The house operates as a system, so when you make certain changes to it.... For example, an easy one that people would understand is when you tighten a home for airtightness, you need to add ventilation. You put in heat recovery so that you still have fresh air, but now you have a more energy-efficient home. But if you thought you were just going to tighten your home without adding that, then you create unintended consequences, such as health issues.
The same holds true for many other things. For example, adding more insulation in airtightness is good for extreme heat, but only if you have air conditioning. It helps your air conditioner be more efficient. What we're seeing with super efficient homes—we have a net-zero energy council; we've labelled over 1,200 net-zero homes in Canada and we're working hard to get this right—is that super energy-efficient homes can also overheat more easily, so you have to adjust. You have to create overhangs. You probably have to put in air conditioning and those types of things.
There are other things you do to prevent fire. There are issues in western Canada with what's called “attic rain”, where one municipality creating a bylaw to stop one thing resulted in another issue.
You need to make sure your co-changes are really well thought through. You do your cross-disciplinary work, so that your house continues to work as intended.