Your Home Might Need External Insulation, Too

If you’re going to replace the siding or cladding on the exterior of your house, you should take the opportunity to add external insulation for a more comfortable house and lower energy bills.

A home that’s warm and cozy in the winter and cool in the summer is a place you can relax—a refuge. A home that’s too hot or too cold all the time will never be comfortable, and can be a drain on your pocketbook: Up to 70% of the energy you pay for in your home is used to heat or cool it, and heating your house likely accounts for about 30% of your utility bills.

How well your home is insulated has a huge impact on its energy efficiency, and on your monthly bills. Adding more insulation to your home can cut heating and cooling costs by an average of 15%, making it a no-brainer home improvement project. Typically homeowners focus on adding insulation inside the house (in the attic or inside the walls), but there’s another strategy to make your home even more energy efficient and comfortable: adding insulation to the exterior of your home.

Upping the R-value

Considering exterior insulation makes sense if you’re going to be removing or adding siding or cladding to the house; the exterior of the home will be exposed anyway, making it the ideal opportunity to improve your home’s overall insulation economically.

Exterior insulation is like putting a sweater on your house, creating an additional barrier to heat loss or gain. It’s a straightforward concept: Once your home’s exterior walls are exposed, you can affix a material like rigid foam insulation or mineral wool batts to the walls, seal it, then cover it with the siding or cladding of your choice. If your home is always chilly in winter or too warm in summer, adding this extra layer of insulation can potentially offer a dramatic improvement.

The benefits depend on the R-value you’re adding, and the climate zone you live in. If you live in Nebraska, for example, you’re in Climate Zone 5, and if your home has 2x6 exterior walls, you’re shooting for an R-value of 19-21. R-values are additive, so any insulation you add to the outside of your house will improve its overall R-value—and a single inch of foam insulation can increase the R-value of your home by 5. If your home already has an R-value of 20, that would represent a 20% increase.

What to consider

One huge benefit to adding insulation to the outside of your home is that you can avoid disruption inside the house. You won’t need to drill holes in your walls or remove the drywall entirely. All the mess and trouble stays outside while the interior of your home remains clean and undisturbed. If you are replacing your siding or cladding anyway, the project shouldn't add to the disruption at all.

There are a few things to consider before you jump into adding exterior insulation, however:

• Cost. The cost of adding insulation to a siding project will depend on how much wall you need to insulate. For example, a 4x8 polyisocyanurate board from Johns Manville costs about $26, so you can measure the square footage of your exterior walls to get an idea of how much the materials will cost. But you also have to factor in complexities like windows, soffits, and other quirks that can add or subtract materials. And then there are the labor costs of installing it, which will vary greatly; you can expect to add a few thousand bucks to a siding project by including exterior insulation.

• Thickness. Adding insulation means everything else gets bumped out by a few inches, which can have an impact on things like trim pieces around window wells or exterior outlet boxes. There are plenty of products that can compensate for these complications, but you’ll want to be sure that your contractor is aware of potential issues and has a plan to address them.

• Moisture. Adding insulation between your siding and the framing of your home can improve the moisture infiltration, because it adds an extra gap between the surface and the interior. However, if you use a rigid foam board that has a vapor barrier of some kind on the outside, this can cause problems because you’ll be sandwiching the insulation between two vapor barriers, trapping moisture below the siding. That’s why it’s vital that you consult with an expert when choosing insulation materials for a project like this.

If you’re already going to be removing the siding or other cladding from your home, taking the opportunity to add some exterior insulation can make a big difference in the comfort level of your house—and the size of your utility bills, all without a lot of mess inside.