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Radiant Floor Heating Insulation: Everything You Need to Know

Radiant floor heating has become one of the fastest-growing industries in the last twenty-five years. Radiant floor heating is reliable, cost-effective (in most situations), and can last twice as long as a similar forced air system, like a heat pump. However, like any heating system, how well the structure is insulated will dictate how well it performs.

If you are interested in installing radiant floor heating in your home, this information is for you.

Do You Need Insulation Under a Radiant Floor Heating System?

Yes, in most cases. Radiant floor heat comes in two basic designs. The first, and most popular, is an electric radiant system. This design will come in two forms: electric single wire, and electric mat. This system is installed under a finished floor, such as tile, laminate, or engineered hardwood. The second design, known as hydronic, is used exclusively in concrete pads.

What Are the Benefits of an Electric Radiant Heating System?

  • Easy Installation.
  • Relatively inexpensive
  • Durable and reliable

What Are the Benefits of a Hydronic Radiant Heating System?

  • Works best in large areas, like a garage or basement.
  • Very cost effective to operate
  • Lasts up to twice as long as a forced air system

What Kind of Insulation Do I Need For a Radiant Floor Heating System?

Insulation is one of the most important parts of a successful radiant floor system. Any heating system will perform at maximum efficiency when the thermal envelope is well sealed. The thermal envelope is an industry term and refers to the encapsulation of any heated space. For example, a home that has insulated floors, walls, and ceilings has a thermal envelope. 

Radiant floor heating relies heavily on insulation as a way of maintaining a constant temperature. Depending on where the system will be installed, you may need to insulate using fiberglass batting, spray foam, or insulated panels. Here is a brief description of each and where it may be used:

Fiberglass Batting

Over one-third of radiant floor heating is installed in a garage concrete pad. As a rule, if a garage has drywall, the walls will contain fiberglass batting. Most building codes require a minimum of R-13 in walls if the garage shares a wall with a heated space.

A good example would be the wall between the house and the garage, if the garage is attached. Building codes will also require that the ceiling of the garage contain R-11 to R-19, if there is a bonus room over the garage.

Spray Foam

Most new homes today will have insulating spray foam as the primary insulating material. This is due to the speed and effectiveness of the installation method, as well as the performance of the foam itself.

Spray foam reduces or eliminates the cracks and gaps that often remain after fiberglass batting is installed. As mentioned previously, the quality of the thermal envelope relies on a sealed airspace.

Spray foam does an excellent job of filling in any gaps in the thermal envelope, as it expands slightly as it dries. In today’s construction, spray foam is usually used on foundations, walls, and even an attic, if the home has vaulted or cathedral ceilings.

Sheet Foam

Before the invention of insulated panels, sheets of styrofoam were often used to create an insulating barrier between the ground and the concrete. These sheets are still available, and usually come in 4’ x 8’ sheets, up to two inches thick.

Even at two inches thick, however, these sheets usually provide no more than an R-8 insulation value. As using these sheets requires the additional steps of installing a vapor barrier and tubing guides, they are rarely used today.

Insulated Panels

This type of insulation is used exclusively for radiant floor heating in a concrete pad. Radiant floor heating of this type uses polystyrene tubing, called PEX, to circulate warm water throughout the pad. Insulation is especially important in this type of installation, as the tubing would otherwise come in contact with the ground and lose the heat.

How Much Insulation Do You Need For Radiant Floor Heating?

This often depends on where the home is and the local building code. With electric systems, R-11 faced batting is standard. In hydronic systems, insulated panels are usually used. These panels come in thicknesses of 1” (R-6 to R-8), 2” (R-10 to 12), and 3”(R-15), and will also include a vapor barrier to further retard heat transfer.

How Do You Insulate Under Radiant Heat?

As a rule, electric systems will be used when the floor to be heated is small, between floors, or both. A good example would be the master bathroom on the second floor. These systems are fairly simple to install and designed for the do-it-yourselfer.

Hydronic systems accomplish the same objective, but since these systems use both water and electricity, the installation process is much more involved. Hydronic radiant flooring is best suited for concrete pads, like those found in a garage or basement.

Both methods require insulation to be installed under the floor to prevent the heat from radiating anywhere but up. Here we will describe the process of insulating under these common radiant heating systems:

Insulating Electric Mat and Electric Single Wire Radiant Heat

Here is a lists of tools you may need:

  • Utility knife
  • Tape and/or adhesive
  • Gloves
  • Eye protection
  • Mask
  • Measuring tape
  • Marker
  • Stapler (optional)

Step 1: Calculate the Insulation Needed

Simply measure the space under the area to be heated and add 12” to the measurement in both directions. This will allow the insulated area to extend beyond the heated area, which will divert the heat directly up.

Fiberglass batting is available in several widths, so most professionals will use one size larger than needed. This allows for easy installation of the staples and makes for a more snug fit.

Step 2: Install the Fiberglass Batting

You can install the batting in a few ways. The most common method is with a stapler. Most professionals will use ⅜” flat staples, with either a mechanical, electric, or pneumatic stapler. You can also use tape, but be sure and use a version compatible with insulation. Using the flap on the facing, staple the batting to the floor joists every 16” or so.

Pro Tip. Don’t be tempted to install the batting upside down because it seems easier. In a radiant floor heating system the vapor barrier, or faced side of the batting, must face the floor being heated.

Step 3: Fill Any Gaps

The efficacy of the insulation is only as good as the seal, so the next step is to cover or fill any spaces or gaps in the batting. This is often done with insulating tape, however, spray foam can also be used. Regardless of the method, ensure that the radiant heating system is as isolated as possible from the space beneath it.

Insulating Hydronic Radiant Heat 

Here is a list of tools you may need:

  • Utility knife
  • Safety Glasses
  • Gloves
  • PEX strap tool, or similar connector

Step 1: Layout

Measure the entire basement floor area. This number will be asked of you anytime you are shopping for materials, so keep it readily available. As a rule, these panels will come in 2’ x 4’ units, with an area of eight square feet. So for example, if your concrete pad is 20’ x 20’, you would need to cover 400 square feet. Dividing by eight square feet, you would need fifty panels.

Step 2: Place the Panels

As mentioned earlier, these panels not only provide a way to align the tubing, but also provide insulation and a vapor barrier. Most installations will use a 2” thick panel, which provides up to an R-12 value. However, some climates may require R-15, which requires a 3” thick panel. 

Installing these panels is very easy. Simply align the first full panel with an adjoining wall and snap another panel to it. These panels connect with interlocking edges, and eliminate the need for additional reinforcement by using the weight of the concrete to keep them in place. Continue until the entire area is covered with the panels.

What Are the Pros and Cons of Insulating Radiant Floor Heating?

Pros:

  • Insulating a radiant floor system not only helps control where the heat travels, but keeps the system at maximum efficiency.
  • Installing most forms of radiant floor heating is fairly simple and reliable.
  • The cost of insulating will lower the operating costs of the system, increasing the return on investment.

Cons:

  • Insulation batting adds additional expense to the project.
  • Since a system goes under the finished floor, access to the space under the subfloor will be required. This may involve some demolition and repair.

Good Insulation Is Key

As with any project involving heating or cooling, insulation is important. Although radiant floor heat is very effective and efficient to operate, using it without insulation will greatly reduce its effectiveness. When maximizing the effectiveness of a radiant heating system, appropriate insulation may be the best investment you can make..

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