How Does Reflective Foil Insulation Work?

What Is Foil Insulation?

Foil Insulation or reflective foil insulation generally refers to products comprising of one or more substrates with a reflective foil backing one or both sides. The reflective facing is usually a very thin layer of reflective aluminium foil, hence the name reflective foil insulation.

History Of Reflective Foil Insulation

Reflective foil insulation is very simple and very effective but it’s not a new idea, and although reflective foil insulation was re-popularised by the space industry, it is not ‘space aged’ either. In-fact reflective insulation has been around for 1000’s of years.

The Beginning: Reflective insulations were used by ancient cultures, there is evidence to show that even the Egyptians would use polished metals to reflect heat out of homes.

1800’s Mass-market: The most popular and probably well-known example of using reflective surfaces and airspaces to insulate is probably down to the inventor Sir James Dewar who invented the thermos flask in 1892.

1900’s Re-popularised: Reflective foil insulation materials were re-popularised by the space industry. Scientists were looking for a way of insulating space suits, and equipment to deal with the extreme heat and cold experienced by astronauts and shuttles.

2000’s Well known uses – today: No doubt you have seen people running marathons and outdoor events. You will also have noticed competitors are often wrapped in foil blankets… but not fibrous insulation and not foam. Ever wondered why, or why we don’t utilise this foil insulation technology in our homes and buildings?

How does Insulation Work?

Most common insulation materials work by slowing heat flow. Radiant barriers and reflective insulation systems work by reducing radiant heat gain. For foil insulation to be most effective, the reflective surface should face an air space.

To understand how insulation works, it helps to understand the basics of heat flow, which involves three basic mechanisms — conduction, convection, and radiation.

Three Types Of Heat Loss

What Is Conduction?

Conduction is when heat travels between two touching objects or through one solid object.

What Is Convection?

Convection is a warm air current, you can see convective heat from a hairdryer or from the surface of a hot road.

What Is Radiation?

Radiant heat is electromagnetic waves which can pass through anything.

Heat flows from warmer objects, airspaces or surfaces to cooler ones, until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and anywhere else that is cooler than your heated area. Heat flow can also move indirectly through ceilings, walls, and floors. The opposite is true in the summer, where heat is transferred from outside into your cooler home.

Why Do We Insulate?

Most people believe that we simply insulate to keep warm. In fact, we insulate for a number of reasons:

  • Comfort
  • Thermal Efficiency
  • Air Tightness
  • Condensation Control
  • Energy Saving

Before we talk about foil insulation, we need to define three thermal values

What Are R-values?

R-values measure a material’s resistance to heat flow. The thermal properties of insulation products are commonly stated in R-values.

What Are U-values?

U-values are a measure of a material’s ability to conduct heat. The thermal performance of windows, doors and buildings is commonly stated in U-values. U-value is equal to 1/R (R-value).

What Are E-values?

E- Values are the measure of ‘emissivity’ which is the ability to emit or transfer radiant heat through a surface – everything has an E-value. The ‘E’ in Low-E stands for Emissivity.

Regardless of how high the R-value on an insulation product is unless it also has a low – E value it has no ability to reflect radiant energy. It will always need to absorb the heat in order to work.

Very high R-values don’t stop heat transfer, they simply slow it down. You cannot ignore the need for good R-values, but by addressing the R-value alone, a large percentage of heat transfer is being ignored.

What is Emissivity?

You may have heard of low E glass in windows. Well the ‘E’ stands for Emissivity and Low-E Insulation works the same way.
Emissivity is the ability to transmit or transfer radiant heat through the surface of a material. Everything has an emissivity value. Clothing, wood, concrete. Cast iron has an Emissivity of .85. That means that 85% of the heat that comes in contact with its surface to pass through it, that is why traditionally radiators were made from cast iron. They were very efficient at transferring heat from one side to the other. Aluminium has a very low natural emissivity value of 0.03. Here are the emissivity values for a list of building materials:

















Aluminium has a very low natural emissivity value of 0.03. This means that aluminium will only allow 3% of the radiant heat that comes in contact with the surface of aluminium will pass through it. Another way to look at this is that 97% of heat will be reflected back to the source.

“If a surface emittance is changed from .90 to .03 the part of the heat flow that is radiation is reduced to 3.3% of its initial value.” 100 Btu/hour – 3.3 Btu/hour

David Yarbrough

“Radiation is the primary mode of all heat transfer. The other two modes… come into play only as they interfere with the primary mode.”

M. Pelanne, Senior Research Specialist for Johns Manville (Energy Design Update, Feb. 93)

While there are all three forms of heat transfer out of a building, it is worth noting that 100% of the solar gain is radiant heat. Consider how hard air conditioner have to work during summer months to deal with that.

See Aluminium -v- Mylar


Why do Airspaces and R-values matter?

Reflective foil insulation products can work without an airspace but to declare what is called an “R-value”, you must install foil insulation with an airspace. Airspaces can be forms in numerous ways and can be as small as a couple of millimeters but airspaces beside a low emissivity surface are most efficient between 6mm – 20mm.

The easiest way of explaining how low emissivity airspaces work is to use a double glazed window as an example. The two pieces of glass on their own do nothing but when the layers of glass are separated by an airspace, it is actually the airspace that becomes efficient. However, if there is a low emissivity surface (pure aluminium) on one side of the airspace, the airspace becomes nearly 5 times more efficient than in a double-glazed window.

Heat cannot conduct through a low emissivity airspace, and small airspaces are too restrictive for convection to occur.

See Our Technical Support Section

Emissivity of Foil insulation

Why Is Moisture Control A Problem?

Warm air carries more moisture than cold air. When warm air cools, the moisture gets left behind. To optimise performance in both new build and refurbished buildings, it is always advisable to install a vapour control layer within a modern building envelope. Warm air carries more moisture than cold air. This is a problem for traditional insulation. When warm air cools down, the moisture from the air is left behind. It is then absorbed by the insulation causing the insulation to become compressed. This happens more so in winter when there is a greater difference between inside and outside temperature. Once this happens, the insulation’s ability to perform drops dramatically. In extreme cases, the insulation must be removed. This can also be a problem if using single layered products. If warm and cold air spaces are separated by a single-layered product or a radiant barrier without a core, there is a greater risk of condensation forming within the insulation system. Standard breather membranes don’t have this problem because they don’t contain any metalized particles. Care should be taken to ensure this risk is dealt with through other methods of environmental control. A closed cell foam core such as ours provides a thermal break between the warm and cold surfaces, dramatically reducing the risk of interstitial condensation. Condensation will not form within the core and can’t travel through the product. The unique Low-E design allows it to self-seal around any nails or screws that puncture it during installation. Moisture problems are virtually eliminated.
Reflective Foil Insulation

How Important Is Air Tightness?

It is now widely accepted that air-tightness is one of the most important parts of building design and there is little point in improving insulation standards unless also addressing the levels of uncontrolled air movement. The first thing to remember is that there is a big difference between breathability and air leakage. Air leakage is the uncontrolled movement of air in to and out of a building which is not for the specific and planned purpose of exhausting stale air or bringing in fresh air. Studies have shown that uncontrolled air leakage can account for over 50% of energy consumption within a building. So whether you are building a Passive House or upgrading an older building, air-tightness is now regarded as one of the most important parts of the design. Air leakage should never be considered as acceptable natural ventilation because it cannot be controlled or filtered and will not provide adequate or evenly-distributed ventilation. It is generally at its most severe during the colder, windier, winter months and has least impact during the warmer, calmer, summer periods. This is generally the opposite of the requirements for ventilation within buildings. Ventilation of a building should rely on a designed strategy based on the assumption that the envelope will be relatively airtight.


Reflective foil insulation can be installed on its own or teamed with a secondary insulation to meet higher R-value standards. It is not meant to replace ALL insulations on the market. Low-E Insulation is designed to: increase thermal performance and airtightness, reduce condensation risks and life-cycle running costs, and most importantly, to tackle issues which have been previously unaddressed by traditional bulkier insulation products. Low-E foil insulation is eco-friendly, won’t rot and deadens sound.