Attic Mounted Air Handling and Distribution Systems

Much controversy and misunderstanding occurs as people add insulation to homes where there is ductwork in the attic. Should the ducts lie on the attic floor with insulation over them or should the ducts be suspended, with insulation under them? Or, should the entire attic be insulated, bringing all the ducts and air handler into the “conditioned space” of the house?

Take a step back and look at what we’re trying to accomplish. An air handler and ductwork is used to suck air from inside the house, heat or cool it, and blow that air back into to the house.  Ideally, this is a “closed system”, meaning that there should be no leaks and only air that gets sucked into the system from the house gets sent back into the house.

Unfortunately, every duct systems leaks. Some leak a little, others leak a lot. When the leak is on the suction side of the system (called the “return” side, because the air is returning from the house to the system), then attic air enters, along with any dirt. The air also carries humidity and temperature from the attic, forcing the system to work much harder to heat or cool the air. Even a relatively minor leak on the return side can reduce the system’s efficiency by 20%-50% when the system is in an unconditioned attic.

Leaks on the supply side of the system, that is, the air being blown back into the house, are not quite as bad as return air leaks. However any air leaked into the attic has to be replaced, and the only place the replacement air can come from is outside. If it’s 20 degrees outside, then every cubic foot of air lost into the attic is replaced by a cubic foot of 20 degree outside air entering somewhere else. This leads to drafts and wasted energy. Worse, during the winter, leaks of supply air into the attic, puts warm, humid air into the attic, setting up conditions for mold, wood rot and ice dams. As you can see, eliminating leaks from the air handling system is critical for your health, comfort and energy efficiency.

Ducts and the 2009 IECC

The 2009 IECC (International Energy Conservation Code), now adopted in Pennsylvania, understands the importance of air leaks, and leak testing has become almost mandatory. If any of the system is outside the conditioned space, then it must be tested with a calibrated “Duct Blaster.” Specifically, the ducts must leak to the outside no more than 8 CFM per 100 SF of floor area when tested under 25 Pascals of pressure. The total duct leakage must be no more than 12 CFM per 100 SF. If tested during rough-in, the ducts may only leak 6 CFM per 100 SF. And, if there is no air handler during testing, the total leakage must be less than 4 CFM per 100 SF.

For testing reasons alone, it makes sense to have the ducts fully inside the conditioned space of the house.

Solution 1: The Conditioned Attic

For an attic-mounted air handler and ductwork, how do you achieve a “conditioned attic?” The secret is to bring the attic into the conditioned space of the house by spraying foam insulation to the gable walls and under the roof deck, forming an air-tight “cap” for the house. In this situation, you don’t use any insulation on the attic floor – you want the attic to be at the same temperature and humidity as the house. In fact, if done as a retrofit, any insulation on the attic floor must be taken up or it could create conditions conducive to mold growth.

 In this situation, duct losses remain within the conditioned space of the house and the attic is warm, reducing the opportunity for moisture or mold problems.

Advantages

1.      Air leaks remain inside the building envelope

2.      Equipment operates in less hostile conditions

3.      Attic stays comfortable, year round

4.      Reduced chance of condensation

5.      Improved indoor air quality

6.      Usually simplifies the insulation job

7.      Simplified attic access when working on equipment or moving around in the attic.

8.      Less concern about thermal bypasses to the attic

9.      Removes need to pressure test the duct system (for code compliance)

Potential Disadvantages

1.      Contractors and codes officers may not be familiar with this construction even though it is accepted by current building codes.

2.      Roofers claim shingle life is diminished. Studies have shown that color and roof orientation roof have more adverse effects on roof life than insulating under the roof deck.

3.      Roof leaks are hidden – if the roof leaks, the water won’t drip down and ruin your ceiling. Instead, it may be trapped in the roof deck until it has a chance to dry out*.

4.      Cost can be high because this technique must only be implemented with spray foam.

* Rather than spraying directly to the underside of the roof deck, I like the idea of maintaining a ventilation channel by installing 2x2’s along the edge of the bays and attaching 2” extruded polystyrened (blue-board or pink-board). This forms a clear air channel from the soffit vents up to the ridge vent, allowing natural ventilation where it belongs.

In this configuration, if there is a roof leak, the water can drain out of the channel and will be visible at the soffit. Natural air flow will help dry the cavity, reducing the chance of wood rot. This also minimizes the opportunity for ice dams because it allows the underside of the roof deck to be flushed with cool air.

Solution 2: Attic Floor Insulation with Buried Ducts

The most common installation technique is to lay ducts on the floor joists of the attic, then blow loose fill insulation all around the ducts. This insulates the ceiling of the living space (floor of the attic) and dramatically improves the R-value of the ducts.

When the attic is insulated in this way, it is imperative that the envelope between the conditioned space and the attic be air sealed prior to insulating. Failure to do so can result in serious moisture problems in the attic.

There is some concern about the attic R-value obtained when insulating in this manner. Because the ducts take up some of the space that would otherwise be occupied by insulation, some mistakenly assume that the attic insulation suffers. In reality, you want the ducts to be as close to the living space as possible, so that any energy lost from the ducts may find its way into the house.

In addition, an R-8 duct is insulated all around, so if the duct is buried in insulation, it will add at least R-16 to the attic insulation. The effective R-value will be even better because most of the duct will be insulated by the loose fill insulation blown around it.

For example, suppose the insulation and duct looks like this:

In this example, we have 2x6 joists, sheetrock on the bottom and a 6” duct wrapped with a ~2” layer of insulation.

The space between the joists is filled with insulation and insulation is blown to mound around the duct. The R-value where there is no duct is about R-44 (12” at R3.7 per inch). In the six inch space where the duct is open, there is about R-28 (R-8 duct plus 5.5” at R3.7 per inch). However, from an energy loss perspective, when the furnace runs, the duct is much warmer than the surrounding air, so it doesn’t matter how much R-value is protecting the ceiling – the energy loss at the location of the duct will be less than the fully insulated areas around it.

In addition, because we have now insulated most of the duct extremely well, the duct losses have been largely eliminated, significantly increasing the overall efficiency of the system.

One could argue that when the system isn’t running, the insulation at that location is compromised and provides only R-28. This is true, but it is easily remedied by adding three inches of blown cellulose on top of the ducts, bringing them up to a total of R-38. Best would be to use damp applied cellulose that has a glue that binds the cellulose into a rigid mass. Doing this prevents the cellulose from falling of the ducts over time.

Advantages

1.      Easy installation of HVAC

2.      Allows use of cost effective blown-in cellulose

3.      Typically high R-value for the attic, also insulates the sides and bottom of the ducts, greatly reducing overall energy loss from the ducts

4.      Ducts are insulated well from the extreme temperatures in the attic

5.      Reduces convective losses through ducts

Potential Disadvantages

1.      Reduces R-value of ceiling structure (but decreases overall energy loss from the ducts)

2.      Increases difficulty of duct maintenance

3.      Requires additional insulation on top of ducts if no reduction in R-value is acceptable

4.      Ducts leak outside the conditioned space of home

Solution 3: Attic Floor Insulation with Suspended Ducts

Another option is to suspend the ducts above the insulation, allowing the full depth of insulation on the attic floor. The primary disadvantage of this method is that it exposes the entire duct system to the full temperature extremes of the attic. When the system isn’t running, convection currents flow through the ducts, creating drafty conditions and energy loss inside the house.

As an example, if there are 100’ of ducts, insulated to R-8, and carrying 50F air in a 150F attic, the ducts lose a minimum of 5000 BTUs/hr. to the attic. When surrounded by insulation, the energy loss is reduced to 1300 BTUs/hr. – about a 75% reduction in energy loss.

For comparison, the added energy loss of the ceiling due to one hundred feet of R-28, 6” wide is only about 50 BTUs/hr. So, on a summer day, the net benefit of burying the ducts in insulation is about 1/3 ton of system capacity (about 5%-10% of the system’s capacity), per hundred feet of duct. During the winter, the difference may be less because the temperature difference between the attic and the hot air in the ducts will usually be less than the summertime differences. However a gas fired furnace on a cold day will lose about the same amount of energy.

Advantages

1.      Provides a full R-value for the ceiling of the living space

2.      Simplifies duct maintenance

Disadvantages

1.      Ducts are fully exposed to the temperature extremes in the attic

2.      Ducts leak outside the conditioned space of home

3.      Additional labor required to properly support ducts above insulation

Conclusions

Attic mounted air handling systems always represent a compromise. Air loss into attic spaces leads to system inefficiencies and indoor air quality issues. Ideally, the system should be brought inside the conditioned space of the house by bringing the attic into the conditioned space.

However, there are times that this is not feasible, in which case, the system gets installed in a “hostile” location. Energy loss from the ducts and air handler are accentuated due to the temperature extremes found in unconditioned attics. This necessitates greater insulation around the ducts in order to reduce heat losses.

The best way to achieve this is to place the ductwork on the attic’s floor joists, then adding loose fill cellulose insulation, preferably mounding it over the ducts to fully enclose the ducts with insulation. Conductive energy loss from ducts installed in this way are significantly reduced due to the greatly higher R-value provided by the surrounding insulation. This results in much less energy loss than suspending the ducts in the attic where they are exposed to the full effects of the attic temperature.

*Calculations contained in this document are approximations based on a variety of assumptions and are subject to error. However, the fundamental principles apply – ducts surrounded by insulation preserve much more energy than those exposed to the full temperature swings found in an attic.