u/halboozus

I recently noticed an area behind a hvac duct that did not have any spray foam installed on it when the house was originally built. The missing spot is on the roof deck behind a hvac duct that was hung from a rafter. Looks like open-cell was used in the house. What is the best route to fill in this missing section myself? Can I use the great stuff cans from the hardware store to do this even though that is closed cell? The missing area is about 1 ft long and 6 inches wide, but it is on the underneath of the roof deck.