BY reducing heat flows into and out of your home, insulation can dramatically improve comfort levels during weather extremes. In winter, once the home has been heated to a comfortable level, it will stay that way with far less energy input than an uninsulated home would require. The same applies in summer. A properly 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. Note though, that any windows with high solar heat gains need to be shaded, particularly west windows, as in hot weather, insulation can slow down the ability of the house to cool down if there are large heat gains from windows.
Heat transfer and insulation 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, floor and ceiling of the house. The type of insulation used to reduce conductive heat transfer is known as ‘bulk’ 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. Bulk 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 and sealed with a spray-on ‘cap’. All these materials are poor conductors of heat and so reduce the rate of heat flow, provided they are installed properly. 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 from using reflective foil insulation, then heat loss or gain through radiation can be greatly reduced. Reflective surfaces such as foil don’t just reflect, they also have low emissivity (the ability to emit radiation, or heat in this case), meaning heat that has entered the material from the nonreflective side is not emitted from the reflective side easily.
This means that foils can work reducing heat flows in both directions, even if only one side of the material is reflective. Convection heat transfer (heat transferred through the circulation of air) is often the undoing of many insulation jobs. Circulating air can pass between poorly installed insulation materials and thus transfer heat into or out of the house, vastly reducing the effectiveness of the insulation. Minimising convective heat transfer is discussed later in this article. Bulk insulation (R1.5, R2, R2.5, R3, R3.5, R4, R4.1, R5, R6, R7) Bulk insulation is primarily used in ceilings, where it is usually installed directly on top of the ceiling between the ceiling joists. Increasingly it is also used inside walls (R1.5, R2, R2.5) and even under floors to further improve thermal performance. Bulk 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. Bulk insulation takes many forms, such as the Ceilink polystyrene foam insulation ceiling panels from Versiclad, which come with a laminated white finished metal face.
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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. Batts are also available in other materials, including recycled and virgin polyester, sheep’s wool, and even combinations of these. All of these materials are available in many thicknesses, with a range of R values, usually as packets of pre-cut batts. They are also available as rolls, or ‘blankets’, that can be cut to shape as required. The installation of batts needs to be done with care as even small holes can compromise the insulation’s ability to reduce heat flow.
Downlights are one common cause of compromised insulation—there must be a certain amount of space around downlight fittings to prevent overheating and fires. To reduce the level to which the insulation is compromised, insulating downlight covers or mitts can be fitted over many types of light fittings. These prevent bulk insulation from encroaching on downlight fittings while providing a reasonable level of insulation. Note that it is important to check that the downlight cover is suitable for the type of downlight. One important point to note with bulk insulation is that its R value rating is only valid if it is installed so that its thickness is not compromised, i.e. it is uncompressed when installed. Many installations of batts don’t perform as well as they should because the installer has compressed the batts when fitting them between the joists or in wall cavities. If you compress a batt to less than its rated thickness then it will not perform at its rated R value. A 110 mm thick batt squeezed into a 90 mm space will lose over 10% of its R value. In plasterboard-lined stud walls, highly compressed batts have been known to push the plasterboard off the wall.
Another type of bulk insulation is loosefill. 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 cellulose 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 fire-retardant chemicals. It is usually ‘blown in’ to the space it is to insulate, often fill right to the top of the gap between the ceiling joists, as some settling may occur with this material over time. An interesting use of this material is to fill the wall cavity spaces in existing homes, thus allowing older homes to have their walls insulated without removal of the cladding.
Any loose-fill insulation will settle over time so the average R value will reduce. R values may decrease by as much as 20% over the life of the product. When first developed, there were reports of this material losing its fire-retardant abilities due to leaching of the fire-retarding chemicals when the insulation was subject to moisture. However, this problem has been overcome, provided the product is made to Australian Standards requirements. Another problem sometimes experienced with loose-fill material R and U values When looking for insulation you will regularly come across the term ‘R value’. The R value of an insulation product is 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 insulation materials work better in summer than in winter and this will be reflected in the ratings. Foil products installed in roof spaces often have a better summer rating than a winter one. To complement the performance of foil insulation in winter it should generally be used in combination with bulk insulation: foil would be used under the roofing and wall cladding, with bulk insulation just above the ceiling and just behind the inner wall cladding. There are now numerous products that combine both foil and bulk insulation, which are suitable for use in walls, under floors and in ceilings.
The R value quoted for foil products is often the total R value of the building element it is installed in, e.g. total R value of a wall, whereas bulk products are marketed on the basis of the R value of the insulation alone. If a foil product quotes a total-element R value, subtracting around 0.5 from this will generally give you the extra R value added by the reflective air spaces. 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! Note that the window industry uses U values to describe the performance of their products. There are different R value systems used in different parts of the world.
For instance, the USA uses a system with much higher numbers for a given insulative value, so be aware that some suppliers may list the American R value to make the product look better than it really is. It should also be noted that some insulations work better in winter than summer, or vice versa and these will often be specified with two sets of R values. Such materials may be more suitable in areas that have cold winters and mild summers, where retaining warmth is more important than excluding it. g Foil insulation often comes as sheets that are stapled in place. Here we can see a different form of foil insulation—Concertina foil batts from Wren Industries, which are used like bulk batts and require no stapling.
Is ‘blowing around’, where air currents in the roof cavity move the material from its original position. At least one company, Cool Or Cosy, has overcome this problem with their SupaCell product by applying a sealant to the insulation once installed. Another material gaining popularity is spraying foam. This is typically injected through gaps or small drill holes into walls where it expands and sets, completely filling the cavity. It is necessary to seal off penetrations through the wall, e.g. wall vents in older houses, to ensure the material does not escape. However, there have been some issues raised in the USA, where this type of insulation is popular, regarding the toxicity of the material and the sickness it appears to be causing in some homeowners. The debate is ongoing, but at least one supplier, Ecofoam, provides a spray foam for walls and floors that is said to be VOCfree and completely non-toxic. Not all bulk insulation is soft or flexible. Some is in the form of rigid sheets that may or may not also include a foil or other material laminated onto one or both sides. They can usually be cut to size, and materials include expanded polystyrene and thermoset foams such as Kingspan’s Kooltherm and Vesiclad’s Ceilink ranges.
Reflective foils There are many different types of reflective foil 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 sometimes reinforced with glass or plastic threads to improve strength. Foil can be used under the roof, in walls or under the floor.
Foil is often installed into walls with the reflective side facing in towards the room. The foil will then help keep heat in during winter with reflectance, and help keep it out in summer by not emitting it from the shiny side. When installed in the roof, foil is usually installed either under the roof battens/joists, creating an air space between the foil and the roofing material, or is fitted directly under the roofing material on the outside of the joists, effectively being sandwiched between the roof material and the joists. In either case, single-sided foil should have the foil side facing downwards (into the roof cavity). This will have the effect of greatly reducing heat ingress in summer due to the foil’s low emissivity, while in winter it will tend to keep roof cavity temperatures higher by reflecting heat back into the cavity. This has the effect of slowing heat loss through the bulk ceiling insulation by reducing the temperature differential between living spaces and the roof cavity. Under floors, foil materials can be fitted under floor joists or on top, but single-sided materials must have the foil side facing down to prevent dust buildup reducing effectiveness. The main advantage to underfloor insulation comes in cooler climates during winter—up to 15% of heat can be lost through floors. A critical thing to remember with foil insulation is that it needs an air gap between the shiny side and the roof or wall cladding, or floor that it faces. When used correctly, foil products can have a very high insulating value, especially in summer.