Duct Insulation and Installation: A Quick Guide for Homeowners and Contractors

Duct Insulation and Installation: A Quick Guide for Homeowners and Contractors

What is the role of the duct system?

Duct systems are used to deliver and remove conditioned and unconditioned air from a building or home. A proper duct installation can improve the indoor quality of a home and increase thermal comfort. To install ductwork properly and effectively, it must be properly designed. 

How to right size the duct?

California Energy Services uses special software to calculate the amount of air needed to cool or heat each room. CES uses what is called Manual D to right size the duct and Manual J to identify the size of the load system.

What is the best design for your duct system?

Using Manual D, CES balances the delivery of air against the friction rate of the ducts and the static pressure in the system. To do so, CES examines the location of the air handler, the distance to the duct, the number of turns the duct makes, and the amount of air needed. The duct material greatly impacts the results of this calculation, as sheet metal ducts have a lower friction rate than flex ducts or rigid fiberglass duct board.

Where are the ducts usually located?

Ductwork is usually installed in unconditioned spaces. However, this can significantly increase overall heating and cooling losses if exposed to unconditioned space or not installed properly. Energy losses in the ducts can range from 10% to 45%.

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How do you solve the problem of ducts in unconditioned space?

The SMUD program requires the duct insulation to be at R-8 and highly recommends deep burying the duct under loose-fill insulation. Deep burying is a cost-effective way to improve duct performance and is considered best practice in home performance. During summer the ceiling of an attic can be 20°F hotter than the floor of the attic, so placing ducts at the top of the attic will increase heat flow from the hot attic to the cooled air inside the ducts. 

How do you properly deep bury ducts?

The top of the duct is at least 3.5” below the surface of the attic insulation level. Each individual section of ductwork must be buried at the same depth. During construction, vertical indicators (rulers) must be placed on buried trunk and branch ducts at a minimum spacing of every eight feet. The rulers serve two purposes: (a) to indicate the insulation level of the attic floor, and (b) to indicate the locations of the deep buried ducts. When ducts are found hanging from the attic ceiling, SMUD highly recommends relocating the duct on top of the attic floor and covering the ducts with insulation until they are deep buried. If you are installing flex duct, you need to have the inner liner pulled tight. This will reduce the friction rate and provide optimum airflow.

If the duct has 15% longitudinal compression, as shown in the diagram, the friction rate doubles; with 30% longitudinal compression, the friction rate quadruples.

california energy services duct infographic

In order to prevent duct kinking, a 90 degree rigid elbow should be installed, which will allow the flex ducts to run straight. 

california energy services deep bury ducts

Sagging ducts should also be avoided. According to Air Distribution Concepts standards¹, the flex duct should not kink more than ½” per foot. The supports need to be at the manufacturer’s recommended spacing, but no longer than 4’. The straps holding up the flex duct need to be no less than 1 ½” wide (see picture below.) 

california energy services duct infographic

The bottom line

It is important to size the ducting properly, insulate it to R-8, keep the duct straight and short and to deep bury ducting under insulation. 

What rebates are available for duct systems?

SMUD Customers

Duct replacement. $500 for one system (see program menu for more details) Duct sealing. $250. Final leakage to outside must be less than or equal to 8%. Reconfigure and deep bury ducts. $200. Ducts in the attic must be straightened and shortened and deep buried under attic insulation. Rebates are doubled for electric heated homes.

PG&E Customers

Additional rebates are available for PG&E customers based on the performance of your system

Contact us for more details about ducting and incentives.