When designing a high-performance roofing system, many builders look at adding a layer above the roof deck to improve energy efficiency and reduce heat transfer into the structure below.

Two common approaches are:

• Installing foam board insulation (EPS, polyiso, etc) above the roof deck
• Installing a radiant barrier like RoofingFoil™ above the roof deck beneath the roofing material

At first glance, these systems may seem similar because both sit between the roof deck and the roofing material. But they work in completely different ways.

• One slows heat down after it has been absorbed.
• The other reflects much of the heat away before it ever enters the roof assembly. 

That distinction matters more than most people realize.

Foam Slows Heat Transfer — It Does Not Reflect It

EPS (Expanded Polystyrene) and Polyiso (polyisocyanurate) foam insulation work through R-value. Its purpose is to resist conductive heat flow and slow the movement of heat through the assembly.

That means the roofing material above the foam still gets extremely hot in direct sunlight. The heat is still absorbed into the roofing system and still builds up within the assembly. The foam simply slows how quickly that heat moves downward into the structure.

This is an important point because many people mistakenly assume insulation “blocks” heat. But it does not block heat, it delays heat transfer. Given enough time and temperature difference, heat still moves through the assembly.

RoofingFoil™ Reflects Radiant Heat Away

RoofingFoil™ works differently because it is not relying primarily on R-value. It is designed to reflect radiant heat energy and it does it very well.

When installed facing an air gap, RoofingFoil™ reflects up to 97% of radiant heat away from the roof deck and the structure below it.

Instead of allowing the roof system to absorb large amounts of radiant energy and then trying to slow it down afterward, the radiant barrier reduces how much heat enters the assembly in the first place. That is a far more direct strategy for controlling solar heat gain.

Reflecting Heat Is More Effective Than Managing Stored Heat

This is where the performance difference becomes obvious.

With foam:

• The roof absorbs solar heat
• Heat builds within the roofing assembly
• The foam slows downward transfer

With RoofingFoil™:

• Radiant heat is reflected away across the air space
• Far less heat reaches the roof deck
• The structure below stays cooler from the beginning

In hot weather, reducing heat gain at the source is often far more effective than allowing the roofing system to become heat-saturated and trying to insulate against it afterward.

Why Air Gaps Matter With Radiant Barriers

A radiant barrier requires an air gap (or air space) to function properly. The space needs to be at least ½" and does not have to be vented (though it can be). Fortunately, many roofing systems already create this air space naturally through:

• Battens
• Counter battens
• Purlins
• Furring strips
• Ventilated roofing systems
• Corrugation patterns of the metal
• Tiles that curve or sit on battens

As long as the foil faces at least a ½-inch air space, it can effectively reflect radiant heat. Larger air gaps are perfectly acceptable, but smaller than ½ inch is not recommended because the reflective performance begins to diminish.

This is one reason RoofingFoil™ integrates so naturally into metal roofing and tile roofing systems.

Foam Can Increase Heat Retention in the Roof Assembly

Another consideration that is often overlooked is heat retention.

Because polyiso and EPS foam insulation slow heat movement, the roofing assembly above it can remain hotter for longer periods of time. Heat absorbed during the day dissipates more slowly because the insulation reduces heat flow in both directions.

Radiant barriers approach the problem differently by reducing how much radiant energy is absorbed into the structure to begin with.

That distinction can help reduce overall roof deck temperatures and lower attic heat buildup during peak summer conditions.

Foil + Foam: A Perfect Pair?

You can combine the r-value of the foam with the reflectivity of a radiant barrier in the same roof and get pretty great results. The key is in how you add the layers. The foam layer should be placed down first, closer to the roof decking, while the radiant barrier needs to be the last layer before the battens or the actual roofing material. 

In this set up, the foil has the required air space above it and will work to reflect 97% of that radiant heat coming off the underside of the roofing. The little bit that gets past the foil layer (about 3%) will then be slowed down as it absorbs into the foam layer, keeping the heat out of the home below better than without it. 

RoofingFoil™ Also Works With Ventilation

Many modern roofing systems are designed to include airflow beneath the roofing material. Ventilation helps remove excess heat and moisture from the assembly while helping roofing materials dry more effectively. RoofingFoil™ complements this design extremely well because it reflects radiant heat while still allowing the assembly to breathe.

Rigid foam, on the other hand, is primarily focused on thermal resistance rather than radiant heat reflection or ventilation performance so if you're using it alone, it's not the best option here. Instead, consider pairing the two as mentioned above.

The Smarter Approach for Hot Roof Systems

Foam board insulation certainly has applications in building design and can contribute useful R-value in some assemblies. But when the primary goal is reducing solar heat gain from a hot roof, a radiant barrier addresses the problem more directly.

Rather than absorbing heat into the roofing system and slowing it down afterward, RoofingFoil™ reflects much of that radiant energy away before it can enter the structure. The effect and benefit of this is immediate and unmatched. If you want to make your roof even more efficient, consider pairing the EPS with the RoofingFoil™, just make sure to layer them correctly. The foil should always be your top most layer and facing an air gap while the foam can be added closer to the roof deck/living space. Together you can create a really effective strategy against heat gain into your home.

That is why radiant barriers remain one of the most effective tools for improving the performance of metal roofs, tile roofs, and other roofing systems exposed to intense solar heat.