Guide to reflective Insulation
GUIDE TO REFLECTIVE INSULATION
There are two main types of heat energy that concern us when considering the insulation of buildings:-
Thermal energy which is the heat contained in matter and is represented by the vibration of the molecules. This is what we feel when we touch something hot and the vibrational energy is transferred to our skin. Transfer of thermal energy requires direct material contact between a hot and a cold substance so that the vibrational energy of the molecules can be physically transferred from the fast moving (hot) ones to the
slower (cold) ones.
Radiant Energy which can pass through a vacuum, (it does not require the presence of matter). Radiant energy exists in the form of electromagnetic radiation of varying frequencies, the frequency range that we encounter at room temperature is in the infra-red.
We receive radiant heat energy from the sun across 93 million miles of empty space. We feel the heat when we absorb the radiation and it is converted to thermal energy causing the molecules in our skin and surroundings to warm up. When things cool down they re-emit radiant energy as well as losing thermal energy through conduction and convection.
Heat is lost from buildings in three ways:-
Convection - when warm air rises and is replaced with cold drafts.
Conduction - when a warm and cold material come into contact with each other.
Radiation - when radiant heat (infra-red) is emitted from a warm building.
When considering how best to insulate a building we should consider all three types of heat loss.
Reflective insulations work by reflecting radiant heat in a similar way that a mirror reflects light. Bulk insulators on the other hand resist the transfer of heat through conduction, by having a low density they reduce the capacity of vibrating molecules to physically transfer this kind of energy.
The functionality of radiant barriers is often misunderstood, especially when people try and make comparisons with bulk insulators, such as glass-fiber which, as we have just seen, work in a completely different way. There have been many attempts at comparing a reflective insulator with so many millimeters for example, glass-fiber and this is not appropriate. It is an attempt at comparing two completely different processes, in this case two completely different kinds of heat transfer.
A bulk insulator will absorb radiant heat energy until it is becomes warm (as the radiant energy is converted to thermal energy) and will then pass this heat onto its surroundings, in this way it will give a temporary respite in protecting buildings from radiant heat GAIN, but will give little or no benefit against radiant heat LOSS.
Reflective barriers will work indefinitely as they permanently reflect the radiant heat, (away from or back into a building) and thereby suffer no cumulative build up of thermal energy, as a bulk insulator will.
Unfortunately for the reflective foil industry the ubiquitous ‘U-value’ tends to measure a materials ability to slow down conducted heat loss and does not take into account the properties of a radiant heat barrier, so comparing ‘U-values’ of a radiant heat barrier with that of a bulk insulator is not going to tell you much and is not a fair comparison. Now if we were to use the percentage reflectivity instead of ‘U-value’ the converse would be true. All Aluminium based radiant heat barriers would score 97 percent and all bulk insulators would score nearer to zero. The conclusion here is that we should not cross compare bulk insulators with radiant heat-barriers.
Now, doubling the thickness of a bulk insulator will markedly increase its resistance to conducted heat transfer, but not so for reflective materials. Two mirrors, one behind the other will not reflect twice as much light and so two layers of Astro-foil will give little additional benefit over one layer. This is because 97% of the radiant heat is reflected away by the first reflective surface, (there will be a marginal benefit by virtue of the additional layer of air bubbles which afford some protection against conducted heat loss). The best combination, is therefore, one reflective layer and another conductive insulating layer, so Astrofoil and bulk insulators should not be regarded as alternatives, rather complimentary parts of a system.
So why do a lot of radiant heat- barriers have layers of bubbles and others don’t?
Well, this is because Aluminum is a good conductor of thermal energy and if it were to come into direct material contact with another dense material, such as brick or concrete, it would act a a conductor of heat, not an insulator. The air bubble layer keeps the metal surface away from the dense materials such as masonry and concrete, allowing maximum benefit from the radiant qualities without compromise from the highly conductive nature of the metal in terms of thermal heat.
Other radiant heat barriers, such as Astrofoil Heatshield are not designed to come into contact with dense building material so do not need the air bubble layer. A typical application for Heatshield would be within the cavity of a timber framed house or in the roof. It comes in a perforated version so that it can allow the building to ‘breathe’. This is important when it is used in the horizontal position above or below floor / ceiling joists as a low of moisture vapour rises by conduction and condensation could otherwise occur.
Specific types of Astrofoil for specific applications:-
1. We have already discussed above the use of Heatshield, and its cousin Heatehield-b-re-a-t-h-e-r-f-o-i-l in roofs and walls where the Heatshield does not come into contact with dense masonry materials.
2. Astrofoil Tuffstuff has air bubbles which are stronger than most in compression and this may be used under concrete floor slabs.
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3. Astrofoi originall may be used in many locations, roofs, walls, commercial,agricltural and domestic buildings where it can span up to 8 feet unsupported, as in-between roof trusses.
4. Astro E is basically a lighter-weight version of Astrfol original. The only difference is that it is slightly thinner and less durable lending it to more domestic applications and where damage is less likely to occur during and after installation, the only advantage of using Astro-E over Astrofoil original is the reduced cost of the material, the insulating properties being practically the same
5. Astrowhite has a single layer of foil, the reverse side is lined with a sheet of white vinyl. It is slightly lighter weight and slightly less durable than Astro-E. It can be used in the same locations and is the preferred choice it is to remain visible and there is a preference for the white appearance internally and, again, for the lower cost.
6. Astrosafe is used to line the inside of heating and ventilating ducts. It has a reflective layer of foil that should face into the duct (when used on the inside) and a layer of Polyester foam which affords some protection against conducted heat-loss. The outside of the ducts can also be lined with Astrofoil original.
Radiant heat barriers can also be used in packaging and transportation. Astrofoil original and tuff stuff can be used as a physical protective material for small packages that require physical as well as thermal protection. Heatshield can be used to line the interior of transportation containers and gives an almost horizontal temperature profile inside the container which may be subject to wildly varying conditions in transit. It can also be used to protect the contents of individual pallets, which may find themselves left outside in direct sunlight for periods of time during transit.
Radiant barriers work to keep buildings cool in hot climates and warm in cold ones. They are often used in very hot countries to protect from the radiant heat from the sun, thus saving on air-conditioning running costs.
Using Astrofoil in Basements and Crawl Spaces is a specialist application and the material has to perform a dual purpose of damp-proofing and insulation. A special range of Astrofoil products has been developed and tested under the ThermalDry brand. These products are discussed in our other guide,’Waterproofing Basements’
For more information call 0800 413 801
Guide written by Raymond Foulkes M.Sc.DMS, MD of Import Supplies Direct Ltd and Basement Systems UK ltd.
Trade and specification enquiries should be directed to raybsuk@googlemail.com
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Copyright 2009 Import Supplies Direct Ltd.
