Insulating Ductwork With Spray Foam Insulation
Many people spend a great deal of money buying new furnaces and new air conditioners that have 98% efficiency ratings. That is great! I am a big advocate for energy efficiency. However, how efficient is a furnace of central air unit if the duct work is full of leaks?
Metal is not easy to work with. It can only be bent certain ways and pieces of metal are put together to form a piece of duct work. The duct work usually does not fit perfectly together and thus there are leaks. These leaks are responsible for about 20% of the loss of efficiency of a heating or cooling system. So how do you fix it? Simple. Seal the duct work.
Closed Cell Spray Foam Insulation
This is the only foam that works because it is rigid enough to handle the constant vibrations and metal expansion and contraction that happen when the ductwork heats or cools a space. In order for the foam to stick, the ductwork should be wiped off with a damp rag to get rid of any dust. Also it will get rid on the oil used to manufacture the duct if it is galvanized metal. The seams should be sprayed first to ensure that all of the air leaks have been sealed. Then the system can be tested simply by running the fan. After the seams are sealed, a flash coat of approximately 3/4 of an inch should be applied to prevent heat loss in the wintertime.
This is a job that you can probably do yourself with a portable spray foam kit. These cost about $400 for 200 board feet. After you seal the duct work, you would have more than enough left over to do the box sills which would add to your efficiency.
Please note that only the supply duct work should be sealed. Sealing the return duct work will not make any difference in efficiency.
The other method of sealing duct work is using a product called Duct mastic. It is a plaster like material that you apply with your hand. Simply put on a rubber glove, dip your hand in the container, and wipe over the leaky areas. Allow 2-4 hours to dry and test for leaks. This is for sealing only and costs roughly $60.
If you are not sure about energy efficiency upgrades or would like to know more methods, hire an energy consultant. They will walk through your building with you to discuss the energy upgrades that will give you most savings. Rebates may be available depending on the state you live in.