How to Insulate Ducts in a Conditioned Crawlspace
- 1). Seal the ducts with duct mastic. Coat all open breaks or connections with the mastic. Allow the seal to dry; the mastic should set after a day. Use a fine hair or lit candle to see if there are still any noticeable gaps or leaks in the ductwork; airflow from any openings in the ducts will make either the candle flame or hair flicker.
- 2). Open the foil backed insulation roll. Calculate the amount of insulation you'll need by multiplying the length of the duct by its diameter, and then multiply the product by 3.5. Divide this number by the width of the foil backed insulation. This quotient is the linear footage of insulation necessary.
The insulation has an insulative rating, or 'R-value. The higher the R-value, the better the insulation. Ductwork should have an insulative rating between R-8 and R-12. - 3). Fix the end of the foil backed insulation to one end of the ductwork with foil tape. The foil backing should be visible when wrapping ducts. Wrap the duct with the insulation in a spiral, like a candy cane, with moderate overlap. Fix the insulation to the ductwork periodically with foil tape.
- 4). Tape the seams of the insulation with foil tape. This and the insulation's foil backing create a reflective barrier from the ambient temperature. After the installation has been sealed with foil tape, check to see if there are any remaining openings or gaps in the ductwork and insulation. If there are gaps, this creates a thermal bridge between the ambient crawlspace temperature and ductwork, greatly reducing the efficiency of the insulation. If there are any openings or gaps, wrap the foil backed insulation over them, and tape the insulation.
- 5). Monitor your energy bills. The efficiency of the insulated ductwork will be apparent after a month. If all ducts in your house are insulated, your energy bills will drop between 10 and 25 percent.