Building codes are strict, but unfortunately building codes aren't often followed closely enough.
I recently noticed in the house I just purchased that a lot of cold air was coming in behind my kitchen cabinets and appliances. I pulled out my range and noticed that cold air was rushing through the gap between the drywall and the floor. Fortunately a can of expandable foam was an easy fix.
This house is only 2 years old. Corners were cut when building which makes this home not as efficient as it could be. I'm hoping to make gradual improvements.
Even though my electricity rate is only around $0.12/kwh and my gas bill isn't high, I'd still like to do what I can both for the environment and to save money.