The Cost of Air Sealing - When is tight, tight enough?

When looking at energy efficient renovations we as Energy Advisors are looking for the lowest cost, greatest savings changes.  Trying to get the biggest bang for your dollar first, because we all know the law of diminishing returns. 

When it comes to air tightness we are often asked how tight should I try to make my home.  We must look at not only the energy saved, but also the cost to reach that airtightness.

One note off the top: Air tightness is also very important to healthy indoor air quality. Leaky homes are uncomfortable and with no control of ventilation, homes get too much in the winter and get dry and drafty, and stuffy in the summer with no wind.   By controlling air tightness AND adding a ventilation system the homeowner gains control of the fresh air in the home and can always get the right amount. 

Some air tightness comes with changes to the mechanical system or window upgrades. The tightness in this case is "free", it was included in the cost of the new windows for example.  Additional air tightness comes with a cost of labor and materials and these can vary widely. Each situation and set of costs must be evaluated on a case by case basis, but here is an example for discussion.

Energy savings from air tightness is a linear relationship. The tighter the home, the more energy saved in a nice consistent pattern.

You could look at this and say, just make the home as tight as possible, it doesn’t matter, but that fails to take into account the increase costs of making the home tighter.

The cost for reaching tighter and tighter levels increases substantially.  But typically remain cheaper energy savings than many other upgrades such as solar panels, air tightness should always be addressed and pushed as tight as possible first and combined with a good ventilation system.

But lets say you've done some sealing work and reduced air leakage 80%, should you go further? Here the costs curve in irrelevant, we only have to look at the next step.

In this example energy model improving from 1 Air Change per house to 0.5 ACH. Although a 50% reduction in air leakage, because its linear only 0.7 GJ of energy is saved for 0.5 ACH reduction.  Same as if you when from 5 to 4.5. If this improvement costs an additional $1500 for advanced air sealing practices, or lots of labor to complete, the cost for energy saved is an incredible $2000/GJ, that is a lot of cost for a little further savings. The math says at that point you would be better off spending that $1500 on more solar panels ($500/GJ saved) or adding a drain water heat recovery pipe ($800/GJ saved). 

These numbers are for example only, but shows the importance of using energy modeling and good costing. An experienced Energy Advisor is an amazing resource in planning energy efficiency renovations. The Canadian Association of Consulting Energy Advisors represents some of the best EA's in Canada. Check their website to find members in your area.