductwork is accessible and new ductwork is being designed and installed.

Benefits

Reducing duct air leakage and improving duct insulation has enormous potential to reduce utility bills and prevent or eliminate associated comfort and health problems. Specifically:
  • Heating and cooling costs can be reduced by as much as 20-30%
  • Comfort can be improved by ensuring adequate delivery and return of conditioned air
  • Downsizing of heating and cooling equipment is possible
  • Entry of mold, radon, dust, and moisture into the house can be reduced
  • The likelihood of house depressurization leading to backdrafting can be reduced

Factors to Consider

Location - Ducts placed within conditioned spaces are more efficient than those placed in unconditioned spaces. If located within conditioned space, conductive and radiative losses, leakage losses, and equipment cabinet losses are reduced or regained into the building space. If possible, locate new ductwork and relocate old ductwork within the house envelope. This means avoiding exterior walls, garages, crawlspaces, and attics. In some cases, it may be easier to alter the location of the insulated and sealed (thermal) envelope so that the existing ductwork is then within the house where leakage is of less concern (i.e., crawlspaces). If it is not feasible to locate ductwork within conditioned space, the ducts should be properly sealed and insulated.
Sizing - With all ducts, care must be taken that the ducts are large enough to deliver the needed volume of air. Smaller ducts tend to be noisier and more leaky than larger ducts due to higher air speeds and pressures. In order to deliver the same volume of air, flexduct and ductboard systems must usually be sized larger than metal ducts as their interior surface is much rougher, leading to more restrictive air flow. Any new ductwork should be sized according to recognized industry standards such as Manual-D, published by the Air Conditioning Contractors of America (ACCA).
Return air - Many homes have one or more supply registers in each room but often have a total of only one or two return registers, usually located in hallways. If interior doors are left open, this arrangement usually works well. However, when these doors are closed, as is often the case with bedroom doors, an adequate volume of air often can not get back to the these centrally located return registers. This causes higher pressures in the rooms with closed doors. This condition greatly increases the amount of heated or cooled air forced out of the house from these rooms. The higher pressures in these rooms may also make it difficult for the rooms to receive enough supply air. The result can be an uncomfortable room and higher energy use. Meanwhile, the rest of the house is at a lower pressure, causing outside air to enter at a faster than normal rate. Backdrafting of exhaust gases from combustion appliances may then result as air is drawn down flues or chimneys in an attempt to equalize the pressure. While undercutting doorways can improve air return, it simply may not be adequate in many cases. Alternate solutions include installing return ducts in each room, or installing transfer ducts or bypass grills which connect the affected bedroom(s) to the hallway, for example.

Installation Issues

In order to achieve a good seal for a long time, it is important that ducts be sealed with mastic and fiberglass mesh (where required). Flexduct should be adequately supported along its length and not pinched. Standard six-inch round flexduct should not generally be used in lengths over 16 feet.
Tests of duct systems may be used to identify leakage sites and to confirm the effectiveness of sealing measures. To test, ducts are pressurized with a fan at a return register or the air handler cabinet.

Material/Equipment Options

Sealing leaks in ductwork involves the use of special duct "mastic" and mesh which is extremely durable, long-lasting, and effective. This work may be done by some insulation or general contractors or weatherization specialists. The cost of reducing leakage is very dependent upon the number, type, and location of the leaks, as well as the particular contractor. A range of $200 to $400 would be typical for retrofit work.
DUCTS
Selection of duct material is based on its price, performance, and installation requirements.
Sheet Metal
  • Most common
  • Durable
  • Can be customized to fit odd sizes/locations
  • Smooth surface offers low resistance to air flow
  • Many connections, joints, and seams, each having potential leakage
  • Must be insulated when located in unconditioned spaces
Flexduct
  • Made with a plastic inner liner inside a tube of insulation, covered with a vinyl vapor barrier
  • Few duct connections and joints
  • Low installation and material costs
  • Easily torn, crushed, pinched, or damaged, with damage to inner lining not visible
  • Has higher resistance to air flow than metal ducts, must be properly specified
Ductboard
  • Made from stiff, high-density sheets of fiberglass with foil facing bonded to one side
  • Insulation is integral to duct material
  • Material costs higher than sheet metal
  • Installed costs may be comparable when sheet metal must be insulated
  • Lightweight and particularly adaptable to attic systems
  • Vapor barrier is part of the duct material
  • Provides excellent sound attenuation
  • Durability is highly dependent on closure method (tapes and mastics)
  • May be damaged or crushed during construction
  • Relatively air tight when properly installed
Transfer ducts/Bypass grills
  • Used in lieu of individual returns for each room
  • Used to connect bedrooms, etc., to a hallway having a central air return
  • Typically made using short pieces of ductwork, or grills above a door or other location
  • Installed in attic or through walls
SEALANTS
Mastic
  • Paste applied to joints and connections in ductwork
  • Becomes hard and very durable when dry
  • Can be used on cracks up to 1/4-inch wide
Fiberglass mesh
  • Used to help seal holes larger than 1/4-inch wide
Butyl-backed foil tape
  • Can be used to seal holes or cracks
  • Can not be used to seal awkward connections, such as where a round duct meets a rectangular one
  • Its long term durability is not known
Foil tape
  • Shown to come loose after just a few years, especially in hot attics
  • Many joints can not be properly sealed with tape at all, such as where a round duct connects to a rectangular duct

New Options

Duct sealing technology which seals the ducts from the inside with a latex-based spray -- will soon be commercially available.

Duct Insulation

For metal ducts, insulation may be installed on the inside and/or outside of the duct. If on the outside, a vapor retarder, usually integral to the insulation itself, should cover the insulation. This is to prevent condensation on the duct which would severely degrade the effectiveness of the insulation and may lead to damage of the house. Remember that insulation does nothing to prevent air leakage -- ducts must be properly sealed before insulating. For ducts of any kind located in an attic, insulation of the ducts can be improved by placing batt or blown insulation over the ducts.
Flexduct and Ductboard
  • Insulation is part of the duct itself
  • R-4.2 insulation is most common on flexduct
  • R-6, R-8, and R-11 flexduct is also available
  • Ductboard is typically made from R-4.3
  • Ductboard also available in R-6.5
Fiberglass Duct Liner (for metal ducts)
  • Used to line the inside of rectangular metal ductwork
  • Made of specially-treated, rigid fiberglass insulation
  • Typical R-values are 3.6, 3.7, and 4.2 per inch
  • Available in ½, 1, 1-½, and 2 inch thicknesses
Fiberglass Wrap Insulation (for metal ducts)
  • Used to cover outside of ducts located in unconditioned spaces
  • Typical R-values are R-3.6, 3.8, and 4.1 per inch
  • Available in 1-½, 2, 2-¼, and 3-inch thick rolls
  • Available with or without a vapor barrier (an outer covering of reinforced foil)
  • Insulated better than duct liner

Remodeling Scenarios

Existing construction

For existing duct work, there are basically three options for improving any given portion of ductwork. Depending on where the ducts are currently located, their present condition, and the costs involved, you can:
  • Relocate the thermal envelope
  • Relocate the ducts
  • Seal and better insulate the ducts
While it is generally preferable to locate the ducts within the thermal envelope, it may be physically impossible or very expensive to alter the location of the thermal envelope or the ducts themselves. If re-location is not possible, ducts should be sealed with mastic and possibly better insulated. This can be a very worthwhile measure even if it is necessary to remove and reinstall the existing insulation.

New construction

At the beginning of a project, consideration should be given to:
  • Location of the duct work
  • Duct type and size
  • Sealing method
  • Insulation method, R-value
As mentioned above, it is preferable to locate ducts within the building envelope. However, this option is not always possible, even with new work. Again, in this case, ducts should be sealed with mastic and perhaps better insulated.

Savings

As mentioned above, sealing ductwork and improving duct insulation can reduce your heating and cooling costs by as much as 20-30 percent.

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