Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.
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There are many options when it comes to running cables throughout your home. However, not all homes are built the same way, leaving questions on where cables can be run.
While it is generally considered an undesirable practice, running cables in cavity walls is possible. However, it is challenging and, more importantly, doesn’t always comply with building codes. Consider the type of wall you are working with, local codes, and the tools you can access when you decide if this method is right for you.
Because of the design of these walls, it can be challenging to maneuver cables through them. Let’s take a look at why it is difficult and what your options may be.
What Is a Cavity Wall?
A cavity wall is an exterior wall of your home or building. It’s called a cavity wall because it has two layers (wythes) to the wall, made of masonry, with an air space between them.
The masonry design uses two layers of brick or blocks with small metal ties and sometimes insulation that fills the air space between these layers. The metal ties are meant to offer support between the layers and keep the proper and even distance between them.
The insulation protects from heat, cold, and noise to some degree. Weeping holes are also added to cavity walls to help moisture within the wall escape. These walls are designed to allow water to flow to the bottom and out through the weep holes.
Adding cables can disrupt this drainage if done improperly. As the water makes its way down the wall, it runs into the cable. When it does, some of the water flows along the cable. Once this happens, it continues that path into your home.
You Can, But Should You?
If you are a DIY home builder, there is a good chance you have dealt with the frustration of running cables in walls. When it comes to wiring and cable management, this can be one of the toughest jobs for any homeowner to tackle.
Cables can be run in walls through a method called “wall fishing”. Wall fishing is the common name for pulling cables through a wall. Here are some reasons why cavity walls aren’t always the best choice for wall fishing.
Reason 1—Building Codes
Due to internal wall construction and materials, the process is not only difficult, but it may not abide with building codes. This is because if it is installed improperly, cables can direct moisture into the home and away from the drainage elements. This means damaged drywall, electrical concerns, damaged carpet, or even mold if things aren’t done right.
In addition to potential moisture exposure, thermal bridging can occur where the cable comes in contact with the interior wall. Long vertical drops can also have a detrimental effect on cables and wires. Unsupported wires will be at risk for unforeseen damage to occur. This may not become visible until many years down the road.
Cables can be damaged during installation if there are obstructions in the cavity wall. Damage to cables during installation can happen easily, especially when metal wall ties or mortar projections are in the way. The damage that occurs from this is unseen until it has already happened and will end up with an issue later on.
The risk of damaging a cable while installing it also increases if there’s something obstructing your path inside the cavity such as metal wall ties or mortar protrusions – these problems should be taken care of before you start pulling wires through.
It may seem straightforward at first, but getting cables between the interior walls and insulation can be difficult. Without proper tools or a fair amount of experience it will take you hours to get through this task. If not frustrating enough already then there is always one more thing that makes an otherwise simple job extremely hard: spaces in your wall cavity are never uniform!
Getting your cables between the interior walls and insulation is essential and can be very difficult. Without proper tools and a fair amount of experience, it can take a lot of time. Even then, it’s a very frustrating task. Here is a video of experts making it look incredibly easy.
In order to avoid any costly mistakes, it is important that you have the proper tools. Proper specialty tools can get expensive quickly so be sure to budget for this accordingly when choosing what to use.
Additionally, while some tools and methods do make it possible to get through the space, obstructions, and insulation safely, it is generally better to run the cables in interior walls if at all possible.
Insulation is not just difficult to deal with, but it is an important part of your home. Disrupting your insulation with fish tape or fiberglass rods can leave uninsulated spots in your wall and create pockets for moisture to build up. In addition to moisture damage, this can result in improper insulation and higher energy bills.
The capacity of the cables can also be compromised when installed near insulation. Cables should be installed where they will not come into contact with thermal insulation unless it is ensured that their current-carrying capacity can get through safely by, for example, increasing the cross-sectional area of conductors.
If You So Choose, What Tools Do You Need?
If you decide to go ahead with wall fishing, these are the essentials.
Since your cables usually run through walls from an attic or crawl space, you’ll need to take accurate measurements to line up your cable path. Measuring distances from joists, doorways, or lighting fixtures helps.
Drill and Bits
You will need this to make a hole through your header or footer, drywall, and wall. Unless you have steel or masonry construction, your drill and wood bits will get you through and into the wall cavity.
If your cavity wall is two masonry layers, you will also need to have masonry bits and a hammer drill to get through.
Masking tape, electrical tape, or even duct tape will work. You will use this to wrap your cable and the pulling device securely together.
This system uses strong magnets to fish cable. The magnetic lead is attached to the cable and dropped into the cavity. The magnetic roller is used on the outside of the wall to pull the lead and cable down to the opening at the base of the wall.
Using these with metal studs or wide masonry is nearly impossible. These are only effective when used in walls with wooden studs. They are also an expensive tool if you’re not going to be using it often.
Fiberglass rods are a good alternative to the Magnepull. They can be pushed past insulation with a practiced hand, give rigid support while still being flexible, and often come with a glow in the dark option.
While flexible, they have their limits in tight places. Bending them too sharply can cause them to splinter.
The fish tape is an old and flexible tool. Contained in a round housing that coils a length of strong but thin, metal “tape” in various lengths. The rigidity and flexibility have made it a time-tested favorite for getting through tight spaces.
While they are rigid, the fish tape flexes easily. This means that they don’t do as well when pushing through constant resistance like insulation. A lot of starts and stops, so you’ll need some patience.
A flexible drill bit has a large bit on a long, flexible shank. These are wonderful tools when you have to drill down through headers or fire stops in very tight spaces. The flexible shank allows the bit to spin as you bend it into corners.
There is a learning curve with using these bits. They often require an extra tool to use them with accuracy.
How Do You Run the Cables?
If you have your tools, it’s time to get started.
1. Measure– Find the spot where you want the cable and measure to a landmark or distinguishing feature you can find from the attic, crawlspace, or basement. This can be a corner, light fixture, pipe, or previous cable run.
2. Make the Holes– After being very sure of your measurements, drill down through the header or up through the footer. If your layers are both masonry, you will have to come in from the header, as the footer will likely be covered in concrete.
After the entry hole is drilled, make your hole in the interior wall. If you are using a faceplate or wall box, be sure to make the hole to the proper size at this time.
3. Feed Cable– Attach your cable with tape or another method and feed it into the hole. Use your preferred device and follow the instructions. When using a Magnepull, one person will need to feed the cable as another draws the magnet down with the roller.
It is important to remember that you need to get your cable between the interior wall and the insulation. Take a look at this video for a demonstration.
4. Go Slow– Carefully feed the device towards the other end. If you hit an obstruction, don’t force it. Back out and start over, so you don’t get stuck or lose the end of the cable from your device.
The insulation will likely bunch up if it gets caught, so you will have to pull back, maybe give it a wiggle, and try again. This may take several attempts.
5. Pull Through– Once you reach the other end, carefully pull the cable through the other hole. Make sure to run it through your wall plate before terminating the ends on your cable.
Ready to Run
So, can cables be run in cavity walls? Yes, but be sure to weigh your options and pick what is best for your situation and as always be sure to check local building codes to ensure that you don’t violate any requirements. Keep in mind all the challenges that have been discussed in this article, and, if at all possible, opt to run the cables within interior walls.
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