How to Reflect Heat with Radiant Barriers

Radiant-barrier insulation performs best in warmer climates, but its energy saving comfort can come in handy during cold winter months, too. It's usually installed in attics on rafters, floor joists, and wall studs to reduce summer heat gain from outside and winter heat loss from inside.

They're more efficient at repelling than conserving heat, so if you live in a colder area, you'll want some thermal insulation materials in your energy mix, too. If your climate gets hot, though, radiant barriers can offer some valuable protection, both for your home and your bank account.

Step 1 - How Heat Transfer Works

Heat generally tends to travel from a warm to a cooler area through a series of processes known as heat conduction, heat convection, and heat radiation.

1 - Conduction

Conduction is when the heat transfers by flowing through materials—drywall, joists, lumber, etc—the same way heat flows along the stem of a spoon sitting in a cup of hot tea to reach your hand.

2 - Convection

Convection occurs as ambient air warms up or gets heated, lightening its density. This causes the warmer air to naturally flow upwards. You can see this at work in the steam that comes from a cup of hot tea, indicating that heat is being transferred into the cooler air.

3 - Radiation

Radiation happens when radiant heat travels away from any surface and warms or heats any solid materials that can absorb its heat energy. It differs from convection and conduction by requiring a medium to absorb and transfer the heat.

The warmth of a campfire, heat from a stove burner, and light from the sun are just a few examples of radiant heat.

The amount of heat flow from inside to outside a building is normally greatly reduced by its insulation material, which readily slows down the conductive and the convective heat flow (though less effectively for convection).

So before considering the installation of a radiant barrier, you should first check on improving the R-value of your thermal insulation, especially in colder climate areas, as it will bring the barrier’s performance at its best—this also being true when operating AC in warmer climate areas.

Step 2 - Choose the Right Radiant Barrier Format

Radiant barriers consist of highly reflective material such as aluminum foil applied onto materials such as kraft paper, cardboard, plastic film, and many other medium types, covering one or both sides of it.

Some types of radiant barrier products are also combined with other mediums such as insulation material or fiber reinforcements to increase their ease of handling and durability, the reflective material remaining as the outer layer.

In Warmer Climates

As the sun heats the roof, the ultraviolet rays make the roof hot through radiant heat transfer, then through conduction, the heat travels through the roofing materials and the roof decking to reach the attic side of the roof.

Solar radiation can account for as much as 93% of heat flow. Hot roofing and decking radiate their gained heat energy onto the cooler surfaces throughout the attic, including the attic floor and air ducts.

For such a situation, a reflective radiant barrier applied underneath the roof structure can greatly reduce the heat energy transfer by radiation from the underside of the roof to other surfaces throughout the rest of the attic space.

In Colder Climates

Installing more thermal insulation is usually more efficient and cost-effective than adding radiant barriers.

Step 3 - Critical Points to Remember

3.1 - Radiant barriers should always be installed in a way to minimize dust accumulation onto their highly reflective surface, as this can hamper reflectivity and performance.

3.2 - If installed on top of attic floor insulation, the foil will be susceptible to dust accumulation

3.3 - An air space of at least 1-inch (25mm) should be provided between the radiating surface and the radiant barrier to give the heat a chance to be reflected back instead of being absorbed by the radiant barrier.

The same air space rule should be applied between the barrier and the floor insulation, as it would also provide extra protection from moisture forming and getting trapped in the fiber insulation, which would occur should the barrier be placed directly on top of the insulation.

3.4 - For maximum performance, the radiant barrier should always be installed perpendicular to the radiating heat (parallel to the surface to cover).

3.5 - Aluminum foil being an electrical conductor, you also have to keep an eye out for the remote possibility of an electrical hazard where a bare electrical wire would come in contact with the foil.

3.6 - There are two ways to install under the roofing deck and incorporating the radiant barriers while the home is being built, while all the structures are opened and exposed, makes it much easier.

3.7 - Installing the barriers in an existing home will, for obvious reasons, represent and involve some extra work. In this case, the radiant barrier installed under the roof deck will need to be installed from inside the attic by stapling the material to the bottom of the rafters

3.8 - To decrease the risk of the barriers’ reflective foil accumulating dust and dirt, thus impeding its performance, the barrier should be installed foil-face down when hung from the roof rafters or laid onto the floor joists.

Step 4 - Prepare and Secure the Attic

Before starting any work in the attic, you should put on safety goggles or glasses and wear a particle mask to prevent insulation fibers and dust in the air from irritating your lungs and throat, while moving around in the attic.

The attic being a space mostly kept empty or used for storage is not a finished area of the house with all its framing exposed and without a solid floor to walk on. If used for storage, move or remove all of it so it doesn’t get in the way causing safety hazards or slowing down the work.

Since a lot of time will soon be spent up there, you’re going to have to get some large boards that are long enough to span across the joists.

Lay these on the joists in your attic and tack them in place to keep them from sliding and moving over while working and creating a solid surface large enough to walk or crawl on. This step can avoid unnecessary repairs to the ceiling below, once you're done in the attic.

Step 5 - Planning Radiant Barrier Installation

With your choice made on the type of radiant barrier you’ll be using under the roof deck, on the walls, and on the floor, or whichever other surface you want to be covered, measure the perimeter of each of those surfaces and calculate the surface area (in square feet or square meters).

Since you’ll probably be using an insulated backing reinforced panel on the floor, add up the total area of each type of material needed.

Plan how the panels will fit, and make sure that they will be fixed and attached to the rafters, wall studs, and to the floor. If you need extra framing to provide an adequate air space above the floor insulation, for instance, figure out the sizes of the lumber and add everything up to your list of materials.

You should also include staples, spare blades for the utility knife, nails, construction tape, etc.

While planning, keep the installation of the radiant barrier on the floor joist for last as your work progresses in sections throughout the attic as you proceed gradually towards the access door.

Step 6 - Cutting Heat Radiant Barrier

Cut the barriers to size using a sharp knife. The drywall T-Square, with the protruding edge of its T-bar at one end, is one practical tool that in conjunction with a sharp utility knife (such as the Olfa) will help you cut your pieces in no time and perfectly straight.

Make sure that they’re all cut to a proper length, and always remember that a slight overlap is better than having it cut too short, thus preventing air leaks. The better they fit, the more efficient your home will be.

Step 7 - Covering the Rafters

After a panel is cut to size, you’ll need to fit them to the underside of the rafters by stretching it out to its full length and securing temporarily at the four corners with staples.

Install with the foil-face down to prevent dust accumulations and make sure that the panel is centered on both of its rafters and adjust if necessary. You can then staple in place to their full length to both rafters using a staple gun. This will ensure that the panels are fitted securely and won't move or drop.

Trim off any excess material and fill any gaps between the panels using caulk and construction tape. Remember that the effectiveness of your radiant barrier will be greatly affected by its installation.

Follow the manufacturer’s instructions carefully for the material you purchased, since different backings will call for different cutting and fastening methods.

Step 8 - Alternating from the Ceiling to Covering Walls

After installing a few pieces on the ceiling, you can start cutting and hanging pieces on the wall studs beginning at the gable wall and covering it completely with the radiant barrier, basically following the same procedure as for the ceiling.

The seams at the corners where walls and ceiling intersect and where two walls meet should also be made leakproof with construction tape or even reflective aluminum tape. You can keep going until get everything covered right up but just short of reaching the access door to the attic.

Step 9 - Finishing the Floor

As you progress with your work, the floor will also need to be covered, making sure to respect the air space of 1-inch between the insulation and the barrier.

The boards previously laid out on the rafters to prevent you from falling through can be shifted on top of the radiant barriers as you progress and nailed permanently in place. They will now be required as the barrier will be hiding the floor rafters from view increasing the hazard of falling through.

Step 10 - Covering the Second Part of the Attic

Once you’ve reached the access door to the attic, stop your work to that first section of the attic. Moving over to the other end of the attic, start the whole procedure over from the other gable wall and apply your reflective lining up to where you reached with the first part, and finish up by sealing all the seams.

When you're done, pat yourself on the back! You can celebrate the improved comfort of your home with a lower electricity bill next month.