Electrical – Using a 16A energy monitor in a 10A socket

As you'd need to maintain those $370/year savings for 54 years to gain any return on your investment you do need to consider other factors.

1. As you point out - you need to work out if it significantly increase the value of your home. Another side to this though is would it make your house significantly easier to sell? In a buyers market if you can offer things that others can't - full insulation, double glazing etc. then you might find a buyer more quickly and one willing to meet your asking price.

2. Pure environmental altruism is, by definition, impossible to put a figure on. Only you can decide whether $20,000 is an "investment" you want to make.

Finally, something that has just occurred to me. By doing this work you might get ahead of any future legislation requiring you to fit double glazing, extra draught-proofing etc. So if you can afford it now you won't have to make the investment later when you might not be able to afford it or when prices go up because there's more demand. The downside to this is that future laws might be stricter or have tighter limits which would mean that you still need to do some work.

Plugging air leaks in you home is a bit like sticking fingers into a leaking dyke. Plug one hole and another one breaks through. Generally, the situation should have improved with new windows and doors (if installed properly).

The technician doing the post-audit should have identified where the air was coming from so you could fix that too.

One possibility is that a fire place flue or external fresh air duct to your funrnace was open during the blower test. This would have let a huge amount of air in. Other things to look for are stuck flapper valves in bathroom and kitchen vents.

Another place to check and seal would be around the sill plate. Often there are a number of small air leaks here. Individually they are not large but in sum they can amount to a significant air leak.