Lance Turner is a technical editor for ReNew magazine. His personal choice for insulation in the home is foil. In this article, he explains why.
11 March 2010 – Insulation improves the comfort level inside your home by reducing heat flows into and out of the building. Reducing heat flows stabilises the temperature inside the house, regardless of the temperature outside. In winter, once the home has been heated to a comfortable level it will stay that way with less energy input than an uninsulated home would require. The same applies to summer conditions. The insulated home will take longer to heat up and if an air-conditioner is used it will use less energy than
one cooling an uninsulated house.
How does it work?
There are three ways in which heat transfers to or from a house: conduction, radiation and convection.
Conduction means the transfer of heat through a substance, in this case the walls and ceiling of the house. The type of insulation used to reduce conductive heat transfer is known as “bulk fill” insulation. This is the most common home insulation and may be in the form of fluffy “batts” made of many materials, including polyester fibre, glass fibre and sheep’s wool. Insulation may also be in the form of loose fill material, such as treated cellulose fibre (usually made from recycled paper), which is simply pumped into the roof or wall cavities. These materials are poor conductors of heat and so when they are installed properly, they reduce the rate of heat flow.
Radiation is a different form of heat transfer. All warm objects radiate heat in the form of infrared radiation. If this heat can be reflected back from where it has come using reflective foil insulation, then heat loss or gain through radiation can be greatly reduced. The main thing to remember with foil insulation is that it needs an air gap between the shiny side and the roof or wall cladding (assuming it has a shiny side facing that direction). If it is fixed such that the wall or roof materials are in contact with the shiny surface then it will not be effective unless it is a double- sided material which has a shiny surface facing into the cavity. In this instance it is not reflectance, but the low emissivity of the reflective material that prevents the heat transfer.
Foil’s effectiveness can also decrease if it becomes dirty or dusty, so it needs to be installed with this in mind. Many foils have only one reflective side and so will reflect heat away only from that side. Foil is often installed into walls on building sites with the reflective side facing inwards. In this instance, the foil will help keep heat in during winter with reflectance, and help keep it out in summer by not emitting it from the shiny side. However, products with foil on both sides, such as the Concertina foil batts from Wren Industries, will have a greater insulating effect than those with foil on just one side, providing that the double- sided foil product is installed in a manner that makes good use of both sides.
Convection heat transfer is often the undoing of many insulation jobs. Circulating air can pass between poorly installed insulation and thus transfer heat into or out of the house
An example of this is the way foil insulation is installed in many house walls. While it will resist radiant energy transfer, air can often circulate across both sides of the foil, as well as through the joins between the sheets. Air also flows through holes in the foil, such as where pipes and cables pass through it.
This circulation greatly reduces the effectiveness of the insulation. However, if foil is used in conjunction with bulk fill (which will stop air currents flowing) the insulating properties are greatly increased, providing an air gap on the reflective side of the foil is maintained.
The problem with this is that most bricklayers are loath to include batts in the walls as they build them. As a result, most newer homes are poorly insulated and require vast quantities of energy to heat and cool to a comfortable level. Putting your foot down with the builder and ensuring that batts are included in the walls can make a huge difference to the natural livability of the home, not to mention energy bills.
One solution to this problem is foilboard, which is expanded polystyrene or EPS sheet with foil bonded to one side of it. This allows the foil and bulk fill to be installed at the same time, thus reducing installation times.
When looking for insulation you will regularly come across the term “R value”. The R value of an insulation product is basically a measure of its ability to insulate, or resist heat transfer. The higher the R value, the greater the level of insulation, so a batt with an installed R value of 1.5 will let more heat through than an R2.6 batt. Some insulations work better in summer than in winter and this will be reflected in their ratings. Foil products often have a better summer rating than a winter one.
To improve the performance of foil insulations in winter they should generally be used in combination with bulk insulation, so foil is under the roofing and wall cladding material, with bulk insulation just above the ceiling and just behind the inner wall cladding.
Another term heard in relation to insulation is “U value”. While R value is an indication of the thermal resistivity of a material, the U value is the inverse of that—how well the material transfers heat. With R values, the higher the number, the better, but with U values, the lower the number the better. It is important not to confuse the two.
Bulk fill insulation
As mentioned earlier, the two main types of insulation systems are bulk fill and reflective foils, though some products use features of both.
Bulk fill insulation is primarily used in ceilings, usually fitted directly on top of the ceiling between the roof joists. This needs to be done with care as even small holes can compromise the insulation’s ability to reduce heat flow. Increasingly it is also used inside walls to improve winter performance.
Bulk fill insulation comes in many shapes, thicknesses and materials, all of which have their pros and cons. The most common materials are still mineral wool and glass fibre. Mineral wool, or rock wool, is composed of tiny fibres made from rock (such as basalt), or sometimes from furnace slag. Fibreglass, as its name suggests, is composed of very fine glass fibres clumped together to form a thick mat.
These mats are available in many thicknesses, with a range of insulation values (R values), usually as packets of pre-cut batts.
Batts are also available in other materials, including recycled and virgin polyester,
sheep’s wool, and even combinations of these two. These materials are also available as rolls, or ‘blankets’, that can be cut to shape as required.
Another type of bulk fill insulation becoming more popular is loose-fill. While arguably in a class of its own, it performs in the same manner as the batts and blankets, that is, as a barrier to conduction.
Loose-fill insulation is usually made from cellulose fibre from sources such as recycled phone books and newspapers. It is reduced to a fine, light and fluffy material that is treated with various fire retardant chemicals. It is usually ‘blown in’ to the space it is to insulate, and can completely fill the gap between the roof joists, as some settling does occur with this material over time. An interesting use of this material is to fill the gaps between wall spaces in existing homes, thus allowing older homes to have their walls insulated without removal of the cladding.
There are many different types of foils available, but most take the form of aluminium foil bonded to a backing sheet. This sheet is often paper treated to resist moisture and fire, and may be reinforced with glass or plastic fibre threads to improve strength.
Some foil insulation comes in the form of batts made from paper or thin card with foil on one or more surfaces. Silverbatts, for instance, have two to four layers of foiled paper separated by short perpendicular walls. They are supplied flat and are pulled open to form batts up to 60mm in thickness which are fitted between or on top of roof joists in a similar manner to bulk insulation.
Another unusual type of foil insulation is the Retroshield and similar products produced by AIR-CELL. It consists of what is basically a heavy grade bubblewrap material, with foil on both sides. The reflective foil reduces radiant heat transfer while the stabilised air space in the bubbles resists conductive transfer. AIR-CELL also make closedcell foam/foil laminations that work in a similar manner.
When used correctly, foil products can have a very high insulating value, especially in summer. When used in conjunction with correctly fitted bulk insulation, the combined insulation abilities can be very impressive.
Pros and cons
While the advantages of correctly fitted insulation are obvious—reduced energy bills and greenhouse gas emissions— there are other factors to consider before deciding on an insulation product.
Over the years there have been numerous claims of the dangers of glass and other mineral fibres, particularly as a possible cause of lung cancer and other diseases of the respiratory tract. There have been many studies conducted around the world, some of which seem to support these claims, and others which do not. That said, mineral and glass fibre are rated as class 2B, possibly carcinogenic materials, by the International Agency for Research and Cancer (IARC).
However, most glass fibre materials used for domestic insulation are now said to be biosoluble, meaning they dissolve in the lungs over two to three weeks. Indeed, some of these materials are supposed to be safe without the use of breathing protection, although installers of these materials still seem to suffer from lung problems—known in the industry as ‘the cough’.
There is no doubt that glass and mineral fibres cause irritation of the skin, eyes and throat, as any fibreglass worker (or indeed, anyone who has walked around inside a fibreglass-filled roof space) can attest. Having worked with fibreglass for some years, I can say that the material is a serious irritant and breathing apparatus is the minimum requirement for handling this substance. I have handled the new bio-soluble materials and they still seem to be an irritant to me—I certainly would never install these materials without the use of breathing apparatus and gloves.
While you would expect that once installed, the glass fibre would not pose a hazard, this can only be said if it is completely sealed into the cavity into which it is installed.
Most homes are not sealed to this degree and have one or more openings between the ceiling and the roof cavity, for fittings such as exhaust fans, manholes, ducted heating and cooling vents, convection vents and the like. What this means is that many homes with glass or mineral fibre insulation may have some degree of contamination of the air inside the living spaces. Virgin polyester, being made from petroleum, is a finite resource. Its production also involves the use of some toxic chemicals, so it certainly has its negative aspects.
Recycled polyester insulation uses far less energy in its manufacture and keeps a vast number of plastic drink bottles from ending up in landfill. The insulation in an average home contains around 1400 plastic drink bottles—that’s a lot of landfill space saved! As far as safety is concerned, both polyesters can be handled without protective clothing, making them safe for do-it-yourself installations.
Another option is sheep’s wool. This material obviously comes from a natural source, but, like all products derived from livestock, has its own associated problems. These include massive land degradation due to clearing for grazing, and movement of the animals themselves. There are also other issues, such as the inhumane treatment of animals during the production process, although the industry is slowly working on some of these problems.
Cellulose fibre insulation has the advantages of being a great use for old newspapers and phone books, although some of the fire retardant chemicals used in these materials are based on the element boron, of which reserves are limited on this planet. These chemicals can also be irritants to skin, eyes and lungs, so breathing apparatus may be a good idea if exposed to these materials for any length of time.
Another safety factor is fire retardance. It used to be that the best of the materials were the glass and mineral fibres. They will not combust and produce little smoke or fumes when subject to a fire. However, in some tests it seems the newer fire retardant cellulose fibre performs as well as or better than mineral fibres. Most of the other materials can be made to burn to some degree, though they all still conform to the relevant Australian Standards and are far less combustible than most other parts of your home.
It would seem that the foil materials are possibly the most environmentally sound products, due in part to the small amount of material required to achieve the end result. Virgin aluminium requires vast quantities of energy to process, but recycled aluminium, which is used in many US made insulations, consumes only around one twentieth of this energy.
So, all in all, there is no perfect insulation from an environmental point of view, but some are definitely worse than others, both in manufacture, installation and living with the product.
My personal choice would be an aluminium foil product used in combination with either polyester or wool batts or blankets, as this combination is unlikely to cause any health problems for residents.
This article is an extract from a longer piece first published in ReNew magazine issue 106, by the Alternative Technology Association.