Cavity Wall Insulation

Should Cavity Walls be Insulated? Pros and Cons

In Technical Details by Giovanni ValleLeave a Comment

Exterior wall cavities offer a unique space to add quality insulation to increase the efficiency of your home and reduce your monthly heating and air conditioning bills. The pros and cons of insulating a cavity wall are numerous. They include all of the positives that come with insulation, such as saving money on heating and cooling electricity and gas, and a more comfortable home that maintains temperature better.

However, as insulating cavity walls has become more common in recent years, the dilemma of whether to insulate the exterior cavity or, alternatively, insulate the interior studs of the wall is often a topic of debate. While insulating the interior walls with batt insulation is easier, there are advantages to insulating the exterior cavity.

By insulating the exterior cavity you gain the added benefit of reducing thermal bridging by blocking the outside elements before they make their way into the interior walls. This is both due to the location (external) as well as the advantage of creating a continuous barrier. Unlike the internal application of wall batt insulation, you do not need to disrupt the insulation by placing it in between wall studs.

However, since cavity spaces in walls are shallow and narrow, the negatives can be that the insulation can get moldy and deteriorate. Cavity walls are effectively two walls (wythes of masonry) that have been placed near each other to produce a single exterior wall. The idea is that the air gap serves as a thermal break and allows for condensation to drain away harmlessly.

This article will go over the pros and cons of insulating a cavity wall, as well as the primary function that a cavity wall was designed to perform, and finally how an insulated cavity wall will work for your home.

What is a Cavity Wall?

A cavity wall is a wall with a hollow opening between two partitions or walls. These walls, or masonry wythes, are commonly referred to as “skins” and are usually made of stone, brick, or some other sort of masonry. The space between the masonry wythes or “skins” is a slight gap, commonly only a few inches in width, that is known as the cavity.

The cavity wall was introduced in the 19th Century in Europe and gained popularity during the 1920s. It was initially used for a particular type of function: air streams could blow into the hollow opening and create a drying action. Also, the cavity allowed areas of masonry, which tended to gather moisture, to drain out this moisture into a safe place. These areas or “weep” holes, allowed for the drying action at the bottom of walls or at the base of windows.

During particularly windy rainstorms, water can be blown up and into the walls of buildings. Having two walls and a cavity in between allows for a resistant layer against wind-driven rain. It keeps the interior of the building dry and safe from damage or moisture build-up. Since cavity walls were designed to “air-out” moisture, this was a great design that limited exposure of the inner walls to rain.

Having a gap where airflow slowly goes through can also be a form of insulation for a cavity wall. This is known as air film and can add a layer of insulation to the building where it is constructed. This gap is also what prevents “thermal bridging” since the wall materials are no longer in direct contact with one another. Both moisture and temperature transmission are essentially blocked by the gap between materials.

Having a cavity built into the walls of a building provides a good opportunity to add insulation to the wall. The air space can be filled or partially filled with various types of insulation in order to keep the home at a consistent temperature. This is easy to do because the gap in the wall is already there.

Are there Downsides to Installing Insulation in a Cavity Wall? 

Cavity walls were not originally designed to be filled with insulation. Instead, cavity walls were built as a barrier against penetrating dampness. However, over time, and with the introduction of rigid insulation, cavities have increased in size in order to accommodate insulation.

Keep in mind, however, that this may not be the case in existing structures, especially if you own an older home. For this reason, it’s a good idea to consult with an architect or contractor before proceeding to insulate the wall cavity to make sure you have enough room to work with.

Retrofitting a cavity wall with insulation that is not done correctly can create problems. If the insulation gets damp and develops mold, this can be problematic. Mold develops because of moisture staying in one place and attracting bacteria and fungus to it, where both will grow.

If insulation is left in a damp or moist, humid condition, mold can develop on the insulation. If the insulation is directly up against the walls of your home, especially at the bottom of the wall, mold can grow severely to the point of entering into your home.

What are the Criteria for Adding Insulation to a Cavity Wall?

Adding insulation to a cavity wall safely and effectively is only possible if your home and the cavity wall meet specific criteria. This is to ensure that no damage occurs to the walls, the insulation, or your home.

The criteria for installing insulation in a cavity wall include having an unfilled cavity space in the wall, having a cavity of at least 2 inches (50mm) in width – preferably wider – having masonry walls in good condition, and having walls that are not overly exposed to wind-driven rain.

Having an Unfilled Cavity Space

First and foremost, the cavity space between the two skins of the wall needs to be empty. Sometimes, in older homes, wood may have been placed in between the area as a spacer, reinforcement, or as prior-insulation. This material needs to be removed before proper insulation can be installed in the wall cavity of your home.

Having a Cavity Space of at Least 2 inches (50mm) in Width

The cavity space needs to be wide enough to allow for a reasonable amount of insulation. If the width is less than 2 inches (50mm), there is not enough space to add enough padding to really make a difference to the maintenance of climate or temperature in your home. There are insulation materials that are less than 2 inches (50mm) widths, should you decide to install insulation despite the air space limitations.

Having Masonry Walls in Good Condition

Installing insulation in a cavity wall requires a dry and safe space so that no damage or mold occurs. It is of vital importance that the masonry of the two skin walls (wythes) that make up the cavity wall is in good condition.

There should be no cracking or breaks in the masonry. Otherwise, this could lead to moisture infiltration or other problems that could damage the insulation and cause it to mold and further damage the walls.

In order to avoid moisture infiltration, the outer masonry wythe should be sealed completely with masonry in good condition, grout, and at joints with caulking. Additionally, flashing should be used in conjunction with weep holes to keep get the moisture out if it manages to make its way in.

Having a Wall that is Not Overly Exposed to Wind-Driven Rain

Wind-driven rain can come up and into the hollow space of a cavity wall. The more exposed a wall is to the full brunt of wind-driven rain, the more likely that water will accumulate in the cavity wall space.

Since the insulation needs to remain dry, it has to be installed in areas only where moisture will not be driven in by wind and rain. An adequately-sealed and maintained exterior envelope is critical under these conditions.

How are Cavity Walls Insulated?

Cavity walls can be insulated in several different ways. The most common way to install insulation in a cavity wall is to have a company come out and do the work professionally by injecting insulation material into the cavity.

If you are able to gain access to the cavity directly, you could try to do it yourself, however, this option is usually only possible during new construction or the reconstruction of your home.

Method #1: Professional Injected Insulation

The cavity wall space can sometimes be installed by injecting insulation from the outside of your home. This process is only done by a registered professional and should make no mess at all. The process requires little damage to your walls and is very useful.

The company that you hire will need to determine where the cavity walls are located. Then, they will have to drill several one and a half-inch full holes about 3 feet apart. Then, the insulation material will be injected into the cavity through the holes.

There are several material types that the insulation could be made out of. The different types of insulation material are polyurethane foam, mineral wool, and polystyrene beads. All of these options are adequate and will insulate your home in much the same way.

The polyurethane foam is much harder to remove once it has been installed, so if you are thinking about making the insulation a temporary fix, consider another material to inject into the cavity of the wall.

Method #2: Rigid Foam Insulation

Rigid insulation boards can be placed in between the two skins of walls in the cavity. In order to do this insulation installation yourself, you will need access to the cavity to slide the foam insulation into space. You will also need to choose the right thickness of insulation that fits the cavity by measuring the air space before purchasing.

This is a method that is only possible if you can get access to the cavity in the wall which, in turn, is only possible during new construction and/or reconstruction. In the case of new construction, the process is fairly straight-forward. Reconstruction would require taking down the internal wythe for access to the wall cavity.

Best Materials for Insulation Injection in a Cavity Wall

There are three primary materials that can be used to fill in the cavity wall of your home, and all of them are relatively simple to use and understand. The three elements that can be injected are called polyurethane foam, mineral wool, and polystyrene beads.

Polyurethane Foam

Polyurethane foam is one of the most popular choices for injecting insulation into the cavity wall of a home because it is easy to install and advantageous as an insulator for your home. The material itself comes in different chemical ingredients and can be mixed to create different effects.

Polyurethane Foam is also known as spray-applied cellular plastic and is made by mixing chemicals to create a foam substance that reacts quickly and expands to create insulation. Because of the expansion of the foam, it is an excellent material for sealing and providing a barrier for moisture. The foam will fill every nook and cranny in the hollow space of the and make an airtight seal. This is the best option for questionable leaking spaces.

The technician installing the polyurethane foam will drill holes near the top of the outside wall of the building and under windows. A hose is used to pump the poly-foam into the holes at strategic amounts so that the foam can expand to the maximum quantity possible and create an airtight and water-tight seal once the foam dries.

Mineral Wool

The mineral is quickly becoming a better alternative to fiberglass insulation. In many ways, mineral wool is a better product than fiberglass insulations. For example, the R-values (an insulator’s ability to be resistant to heat) is much better than that of fiberglass.

Mineral wool is also made from about 70% recycled material, compared to about 20% recycled material that fiberglass is made from. The content is hydrophobic, so it won’t absorb moisture and cause much risk of molding. 

The material of mineral wool can be hosed in pieces, just like the foam. Unlike the foam, it is a dry product that is hosed in chopped up pieces. The installation is complete once the injection is done, and the holes where the mineral wool was injected are made whole again to the best of the abilities of the installer.

Polystyrene Beads

The polystyrene beads are the most affordable option when it comes to insulating the cavity wall. The beads are injected and fill in the cavity and then solidify as a mixture after the installation is complete. Companies who install this pre-formed bead install it with an adhesive mix that will solidify the beads into the area of the cavity after drying.

The beads are completely water-resistant. Any water on the outside will drain to the bottom of the wall without harm. The beads are fire resistant and will not ruin the fire-resistant properties of the wall. 

Just like the other procedures, the polystyrene beads are injected with a hose or gun into the cavity of the wall through holes on the outer wall. Then the holes are closed up after injection.

How is Insulation Injected into the Wall Cavity? 

There are several steps that a trained technician who is installing the insulation in your wall cavity should follow in order to be in compliance with regulations. These steps include assessing the suitability of the space where the insulation will be installed, preparing the area for injection of the insulation materials, injecting the elements cleanly and carefully, and repairing any damage or mess made by the holes in the outer wall before a final installation check.

Most installers of insulation have a criteria checklist, much like the one detailed earlier in this article. The check on the outer wall and inside the home and cavity can be done by a technician who is looking for structural integrity and the cavity volume.

The prep work that needs to be done by a technician before the injection of the insulation materials takes place is the same for most jobs. The holes in the outer wall need to be drilled somewhere near the top of the wall and under the windows. Where the cavity meets another wall’s line needs to be sealed so that the insulation is appropriately airtight, and there is no structural damage.

Injection of the Insulation Materials

A pressurized gun is used to shoot the insulation materials into the holes in the outer wall. The gun will be pushed into the recess of the cavity wall so that the content is able to fill in all space in the cavity. The hose or gun is slowly pulled up to ensure a good seal and consistent filling down throughout the cavity wall. When the material is filling, keep in mind that some materials expand. So if it seems it is not fully loaded, it will be once the material expands in the cavity.

Repairing Holes and Cleaning any Mess

If the insulation comes down on the outer wall or is visible around the outer wall holes that were drilled, an installation technician should clean these places as well as possible. However, it is always best to avoid overfilling since some of the insulation materials tend to expand. Then, the holes will be repaired and sealed as best as possible with a waterproof and uniform sealant that is aesthetically and structurally done well.

The Final Inspection

The final inspection should assess whether the expanded insulation is settled into the cavity thoroughly. Also, the technician needs to make sure that there is no leaking of material into other places near the cavity. Finally, the holes on the outer wall should be checked for integrity. To check the holes for integrity and waterproof, the tech may need to run tests, but this is a standard procedure. 

The completion of insulation installation into the cavity wall of your home should bring some peace of mind for you. Not only will the materials that are used for insulating lower your energy bill and make your home more comfortable, but as long as the content is water-resistant, it will reduce water damage to your home and prevent unwanted air drafts from between cracks and wall lines.

Cavity Wall Insulation Lifespan

Most insulation materials for use in cavity wall construction are guaranteed for the life of the building. They are made to be hardened or left in place in the crevice of the cavity wall until the wall needs to be replaced. In this way, they add to the structural integrity of your building and will be a welcomed part of your home for decades to come. As always, check with the manufacturer’s specifications to ensure that it comes with a lifetime warranty.

The more your home is insulated, the better it will hold heat in, and the less energy it will take to heat or cool off your home. The walls are the place where the most heat is released from your home (compared to the ceiling and floors), so if you are thinking about getting other walls insulated, it may be more cost-effective to do all your walls at once.

Nearly 35% of a home’s energy and heat is lost through the walls. By insulating a cavity wall, you can reduce that to only 15% lost through the walls of your home. That means with a $200 heating bill, you may be able to reduce your bill by about $40 each month!


Having your cavity walls insulated is a job that is best left to professionals. This is because holes in the outer wall of your building are required, and the materials are messy if you don’t have the right tools to fire them into the cavity. It’s always best to do the job right the first time to avoid having to redo it at a later date.

If the cavity wall is accessible and meets the criteria for installation, it’s a good idea to install it prior to the completion of your new home or alteration. On the other hand, if your home is already built, you should consider having the insulation injected into your cavity wall because it can protect your home from moisture and lower your energy bills substantially!

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