We live in a split-level zero lot line home. We just had the waste pipe coming from our upstairs bathroom repaired. It runs down through the wall between our garage and downstairs family room. The pipe had cracked when the house "settled" and the stud that it passes through put pressure on it. The pipe is fixed, but the drywall is missing on the garage side of the wall where the plumber accessed the pipe. The new section of pipe sticks out a bit ****her than the old pipe, so drywall will not fit flush over the opening. To complicate things, the section of wall the waste pipe runs down through is also our cold air return (originating from a vent on the baseboard of the family room side). The air return is approx. 32" wide with a stud down the center and the waste pipe running through one half. Right now I can see through the air return vent to the garage. I have plastic sheeting covering the opening temporarily. My questions are: Can I cover the garage-side opening with plywood instead of drywall? What about insulation? The wall is insulated except for this section since it is the cold air return. My concerns are keeping the cold garage air (it was 5 below zero last night) away from the pipe so it won't "freeze", and also sealing the cold air return effectively. Any suggestions would be greatly appreciated!