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I recently visited a crawlspace and found a host of problems that, individually, most likely would not have created a big problem. But, as the handful of different issues came together they added up to a real problem for the homeowner. Air leakage from furnace ductwork plays a critical role in determining crawlspace environments.
Furnaces can be located in attics, in the living area of the house, or in basements. For homes with crawlspaces, the warm supply or cold return line will often run below the floor. If your heat comes from registers that are on the floor, then the heat supply lines run under the floor through the crawlspace. You have what’s called a downdraft furnace.
While this is generally fine, the installers failed to do one significant thing when they installed the ductwork in my problem crawlspace. They never sealed the joints in the ductwork.
Obviously, the ducting leaked. It leaked so bad that whenever the furnace turned on you could feel the air blowing out of the crawlspace hatch. Warm air filled the insulated crawlspace all winter long keeping it around 55+ I would guess. I didn’t have a thermometer so I’m guessing based on the amount of sweating I did while crawling around.
It used to be that foil tape was used to seal the joints, but this has fallen out of favor for many installers. Today, HVAC installers primarily use a type of mastic sealant that is brushed on (I have also seen similar products sprayed on).
Before this ductwork was sealed, whenever the furnace came on the spider webs hanging on the floor joists would start blowing all over. After it was sealed the air in the crawlspace remained still. And, no more air came pouring out of the crawlspace hatch.
This is a photo of the same area that was not sealed in the previous picture.
If you look closely, in the first picture the ground is bare dirt. In the second picture you can see that a vapor barrier was added. The moisture coming from the bare dirt was being heated by the warm air coming from the ductwork. The warmer the air, the more moisture it can hold.
Continuous warm moist air all year long is a great breeding ground for mold and wood destroying insects. Not to mention the spiders higher up the food chain!
Here in Ohio the winters are pretty cold and insects and mold should not be active. Winter is a time for drying, but the joists in this crawlspace were maintaining over 15% moisture content. I really wonder what it would be during the humid summer months? So, make sure any ducting in a crawlspace is sealed. And while you’re at it put a vapor barrier down as well or you can end up with unwanted house guests.