Why the Foil Side of BlueTex™ Needs an Air Gap to Work as a Radiant Barrier

Radiant barrier insulation is one of the most effective ways to stop radiant heat transfer in your metal building—but only when it’s installed correctly. Whether you’re insulating a metal building, a carport, a garage, or a roof deck, this air space is not optional if you want the foil to block 97% of the radiant heat, it’s essential. BlueTex™ can be used as just a vapor barrier for condensation control or for covering up old insulation in an older building, but if you want it to help with heat control, you need to make sure you're installing it correctly. 

How Radiant Barriers Actually Work

The foil side of BlueTex® Insulation isn’t like traditional metal building insulation that slows down conductive heat transfer. Instead, the foil surface of BlueTex® reflects radiant energy (heat) back toward the direction it's coming from. Foil has the unique ability to do this via reflectivity and emissivity - that means that no matter where the heat is coming from (the sun or your propane heater), aluminum will stop the transfer of that heat.

But here’s the catch: foil only reflects radiant energy when the foil faces an air space. If it’s installed directly against a solid surface like plywood, metal, or drywall, there’s no radiant transfer to reflect—only conduction. Without that air gap, the foil’s radiant barrier properties are essentially lost. You need the air gap in order for the heat to be in radiant form, and that's how we stop it.

What size air gap does the foil need?

We recommend between ½ to ¾ inch minimum air gap. It can be larger, but not smaller than that. 

The Air Gap Does Not Have to Be Vented

A common misconception is that the air gap needs to be ventilated to work. That’s not true. While a vented gap can help with moisture control or overall building ventilation, the radiant barrier doesn’t depend on airflow.

What matters is simply maintaining a minimum ½ to ¾-inch air space adjacent to the foil. This gap allows the foil to block radiant heat transfer, rather than conduct it through the material it’s touching.

Practical Ways to Create and Maintain an Air Gap

The good news: there are many simple, effective ways to create this required space during installation. A few proven options include:

• Hat channels – Lightweight and durable, hat channels can provide a consistent gap across large areas. You can install BlueTex® under or over your hat channels to create an exterior air gap before the metal siding/roofing goes on. 
• Purlins – Common in metal building construction, purlins naturally create space for the radiant barrier to face. The best time to utilize purlins is during new construction as the building is going up.
• Plastic HDPE spacers – We created these small spacers that are easy to work with and ensure the air gap is maintained even in tight spaces. They have an adhesive backing that sticks to the foil and keeps the exterior metal from touching it. These are great for new metal building construction.
• Wood or metal furring strips – A traditional and highly effective way to secure insulation while preserving the gap. These can be added on new and existing construction projects.

No matter which method you choose, the key is maintaining that air space across the entire reflective surface. You can space out your spacers as close or far as you would like, but we recommend you aim for about 2' apart, or whatever other space your metal manufacturer suggest for stability of the metal panels. Gaps that are too small or areas where the foil is pressed directly against a solid surface can dramatically reduce performance as a thermal shield. 

No Exceptions: The Air Gap Is Critical 

If your goal is to stop radiant heat flow, this air gap is not optional. Without it, the reflective foil can’t reflect the heat. It simply becomes a conductive surface—like any other piece of metal or foil and won't cut back on heat gain/loss as you want it to.

By ensuring a proper air space, you’re allowing BlueTex™ to do what it’s fully designed to do: reflect up to 97% of radiant heat, create an interior vapor barrier for moisture control, and keep your building cooler, more comfortable, and more energy efficient.

In short: If you want the foil to work as a radiant barrier, give it the air space. Whether that's done through hat channels, furring strips, or spacers, that gap is the secret ingredient to high performance. No air gap = no radiant barrier benefit/effect. If you have questions about an installation, or aren't sure how to maintain an air gap in your space, contact us and we'd be happy to help see if BlueTex™ is a good option.