"duct tape" - this is what it is actually designed for! Though actually I would use a good metal tape like:
However, would that explain 30% difference in energy usage? No. Maybe 1%, if that, since any leaked air is still going into your house and not to to the outside.
The typical utility company comparison is based on very broad comparisons. At best, they will compare single-family-home to single-family-home and townhouse to townhouse. I've never seen any that actually take square footage or other factors into account.
Your 30% difference could easily include one or more of:
- electric vs. gas hot water
- electric vs. gas heat
- electric vs. gas kitchen appliances (e.g., I have a gas cooktop, but the house originally had electric and most of my neighbors still do)
- number of people in the house (more people = more computers, lights, cooking, etc.)
- typical temperature setting for air conditioning and whether you set back the temperature during the day (well, in the old days when people went to school or worked outside the home...)
- quality of windows
- lighting - incandescent vs. fluorescent vs. LED
and many other things. Bitcoin mining in your basement?