How to Combine Spray Foam Insulation and Radiant Barrier

Whether you’re building a new home or improving an old one, you can use a combination of spray foam encapsulation with radiant barrier foil to keep the heat out and improve the comfort within your home. To get started, you’ll need to encapsulate the roof deck with 6″ open-cell foam, and then insulate the attic and all the exterior walls. Foam alone is not enough though to keep the radiant heat at bay, so you also need a good radiant barrier foil.

The Air Gap Problem

By combining radiant barrier foil with spray foam insulation you create two defenses for reducing heat flow into your home. Although using them together with a normal roof is not as easy because the shingles are connected to the roof deck, which is connected to the foam, and there is no air gap in which the heat energy can become radiant heat.

Radiant barrier foil is ineffective when it doesn’t have an air space on one side. So you can’t just glue it to the shingles and add the foam afterward. The barrier reflects only radiant heat, which forms only in an air space. When you have an air space that allows radiant heat to form, you can use the barrier to reflect almost all the heat, not just merely decrease it, as insulation does.

The Solutions

If you have a raised roof made from metal or concrete, you can add the radiant barrier below it, but over the roof deck. If you don’t, things are a bit more complicated and can get expensive. For example, a raised roof is two or maybe even three times the cost of an average roof, and you probably won’t be eager to change your roof just to be able to install radiant barrier.

One idea is to can create a double-deck roofing system as an affordable option. Another approach is to add the foil via the cathedral ceiling method and then add spray foam on top of the foil. Read below for more info on each method.

Option 1: The Double Deck System

1. Install the bottom deck, leaving a 10″ gap from the fascia board to allow air to come in from the soffit vents and flow between the two decks of the roof.

2. Add the radiant barrier foil over the roof deck, ensuring that the foil side faces up. A double-sided 48″ barrier is recommended.

3. Use a marker to mark the rafters on the foil.

4. Install 1’x2′ furring strips, leaving a gap of at least ¾” between the roof decks.

5. Install a foil-backed roofing deck material of your choice over the furring strips, creating the second deck, which you will tie to the fascia boards.

6. Add the roofing felt and the shingles.

7. Spray foam the bottom of the first deck from inside the attic space as usual.

8. (Optional) Add ventilation. The rather limited air space between the two decks doesn’t require much ventilation, but you can consider installing effective yet affordable wind turbines that will refresh the air between the decks, cooling and drying them.

The result will impress you. During hot summer weather when the temperature hits 100º and the shingles easily reach 170ºF, the lower deck will have an impressive bottom surface temperature of under 110ºF. That’s because the air gap you’ve created enables the radiant barrier foil installed under it to eliminate conductive heat and transform the rest of the heat into radiant heat, almost all of which can then be reflected.

Building a double-decked roof in order to create an air gap as described above and then using the combined power of spray foam and radiant barrier foil will increase your building costs a little. But your home’s energy efficiency will be increased dramatically, reducing your energy bills and at the same time increasing the comfort level in your home. It works for any type of house. Do it and you won’t regret it!

Option 2: Retrofitting the Roofline from Inside the Attic

1. Install the single-sided radiant barrier foil according to the installation instructions on this page for Cathedral Ceiling Installations and Wall Installations (they are basically the same steps, just different angles) with the foil side facing the outside, toward the sun.

2. When it’s time to add regular insulation, forego the traditional batt insulation and instead spray foam on directly on the foil. *This only works because in Step 1 you’re going to create the critical air gap that the foil requires to work so on the inside you can add material right on top of the foil.*

If the spray foam is having trouble sticking to the foil layer, use a medial layer of foam board. Wedge the board into the cavity right up against the foil and then spray the foam on to that.

3. Finish out the space if this is an interior area or if it’s an attic, just leave it open.