Adding insulation to your exterior walls can increase your home’s energy efficiency significantly, saving you energy dollars every year. Some older homes were insulated with poor performing and outdated insulation materials that allow heat and cold into your home.
Adding insulation to an exterior wall can be complex and time consuming, but using the correct insulation materials for the job makes it easier. The size, number of floors, and location of the exterior walls can affect which strategy you choose.
Here we hope to offer a guide to insulating exterior walls, and discuss the most common methods used in modern construction. We’ll also offer practical tips for adding insulation to an exterior wall the way the pros do it.
Which Types Of Insulation Are Good For an Exterior Wall?
First, we should mention that insulating an exterior wall is much easier when the home is under construction. If your home is being built and you need some guidance on which insulation to use, check out this article.
Exterior wall insulation does more work than interior wall insulation, so there are minimum thicknesses required. Building codes may vary depending on where you live, but here’s a general guideline for deciding how much insulation you’ll need for an exterior wall:
- 2” x 4” Wall thickness: R-13 to R-19
- 2” x 6” Wall thickness: R-19 to R-21
Some home designs have even thicker walls. Homes using passive house building design may have walls up to 36” thick and contain as much as R-60 in insulative value. Passive home designs are built to have a net zero carbon footprint, so they create as much energy as they consume.
Here’s a list of the most common types of insulation added to existing exterior walls, and the pros and cons of each:
Spray Foam
Pros:
- Offers the most R value by volume
- Does the best job of sealing tiny air gaps and leaks
- Doesn’t sag or compress over time and lose its initial R value
- Can last as long as the home itself
Cons:
- Both the materials and installation are among the most expensive options.
Loose Fill Fiberglass
Pros:
- Loose fill takes advantage of gravity and allows the material to gently float to the surface. This traps air pockets with the fiberglass fibers, creating the R value without significant weight
- Relatively inexpensive, especially if you perform the work yourself
- The tools are often provided for free
- Readily available at any home center
- Has a high ignition point
Cons:
- Can absorb moisture, leading to mold and mildew
- Can settle over time, leaving an uninsulated space at the top of the wall
Loose Fill Cellulose
Pros:
- Relatively inexpensive
- Can be done DIY
- Tools can often be rented instead of purchased
- The colder the outside temperature gets, the more effective the insulation becomes
- Offers very good resistance to flame
Cons:
- Settles over time
- Can absorb moisture if air gaps are not sealed properly, leading to water damage and mold
Loose Fill Stone Wool (Mineral Wool)
Pros:
- Excellent resistance to flame
- Lasts virtually forever
- Can be added to as needed
- High resistance to mold and mildew
Cons:
- Expensive
- Tool rental less common than for fiberglass or cellulose
Stone Wool Batts (Mineral Wool)
Pros:
- Excellent resistance to flame
- Lasts virtually forever
- Can be added to as needed
- Can be installed using compression, adhesives, or mechanical fasteners
- High resistance to mold and mildew
Cons:
- Expensive
- Often more time consuming to install due to the accuracy required
Rigid Foam Boards
Pros:
- Inexpensive
- Easy to work with
- Lightweight
- Available in several R values, which can be adjusted depending on the thickness
Cons:
- Difficult to install unless the exterior material is removable, or the interior walls are open
Depending on the exterior construction, you can use rigid foam board insulation, spray foam, batts, or loose fill insulation in an exterior wall. If the exterior covering is removable, such as vinyl siding, hard board, or fiber cement board siding, rigid foam boards and batts are an inexpensive option.
Homes with exteriors that cannot be easily removed, like brick, stone, and stucco, often forego batts and foam boards for loose fill or spray foam. Loose fill or spray foam can be installed from the inside, but this option requires repairing and repainting the drywall. Spray foam can also be installed from the outside, but the process requires drilling holes that must be refilled.
Which Types Of Insulation Can a DIYer Install?
If you have a good set of tools and experience in DIY projects, you can learn to install any of the common insulation materials, like foam board insulation and spray foam. Installing batts and operating a hopper are within the skills of a dedicated do-it-yourselfer looking to expand their skills.
Loose Fill Fiberglass and Cellulose
Loose fill fiberglass and cellulose is done by making the material airborne using a hopper, and allowing the material to loosely fall to the surface. This method allows the loose fill material to trap pockets of air between the fibers, creating the thermal barrier.
Installing loose fill insulation can be a little tricky at first because you are directing the material, as well and turning the blower off and on. Be careful when operating a hopper for the first time, as you may be on a ladder, walkboard, or scaffolding.
Loose fill insulation can be installed by yourself, but having a helper feed the hopper is the best option. You’ll avoid trips up and down the ladder, leading to early fatigue. Also consider the temperatures you’ll be working in and take frequent breaks.
Foam Insulation Boards
Foam insulation boards can be an option for insulating an exterior wall if the exterior facade is removable. Siding is generally the easiest to remove and replace, but masonry exteriors are not. If you want to install foam board insulation in an exterior wall covered by masonry, you’ll need to install the foam boards from the inside.
Most forms of siding, like vinyl, hardboard, and fiber cement, are simple to remove and replace. Sometimes it is easier to remove the siding and wall sheathing from the exterior and install rigid foam insulation boards from the outside than to install them from the inside.
Foam insulation boards can be attached with button cap nails or adhesives, and are placed between the wall studs (the vertical lumber that makes up the wall), not on top of them. Foam insulation boards can be combined with other forms of insulation, like batts and spray foam.
Foam boards can be built up, making it easy to control the R value. Adding the bead of adhesive between the boards not only bonds them together, but also traps air that acts as an insulator. This method is also useful for reducing noises that can transfer through the framing.
Spray Foam
Spray foam is a common choice for adding insulation to an exterior wall because it can be done from either side. Exterior installations usually rely on a horizontal row of holes, through which the liquid foam is pumped.
Installing spray foam from the outside often requires at least two sets of these holes. The foam could over- expand inside the wall, so most pros pipe the foam into the bottom half, and then come back and finish the top half.
Installing spray foam insulation from the inside is very simple, but practice is required. The spray nozzle mixes two chemicals together, which forms the expanding foam. You’ll need some experience before installing spray foam yourself to avoid overfilling the wall voids.
Stone Wool (aka Mineral Wool or Rock Wool)
Stone wool can also be blown-in like fiberglass and cellulose. Stone wool has the advantage that it does not hold as much moisture as fiberglass or cellulose, so mold is less likely to grow. The material is spun from actual rock, so as an insulator, its resistance to fire is unmatched.
Stone wool is a bit more expensive than other forms of loose fill, and most installations require a professional. Installing stone wool requires the use of personal protective equipment (PPE), and the training to use it.
Stone wool is often installed using compression, instead of a fastener. This method requires very accurate measuring to avoid wasting the material. Stone wool can also be glued or attached with button cap nails, but this method can allow air gaps that leak conditioned air.
What Are the Most Common Mistakes Made When Adding Insulation to Exterior Walls?
Those new to adding insulation to exterior walls tend to make the same mistakes, such as:
- Inaccurate measuring. Inaccurate measuring creates waste, which can end up in a landfill. Measuring and cutting accurately ensures you’ll fill in the air spaces that leak air, and reduce the amount of waste you generate.
- Selecting inappropriate materials. Choose a form of insulation that provides the R value you need, but still fits within your budget.
- Underestimating the size of the job. Underestimating the size of the project can cause you to abandon the project due to time and cost constraints. Have a backup plan (like hiring a professional) in case the unexpected happens and you cannot finish the job.
- Sloppy work. Accuracy, especially with materials like stone wool batts, is critical to the performance of the insulation. Don’t rush, or get sloppy, in your work and the insulation
- will perform at its highest potential.
How Do I Know When to Call a Professional to Add Insulation to an Exterior Wall?
There are situations when the job is too complex or large to perform the work yourself. Exterior insulation projects can require advanced skills, tools, and other machinery that are not feasible to own or rent. In these instances hiring a pro will be less expensive in the long run, plus you’ll get a warranty or guarantee.
For instance, homes with stucco, stacked stone, and brick are more difficult to insulate from the exterior than others. Often, these jobs require too much time for a DIYer to commit, so they go unrepaired until more damage occurs.
Adding insulation to exterior walls can be performed by a do-it-yourselfer. But when the job will require an investment in tools, training, and hired help, the most cost effective option can be to hire a local insulation company you can trust.
As mentioned earlier, experienced homeowners have learned that catching and resolving exterior wall insulation problems early is the least expensive option. The pros will get the job done quickly, expertly, and probably offer a warranty or guarantee.
If you need professional help right away, requesting a free estimate from a local insulation company is a great first step.
