Foil Insulation Frequently Asked Questions
Author: Mac Slodyczka Date Posted:11 September 2013
What is radiant heat?
Radiant heat is energy. Radiant heat is the heat that is transmitted from a heat source through space. It is the heat we feel from distant objects like the sun or a fire. Radiant heat is also called infra-red heat or infra-red energy. Around 80% of the heat transfer in your building envelope is via radiant heat energy, which heats up your house in summer.
Bulk Insulation and R-values?
Material R values are determined in a laboratory by placing a product between two fixed temperature plates set at 33 and 13 degrees Celsius (a temperature difference of 23 deg) and measuring the heat transfer that takes place over a four hour period. The heat transfer measured is then presented as the material R-value.
The problem is that the quoted R values are only valid for the standardised conditions in which the test took place. If the temperature difference is increased, the R-value decreases. This is the case in real life conditions, where temperatures of up to 50-70 degrees are common in roof spaces during summer. In these sorts of conditions the actual R-value, your bulk insulation would be much lower than what is quoted.
Also, keep in mind that no matter how high the R-value might be, bulk insulation does not stop any heat flow at all. It slows it down by absorbing it, but later, when the material has reached its thermal capacity, it will give this energy off as heat.
How Radiant Insulation Works?
Radiant insulation like our RadiantShield product works differently to traditional bulk insulation and doesn’t merely slow down or ‘resist’ heat transfer. Radiant insulation blocks radiant heat (up to 97% is reflected) by reflecting it back towards the direction it came from. This ensures that the heat never reaches the insides of your house.
A simple test to prove the effectiveness of reflective insulation is to place a layer of bulk insulation (such as a fibreglass batt) between yourself and a source of radiant heat (such as an oven with the door pulled down or near an open fire). After a short time, you will find that your side of the batt is warm to touch and it will be radiating heat at you.
Then place a sheet of foil between you and the source of radiant heat. You should find that nearly all of the radiant heat has been blocked and very little reaches you – even though the foil will be warm to touch. It will stay this way.
It is the foil that does the summer insulating, not the bulk insulation.
What is the R-value of your material?
As already explained, R-values are a measure of a material's resistance to conductive heat transfer. Radiant insulation works on a totally different principle and thus simply knowing the R-value of a reflective material is useless.
In the case of reflective insulation, Total R-values must be used. The Total R-value simply indicates the thermal performance of an insulation system, determined by a calculation process of airspace values and individual R-values of building materials.
Doubling the product doubles the benefit?
This entirely depends upon the method of installation. If by doubling up one means just adding a second layer of reflective insulation without increasing the number of air spaces, that is if both layers of insulation are in contact with each other, essentially the only increase in thermal value would be the intrinsic R-value of the added layer. In the case of a RadiantShield, that would represent about an added R0.15. This would not represent an economical insulation option. If, however, doubling up were meant to mean dividing the wall cavity into 3 separate and equal air spaces in place of 2 with only one insulation layer, then the result would be an increase in the total R-value.
There is no way a product that thin provides many benefits!
The thickness of reflective insulation has much less to do with the overall thermal performance than does the reflective, (low emittance), the surface facing the adjoining airspace. With respect to reflective insulation materials, the application, including the size of the adjoining airspace(s) and direction of the heat flow are the major contributors to the R-value and thermal performance of the insulation system.
Radiant Barriers and Reflective Insulation only work in hot regions. They are not suitable for use in cold climates.
Reflective insulation products excel in hot climates and often are the first choice for insulation in those regions. However, they also provide significant benefits in cold climates where they are used alone and/or when used in combination with other insulation materials in specific applications.
Does dust accumulation on the foil surface render foil insulation useless?
Remember that an aluminium surface has both reflective and emissive properties. Reflectivity is the ability to reflect radiant energy and emissivity (the reciprocal of reflectivity) is the ability to not release radiant heat.
Over time, there will be some dust accumulation on the top surface of the foil. In practice and through various studies it has been shown that a significant amount of dust is required to reduce the reflective performance of a foil surface.
However, with double sided foil products, there will always be a foil surface facing downwards which won’t collect any dust. This is where the emissive property of aluminium foil is important to us. Theoretically, even if an upwards facing foil surface is covered with dust and starts to absorb all radiant heat rather than reflect it, only 3-4% of this energy will be further transmitted downwards through the downwards facing foil surface. This is because the downwards facing foil surface, as a function of its physical property, can only ever release 3-4% of it’s absorbed energy via radiant heat.