Is it a good or bad idea to insulate house walls if the exterior is brick and the interior is plaster?

Dear Angie:

Is it a good or bad idea to insulate house walls if the exterior is brick and the interior is plaster? It's been recommended that we insulate our walls. The house was built in the 1940s. The interior walls have some very fine cracks, not a lot, but a few. We're concerned about what could happen if the plaster is drilled. And we're wondering just how much energy would be saved, as we have a high efficiency gas furnace, Energy Star windows and doors, and R38 insulation in the attic.

- Colleen D., Seattle

Dear Colleen:

Congratulations on the steps you've taken to make your home more energy efficient. You've invested a lot of time and money already in so many different ways, but if you have uninsulated exterior walls, you likely haven't fully sealed your home's "thermal envelope."

Adding insulation in plaster walls with a brick exterior can be a challenge, especially if there are large cracks in the plaster. Chances are you'll need a professional to address any large cracks before and after the job.

An experienced insulation professional should enlist the help of a good plastering pro. Together, they should be equipped to handle most retrofit scenarios.

I recommend you talk to three insulation companies in your area with good reputations and enlist their advice. Ask how they would handle your situation so you're sure they have the experience and skills needed for the job.

As for how much energy you save, adding insulation to those walls could make a significant difference and pay for itself within the first year or two.

Because you're clearly investing in making energy efficient improvements to your home, I recommend you have a professional energy audit done.

An auditor - one who is independent and doesn't sell insulating products or other products or service - will perform an unbiased evaluation of your home's energy usage and where you're losing that energy. He or she can tell you which walls should be insulated and which shouldn't and provide data supporting the savings from making those changes.

Energy audits can cost from $300 to $800, depending on the scope of the testing, but many utilities offer discounts, rebates or incentives to have an audit done. Seattle City Light, for example, offers discounted audits to its customers.

If you have combustible appliances, be sure your auditor performs combustion safety testing to ensure the changes you make aren't sealing up the home so much that carbon monoxide backdrafts into the home.
