In your own home, the results of air flow through exterior walls may be observed through feeling the difference in the floor temperature on a cold day using bare feet on the floor near an outside wall vs. in the center of the same room. In an older home, the difference in temperature is often quite extreme.
Air sealing has been an important consideration in residential work for longer than the 22 years I have been a carpenter. There has been much trial and error on the path to discovering appropriate methods to insulate and air seal an older home. Insulating and air sealing is as much of an art as it is a science (more in some homes than in others).
Newly constructed homes can be designed to integrate insulation into the structure for efficiency. It is fairly simple and inexpensive to air seal in new construction. In remodeling projects on older homes it is much more complicated to air seal. There can be air leaks from areas that are not in the scope of work and one must weigh the cost of addressing these areas with the expected return. At a minimum, it is crucial to address what is accessible and not overlook or disregard opportunities simply because it is difficult to access an effected area.
In remodeling attic spaces the we have had the opportunity to try different methods of air sealing, the thermal insulation is relatively simple once drafts are stopped. One approach that we found to be very effective is known as “hot roof.” It provides one plane of insulation, the underside of the roof, rather than trying to waeve the envelope around walls, floors, and ceilings. This approach still requires stopping airflow through wall spaces from the basement to the upper levels (AKA "stack effect.") One could run around for days on end caulking and sealing all the seams and penetrations in the top of the walls, which would have great results provided there is access to all the required leaks, and the budget to pay for the labor. In some cases, that is exactly what is required, especially in cases when areas not otherwise involved in the renovation are negatively impacting the efficiency of the insulation. However, we’ve found spray-in-place foam insulation to be a fantastic one stop solution. It both stops the movement of air and it insulates. It can usually be installed in hard to reach spaces, and has the ability to offer almost twice the r-value of fiberglass batts. Its cost to overall efficiency ratio is difficult to compete with as well.
Overall there is no one method that is the perfect solution. Every home needs to be approached with a fresh set of eyes and an open mind while drawing on the past successes and less than successes. The method outlined here seems to work well in the older homes in our area but its critical that we remain observant and honest in the process to figure out where it will not work and where ther may be better, albeit more challenging, means to the same end.