Why your AC ducts in the hot summer attic just can't cool your home enough in Massachusetts!

by Insulation Geek | Jun 15, 2017 | 0 Comments

Imagine carrying a couple of soft serve cones on a hot summer day, from an ice cream truck to your kids waiting on a park bench some distance away. Would you walk under the hot sun or in the tree shade to keep your prized possession from melting away? This is a no-brainer but when it comes to making a similar decision about air conditioning, builders have been choosing differently.

Air conditioning HVAC ducts have traditionally been installed in unconditioned attics. At first glance, attics appear to be the perfect place to tuck away the clutter and a good use of an otherwise wasted space. Ducts are often installed up against the roof deck by displacing or disturbing the existing fiberglass batts insulation, on the attic floor or somewhere in between.

Now consider this. During summer, the roof deck temperature can exceed 150°F and the temperature inside the attic can breach 140°.

Within the duct, cool air flows at a temperature of about 55° F. This creates a huge temperature gradient of up to 95° F which pushes the heat into the cool ducts heating up the air inside. This leads to loss in efficiency of your air conditioning system and results can show up in your energy bills or as temperatures beyond the comfort level inside your house. Similarly, during winter months you have a freezing cold attic with warm air flowing through the ducts inside. Again, a large temperature gradient is created that works against the air conditioning system. Heat Transfer 101 – heat will flow from a higher temperature to lower temperature.

Fitting air ducts up against the hot roof deck by cutting up insulation is the worst decision that can be made when installing air conditioning. In fact locating air ducts anywhere inside an unconditioned attic is a bad idea. It’s like carrying ice cream under the hot sun and hoping that it would still be frozen solid when it reaches its destination.

The coldest fluid, travelling from the air handler to the conditioned space inside the home, is being made to pass through the hottest area in the house. Hundreds of square feet of the ductwork surface is exposed to high temperatures inside the attic. The 55° F air inside gets warmed up significantly before reaching its destination. If you have duct leakages, the warm air is leaked into an unconditioned space further compounding the energy waste.

David Roberts and Jon Winkler working at the National Renewable Energy Laboratory of the U.S. Department of Energy have studied the impact of duct location on cooling load, peak demand, and energy cost in hot climates. In their paper, Roberts and Winkler have modeled the savings possible through relocation of the ducts from an unconditioned attic to a conditioned space of a 2-story, 2500 sq. ft. slab-on- grade home. The study was conducted in Houston, Phoenix and Las Vegas but the qualitative results are equally valid for hot summers in the Massachusetts area.

The study reveals that relocation of air ducts from the attic to conditioned space inside the house can reduce the required cooling capacity of your air conditioning system by up to 24%. This means reduction in your upfront investment in an air conditioning unit and replacement cost at the end of its 15 year estimated life. The annual electricity usage for cooling goes down by about 16% which means an equivalent amount of savings in your air conditioning energy bills. The savings from reduced energy usage during winter months is in addition to this figure. Duct relocation also helps reduce peak cooling demand by about 22% which improves the load factor for your local electric utility.

The savings possible from relocation of the air ducts when compared to other energy saving measures clearly shows that moving your air ducts out of the unconditioned space to a condition space is by far the best thing you can do to improve energy efficiency in your home. Moving air ducts out of the unconditioned space is far more effective in reducing energy bills than adding insulation to walls, installing better windows, and using more efficient air conditioners.

What are alternatives to installing ducts in an unconditioned attic?

There are two main approaches to resolving this issue.

The first approach is to turn the unconditioned attic into a conditioned space. This is called cathedralizing the attic and involves moving the thermal and air barrier to the roof deck from the attic floor. The roof deck is insulated with closed cell spray foam insulation of required thickness to form a thermal barrier against the hot-roof. The insulation blocks the transfer of heat from the hot 150°F roof to the attic lowering the inside temperature and hence reducing the temperature gradient across the duct walls.

The second approach is to move the ducts out of the attic and into conditioned space or increase the insulation of the HVAC ducts. Available relocation options include use of drop ceilings, raised floors and soffits below the attic floor. In two-story homes, the space between floors may also be used for this purpose. Another option is to use a scissor-truss to create a conditioned space below the attic and above the living space. Air ducts should also be insulated to reduce heat loss. If your ducts are uninsulated or poorly insulated seal them and add insulation to keep the air inside at the desired temperature as it moves through the system. We have even applied closed cell spray foam insulation around HVAC ducts in an unconditioned attic with insulating the roof wasn’t an option for a myriad of reasons. The R-value of duct insulation today ranges anything from R-3 to R-10 or R-12 so applying 3 inches of closed cell spray foam (SPF) onto the ducts as an insulation works out very well to providing R- 21 insulation to the attic ducts in the unconditioned attic.

At the end of the day, installing air conditioning ducts in unconditioned attics is a bad idea. It increases your investment in air conditioning equipment and inflates your energy bills. So this summer, if your energy bills are going through the roof, consider the possibility that it might be because energy is literally going through your roof. Consult a professional to find out if relocation of air ducts or spray foam insulation on your roof deck can help reduce your energy bills. In addition to converting an attic into a living space, this is one of the main reasons that we insulate the roof, in our Massachusetts homes – when we have an HVAC unit in the attic and thus ducts, that is.

Do read our other articles to learn about why we install closed cell spray foam insulation in the attic roofs vs installing open cell spray foam insulation in the attics. This applies to insulation solutions for ducts as well.

For more information on the benefits of install insulation in your attic to condition it or upgrading your home installation to the most current Massachusetts Building Insulation Code, and the ways in which this can be achieved, call Mass Energy Lab Insulation at 617-902- 2744 or visit our insulation services page. We’re home insulation geeks and would love to hear from you!

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