Graphic of an outline of a house with the caption "How to Add Insulation to Existing Walls"

If your heating and air conditioning costs are out of control, you might not have enough insulation in your walls. Insulation is typically built into a house when it’s under construction. But thankfully, there is a way to go back and add it to the walls of older houses that need more insulation.

Most often, this is a project you would hire a contractor for. But it is possible to DIY if you have the skills and access to the right tools.

How Do You Add Insulation To Existing Exterior Walls?

To add insulation to any exterior wall, holes are drilled between the studs of the existing walls. Then, spray foam, cellulose, or another form of loose-fill can be blown in to fill up the wall cavities. Finally, the holes must be plugged up and refinished to match the rest of the wall.

Types Of Insulation

First, let’s take a quick look at the different types of insulation that are commonly used so you can be better prepared to find what works best for your house.

  • Fiberglass: This is made from fine glass fibers, so you will need to wear protective gear if you install it yourself. Fiberglass is one of the most common types of insulation, and it is naturally fire-resistant.
  • Cellulose: This is typically made with recycled paper products. A chemical is also added to make it fire and insect-resistant. Cellulose is eco-friendly and affordable.
  • Natural fiber and mineral wool: These are less common but have similar properties to cellulose and fiberglass.
  • Foam: There are many types of foam insulation, but spray foam is most often used in existing walls. Foam is not fire-resistant.

The right insulation choice for your home depends on where you live and the construction of your home, among other things. (Read our comparison of cellulose vs. spray foam, here) Of course, it’s possible for a single home to have multiple types of insulation in different areas.

How To Add Insulation To Wood Or Drywall Walls

Whether your house has a wooden exterior or drywall on the inside, this method works for both.

First, you are going to look for the studs in the wall and make sure to mark where they are. The studs are the upright wooden beams in your walls that support your home. Once those are marked, you should drill a two-inch hole near the top or middle of your wall between all the studs. If you drill it in the middle of the wall, you will have to add another hole near the top, but not until after the middle hole is filled.

Photo of an unfinished room with the wall studs exposed and uninstalled doors leaning against the walls. Wall studs have to be located in order to add insulation.
On unfinished walls, it’s easy to see the studs. It’s also easy to see why you have to drill between each of them in order to fill up the whole wall with insulation.

Now that you have your holes, you have two options to insulate your house. You can blow in insulation or spray foam. As we mentioned above, fiberglass and cellulose are two of the most popular loose-fill insulation materials. But insulation is also made from other natural fibers as well as mineral wool. You can mix insulations in your house, or you can stick with the same type of insulation that is already there. For instance, if your home has old fiberglass insulation that has settled, you can add more fiberglass to keep the type of insulation the same.

You can also get spray foam. Instead of letting gravity pull the loose insulation down, you can easily fill in all the nooks and crannies of your walls with expanding spray foam. It will also harden and won’t settle later. However, it costs a lot more.

Whichever insulation you choose, you will use a tube to blow or spray the insulation into your walls through the holes you created. Once each wall cavity is full, some people will use a premade wooden plug to clog the hole. You can also fill it, smooth it over, and paint it to match the exterior wall. If you did decide to start with holes in the middle of the wall, it is after this step that you would go back and cut more holes at the top to fill in the rest.

Here’s a video explaining the process to help you visualize it:

Screenshot from  video explaining the process of adding injection foam insulation to existing walls
Courtesy of RetroFoam of Michigan

How To Add Insulation With Standard Siding Walls

Now, the standard siding is those long panels that are layered on the outside of homes. This is similar to the installation process for wood and drywall. First, you are going to go halfway up the wall and remove two of those long panels. Then you will be faced with wood, concrete, brick, OSB, or drywall backing. Find and mark the studs before drilling 1.5 to 2-inch holes in between the studs. Be careful not to drill into the studs because it could damage the supports to your house. Watch out for pipes, too!

Next, you are going to use your blow-in or spray insulation and fill the wall cavities all the way up to the holes. Since this part will be covered by the siding you removed, you really just need a wooden plug for the hole. Finally, you can put the siding panels back on and repeat the process for the top half of the wall.

How To Add Insulation To Brick Walls

If you have a brick house, this is where it gets a little bit tricky and more complicated. First, you are going to split the wall into three sections: the top, middle, and bottom. In the middle section, drill three 1/4-inch holes in the mortar joints.

The mortar joints are where the corners of the brick meet. Drill the holes, and then fill the wall with insulation. To get behind the wall, you’ll need a tube that goes into the hole you made. That’s because these holes will be deeper to get through to the other side of the wall. You are not going to drill holes in the brick, just the mortar at the corners of the bricks. These holes should be as deep as the mortar layer.

Now you can fill in the hole with more mortar to seal it up. The best part is that the holes you fill will match the mortar, with only a slight difference in the color from sunlight and aging. You’ll then repeat this process for the remaining existing holes along the wall.


Did you know it’s more eco-friendly to retrofit your old home than to build a highly efficient new one? Whether you hire a contractor or do it yourself, insulation is a huge step in making older homes more energy-efficient. You’ll enjoy a more comfortable home year-round and your bank account will feel the difference as well!

Leave a Reply

Your email address will not be published. Required fields are marked *