Is It Safe To Put Insulation Over Electrical Wires In The Attic?

We’re all familiar with the fact that electricity generates some amount of heat as it travels through cables. For this reason, some homeowners understandably get concerned that insulating wiring, especially in the attic, might cause overheating and potentially start a fire. Is this the case?

If your attic has electrically insulated wiring, it’s safe to lay insulation over it. All modern wiring types, such as Romax and THHN, are electrically insulated. However, older, uninsulated types like knobs-and-tube (K&T) should never come in contact with attic insulation.

Modern wiring is designed to handle the heat created by the electrical current, and adding insulation will not increase the risk of a fire. But the type of insulation and the installation method are important factors to consider. Let’s take a look at how to insulate your attic with electrical wiring safely.

Can You Cover Electrical Wires With Insulation?

Electrician working with wires at attic renovation site

To understand how insulation and electric wiring interact, it’s essential to know a little bit about how each part works.

Thermal insulation is usually made from fiberglass, cellulose, or foam. It traps the air and slows down the movement of heat, keeping your home warm in the winter and cool in the summer.

There are different materials with different fire ratings and R-values, a measure of how well the material resists heat flow. But generally, all of these materials are treated to make them fire-resistant.

Now coming to electric wires — wires are made in layers, and electricity flows through the inner metal layer, encased by protective insulation like PVC. The heat and current stay inside the wire, so any object touching it isn’t affected. The same is true for insulation wrapped around wire.

Since the insulation doesn’t come into contact with live wiring due to electrical insulation, it will not overheat or catch any electric spark.

However, knob-and-tube or cloth-covered wiring, popular back in the 1920s-40s, didn’t have any insulating cover. Although cloth wiring is comparatively safer as long as it’s not damaged, it can still overheat if insulation is installed over it.

If you have one of these older types of wiring in your attic, you can’t cover it with insulation. But they’re almost extinct; it’s rare to find such wiring in any home today. Still, it’s a good idea to check if your wiring is too old for insulation.

So, to summarize, you can safely put insulation over electrical wires in the attic as long as your home has electrically insulated wiring and you know the proper installation methods. In any case, consult with a professional insulation installer to get the best advice for your specific home.

What Is Electrical Insulation?

In the context of electrical wiring, insulation is a material that is used to cover the live wire. It’s always a non-conductive material, meaning electricity cannot flow through it. This is important because it keeps the heat and current inside the cable, preventing any effect on objects coming in contact with the line.

You can easily identify electrical insulation in your home; it is the solid rubbery or plastic cladding covering the metal wires.

The most common types of insulation are PVC (polyvinyl chloride), PE (polyethylene), and Nylon. All are electricity resistant and have a high R-value, meaning they effectively slow down the movement of heat.

Many modern electric insulations are now coming with additional safety features, such as the following:

  • ECTFE (ethylene chlorotrifluoroethylene) is resistant to chemical corrosion.
  • XLPE (cross-linked polyethylene) has increased resistance to heat; it can stand temperatures up to 195˚F.
  • PVDF (polyvinylidene fluoride) or Kynar is flame resistant, UV resistant, and has the highest mechanical strength.

So, as you can see, electrical insulation is an important factor in protection against electric fires. A few millimeters of insulation can block hundreds of volts of electricity and keep the electric supply safe. Modern insulations also prevent corrosion and increase the life of electrical wiring.

Types of Thermal Insulation for Your Attic

Attic thermal insulation

Depending on the climate and space in your attic, you probably want to find something both cost-effective and thermal-efficient for insulation. Since insulation comes with an R-value rating, you can make an informed decision. The higher the R-value, the better the insulation.

Let’s review the four most common types of insulation.

Fiberglass

Made from molten glass spun into fine fibers, this is America’s most popular type of insulation. It is effective in both hot and cold climates and has a high R-value. Fiberglass is also resistant to fire, making it a good choice for attics. Check out Pink R-13 Faced Fiberglass Insulation (on Amazon).

Cellulose

Cellulose is made from recycled paper or cardboard and is a good choice for environmentally conscious people. Cellulose has good heat resistance, and the fibers are further treated to make it more fire-resistant. It also has a very high R-value.

Some companies also make natural cellulose insulation from sheep’s wool, cotton, or straw. But they’re expensive and not as common.

Mineral Wool

This type of insulation is a mixture of natural minerals, most commonly slag wool, rock wool, and glass wool. Slag wool is produced as a by-product of the steelmaking industry and now makes the most common type of mineral wool.

It is also effective in both hot and cold climates and has a high R-value. In addition, all mineral wool insulations, such as this one from Crouse-Hinds (on Amazon), are resistant to combustion.

Polystyrene and Polyurethane

Both of these materials originate from petroleum products. They’re highly environmentally friendly products and have a very high resistance to temperature. Polystyrene is commonly used in spray foam insulation, while polyurethane makes the common type of rigid foam board insulation.

Other features for good insulation may include water resistance, mold resistance, and soundproofing. When choosing the proper insulation for your attic, make sure to consider all these factors. 

How to Install Insulation

Now that we’ve covered the basic materials, let’s discuss the types of installation. Each of these materials can be installed in a few different ways:

Loose-Fill Insulation

Fiberglass and cellulose are used for this type of insulation. You’ll need a blower to blow the filling into all the nooks and crannies. Sometimes the material is poured over the floor. It’s a great choice for refinishing an attic but can be a bit messy.

Since loose-fill automatically fills all nooks and crannies, it’s effortless to lay it over electric cables and is an excellent way to add insulation without disturbing the wiring.

Batt Insulation

Batts, or blankets, are thick fluffy sheets that are cut according to the size of the spaces in your attic. They are installed by stapling or tacking them to the frames of the attic opening. Each batt can be 3 to 7 inches thick, and you can use multiple layers to add more insulation.

Thermal blankets are made from fiberglass, wool, or cellulose and are a little tricky to install around electrical wiring. However, this is the best type of insulation to put in new construction when the framing is open and accessible.

Spray Foam Insulation

Made up of a mixture of Polyurethane and other synthetic polymers, this insulation is sprayed in place with special machines. The mixture expands up to 70 times its original volume filling all the gaps and crevices, and hardens almost immediately.

Spray foam insulation is a good choice for areas that are difficult to reach, like the attic floor or around ductwork, and it also covers uneven surfaces well.

But, as you might imagine, it also needs special equipment and protective gear to install and should be done by a professional. It is the most expensive type of insulation but also the most effective.

Foam Board Insulation

Foam boards have been the most popular and efficient type of insulation since the ’70s. They are also made up of polyurethane or other polymers sandwiched between two rigid layers of plastic or metal. The foam blocks are installed in the same way as the blankets, but they need less thickness since they have very high efficiency.

The foam board has double the R-value of any other insulation, which makes it a suitable choice for insulating every part of your home. They are also highly resistant to electric fires, so safety around wires is not a problem.

Each of these insulation options is safe for electrical wiring. So it all comes down to what type of insulation will work best in your attic and how you can install it.

How to Add Insulation Around Wires in the Attic

It’s important to be aware of the local building codes and National Electrical Code (NEC) when doing insulation work. Every state allows thermal insulation over electrically insulated wiring, but restrictions apply on junction boxes, recessed lights, and other electrical fixtures in the attic.

Make sure to check with your local authorities to see what is allowed in your area. In general, you should never cover a junction box with insulation; it needs to be visible and accessible.

It is easier to insulate a brand-new attic since you have visible access to the frame and wiring. On the other hand, it might become a bit tricky if you’re adding new layers over the existing insulation. We’ll try to explain the process for both scenarios.

The first step of insulating is measuring the amount of insulation you need. It’s done by multiplying the R-value of the insulation by the depth of insulation you want. For example, if you’re using batt insulation with an R-value of 3 and want a total insulation R-value of 30, you need to install 10 inches of insulation.

After you have measured and bought the insulation, it’s time to start installing it. Don’t forget to cut the power before starting.

  • Loose-fill insulation is the easiest type to install in both new constructions and retrofits. All you have to do is fill up the spaces by blowing the fibers, covering until the insulation reaches the desired thickness, burying the wiring deep inside it.

    This process is done with a special machine called a blower. You can handle it yourself or hire a professional. Make sure you use protective gear because fiberglass insulation has tiny glass shreds that can irritate skin and eyes and even get into your lungs.
  • Batts come in rolls cut to fit standard wall and ceiling cavities. However, always measure your attic spaces to get the right size. The batts should fit snugly between the framing without leaving any gaps or squeezing. If you have to cut the batts, use a sharp utility knife in straight lines. For fitting wires, make a small straight slit in the center of the batt and push the wire into the slit.

    Another method is to split the batt into two layers with your hands and then slide one layer under the wire and the other layer on top. Staple the batt to the framing every 12 inches and keep adding layers as needed.

    Again, make sure you use protective gloves and appropriate clothing while handling fiberglass batts. For repairing or installing over existing insulation, carefully remove the old insulation, section by section, and replace it in the same way.
  • Foam board insulation is usually only used in new construction because it needs more accessibility to the framing than batt or blown insulation. But because they’re thinner, foam boards easily fit behind electrical wires.

    Simply cut down the board to your desired shape and size and slide it into the space between the framing and wires. Then keep adding more boards until the insulation reaches the desired thickness. They’re highly favored to insulate around circuit boards and junctions because they’re non-combustible.
  • Spray-in-place insulation is another method suitable for both new construction and retrofit. It comes in a can like spray paint; you attach it to the compressor and evenly coat the space with insulation.

    Since the spray expands exponentially, professionals recommend leaving a two to three-inch space between the insulation and electrical wiring.

    This prevents extra pressure on the wires, but it’ll usually be safe even if you directly coat the insulation over the wiring. There is a significant risk of inhalation, so using a respirator is highly important when doing this type of insulation.

There you have it. Whatever insulation you choose, make sure to check with your local authorities or an electrician to see what is safe and allowed in your area. And always use protective gear when handling insulation, especially if it contains fiberglass. Stay safe… and warm!

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