Insulating Furnace Ducts in Crawlspace

Insulaing ductsOnce again, Melody homes did yet another crap job, this time “forgetting” to insulate the duct work for our central air furnace which happens to run through a ventilated crawlspace. The duct work spiders off into many different directions to supply heat to our house, of which about 100′ runs through our crawlspace. So, during the winter the crawlspace is the same temperature as outside; about 20°F. Whenever the furnace is off, the duct work cools off to about 20°F and takes a good 30 seconds to heat up. As a result, we get an initial “cold blow” through the registers for about 30 seconds, pumping cold air when it should be warm. Even after the ducts heat up, quite a bit of heat is being conducted from the duct work into the surrounding air which happens to be in a non-living crawlspace. This is a huge waste of energy.

So my obvious solution was to insulate the duct work, something Melody Homes should of done when the house was first built. Before you insulate your ducts though, I would strongly suggest you seal joints and holes with mastic to insure maximum efficiency. Once the mastic was applied, I tried two different methods of insulating my duct work.

I bought R-8 duct wrap which is a 2″ thick roll of fiberglass insulation surrounded by a shiny aluminum like sheet that somewhat generically fits around 6″ duct work. It comes in 5 foot lengths and is usually slipped around the ducts before they are installed. If installing the insulation after the ducts are in place, some cutting will be required. My second method was to purchase R-19 fiberglass batts at 5 foot lengths, cut them to the right size, and wrapped them around the ducts. This was a bit more work but I caught the hang of it and the installation went pretty quick. Strangely, the fiberglass batts only cost a little bit more money than the 5 foot spans of R-8 duct wrap.

I secured the R-8 duct wrap and the R-19 fiberglass batts with white nylon string. Although MacGyver can make a pig fly with duct tape, it really has very few applicable long term uses. In cold air, duct tape will lose its adhesiveness in a relatively short period of time and basically fall off whatever it is secured to, so stay away from it if at all possible (I learned the hard way). There are other kinds of tape that have a wire mesh, which makes the tape stick a little better, but it’s more expensive and really isn’t worth the money. I was on a shoe-string budget (pun intended) and string was a lot cheaper. It will also last much longer although it doesn’t look as “professional”.

In the end, it’s tough to know which insulation method worked best. I have to believe the R-19 batts offer more insulation than the R-8 duct wrap, but the question becomes, is it noticeable and worth the small extra cost? That’s tough to answer because all the runs are different lengths and have different amounts of air that pass through them. Whatever the case, the cold blow has been greatly reduced although not eliminated. Instead of it taking 30 seconds for warm air to come out of the registers, it does so in about 10 seconds. The crawlspace is also much cooler when the furnace is running than it was before, so I know that less heat is being conducted into the non-living spaces. This means more heat is making it into the parts of the house where I want it. So the effort and time to insulate the ducts was well worth it.


Leave a Comment