Building Envelope  »  Thermal insulation

Insulation materials can come in various forms whether mineral synthetic / petrochemical or natural materials. Mineral wool is a common insulation material used for buildings and a number of other typical insulation materials exist, including aerated concrete, cellular glass, expanded and extruded polystyrene (EPS and XPS), polyurethane, cellulose fibres and fibre boards. More innovative insulation concepts (can be clicked upon for further information, see second text box below) include the dynamic insulation that has been in use in Scandinavia and Austria for some time as well as transparent and translucent insulation and vacuum insulation panels and nano fibres. The insulation material should provide long-term performance, as it is rather laborious to replace. Early deterioration of the material will not only lead to an increase in energy bills but it might also damage the building structure through condensation (link to ‘thermal bridging’). Close attention must also be paid during the construction phase. If not properly installed, the potentially high benefits of the insulation can be lost (Harvey, 2006).

Managing condensation
In all climates where Insulation is used care should be taken to prevent condensation between the exterior and conditioned interior due to the variations in humidity levels. Depending on the material, condensation can drastically reduce the functionality by decreasing the insulating properties and damaging the construction materials. Care should always be taken that insulation is protected from water and remains dry, as water is a good conductor of thermal energy. In addition to this water within the envelope can lead to mould and mildew, which pose a health hazard.

Reflective barriers
Insulation can also be further improved upon through the use of a reflective surface to reduce thermal radiation. This is either through a reflective barrier or reflective insulation. Reflective barriers are highly reflective films with out insulation. Reflective insulation is a combination of reflective material and insulation usually with a reflective surface on sides of the insulation material. Radiant barriers should have an extremely low emittance and high reflectance. The reflective surface of both needs to have an emissivi-ty of 0.1 or less and a reflectivity of 0.9 or greater. The effectiveness of such reflecting surfaces is however, highly dependant on the direction of the heat flow. In addition to this a reflecting surface should be combined with an air gap of at least 25 mm as reflective surfaces in close contact to another material have no effect on heat transfer as this can take place through conduction. With age the effectiveness of such insulation is also reduced as the accumulation of dirt and dust on such surfaces reduces its effectiveness. Foil laminates can be used to great effect in open systems (where in insulation is used) to reduce heat gain in a building interior through radiant heat transfer. They can reduce heat transfer to the building during the day yet release functioning as a heat diode. In hot climates reflective foil laminates can be used to great effect.

The following figure shows the amount of insulation necessary to maintain a certain internal temperature for a given outdoor temperature. The first few millimetres thickness of insulation has usually the maximum impact on the U-value and beyond a certain thickness of insulation the U-value reduces only minimally. This table shows varying thickness of insulation, its impact on the U-value and the corresponding internal surface temperature of the wall. The thickness of the insulation should be more than 160 mm since anything less would cause an uncomfortable indoor surface temperature of the wall. The amount and thickness of insulation depends on the energy efficiency level of the building that one is aiming for. The exact thickness needs to be calculated during the planning phase. To meet the Passive House standard of no more than 15 kWh/m²/year of heating energy, however, the following indicative U-values (W/m²K) are required in three different locations (Harvey, 2006):

Close attention must be paid on site and training be given, in order to make sure that the full potential of the insulation is reached. A study on the implementation of insulation a per California Energy Commission Envelope Protocol for housing, and qualitative assessment of the rate of compliance in 30 new energy-efficient houses in California shows that insulation has not been installed properly in all the surveyed houses. The report lists several key installation features and the percentage of houses in which these are not properly implemented. Installation of insulation needs special training (Harvey, 2006).

These include products based on silicon and calcium (glass and rock).

Mineral wool and glass wool (fibre)
Mineral or rock wool is produced by melting a base substance at high temperature and spinning it into fibres with a binder added to provide rigidity. It is vapour and air permeable due to its structure. Moisture can build up in the insulation reducing its insulating value and it may degrade over time. Lambda* value 0.033-0.040 W/mK. There is a health issue with fibrous materials. Some cause skin irritation and it is advisable to wear protective gear during installation. Loose fill fibre insulation should not be ventilated to internal habitable spaces. There has been the suggestion that fibrous materials constitute a cancer risk. However, they are currently listed as ‘not classifiable as to carcinogenicity in humans’.

Cellular glass
Manufactured from natural materials and over 40% recycled glass. It is impervious to water vapour and is waterproof, dimensionally stable, non-combustible, vermin-proof and has high compressive strength as well as being CFC and HCFC free. Lambda value 0.037-0.047 W/mK depending on particular application.

Vermiculite
Vermiculite is the name given to a group of geological materials that resemble mica. When subjected to high temperature, the flakes of vermiculite expand (due to their water content) to many times their original size and become ‘exfoliated vermiculite’. It has a high insulation value, is resistant to decay, odour-less, and nonirritant.

Materials derived from organic feedstock based on polymers. They are confined to cellular structure.

EPS (expanded polystyrene)
Rigid, flame retardant cellular, non-toxic, vapour resistant plastic insulation, CFC and HCFC free. Lambda value 0.032-0.040 W/mk.

XPS (extruded polystyrene)
Closed cell insulant, water and vapour tight, free from CFCs and HCFCs. Lambda value 0.027-0.036 W/mK.

PIR (polyisocyanurate) as an improvement to PUR (polyurethane)
Cellular plastic foam, vapour tight, available CFC and HCFC free. The main concern here is the toxic cyanide fumes if burned. Lambda value 0.025-0.028 W/mK.

Phenolic
Rigid cellular foam with very low lambda value, vapour tight, good fire resistance, available as CFC and HCFC free. Lambda value 0.018-0.019 W/mK. In general, cellular materials do not pose a health risk and there are no special installation requirements.

Vegetation-based materials like hemp and lamb’s wool, which must be treated to avoid rot or vermin infestation.

Cellulose
Mainly manufactured from recycled newspapers. Manufactured into fibres, batts or boards, treated with fire retardant and pesticides. Lambda value 0.038-0.040 W/mk.

Sheep’s wool
Must be treated with a boron and a fire retardant. Disposal may have to be at specified sites. Lambda value 0.040 W/mk.

Flax
Treated with polyester and boron. Lambda value 0.037 W/mK.

The innovative concept of dynamic insulation began with ceilings in Norway in the 1960s and has since been widely used throughout Scandinavia and Austria, with isolated examples in a number of other countries. The concept has also been investigated through experimental test facilities (Baker, 2003). Normally, heat diffuses outward through the building insulation. This can be counteracted by drawing the required outside ventilation air into a building through the wall or roof insulation.

Air is drawn into a cavity between the insulation and the wallboard, and distributed through the house via ducts and vents. In a demonstration house in Switzerland, the incoming air thus warms to 16°C during its passage through the insulation, and to 19 °C as it passes through the ductwork inside the warm floors. The room air temperature is 22 °C, which is the temperature at which exhaust air is collected. Heat is extracted from the exhaust air with a heat pump, which cools the exhaust air to -2 °C and creates water at 45 °C. This water is stored in a hot-water tank used for radiant floor heating, maintaining a floor temperature of 25 °C, which in turn is sufficient to maintain the 22 °C room air temperature. A supplemental heat source is used when heat extracted from the exhaust air is not sufficient, which will be the case when the final exhaust air temperature is warmer than the outside air.

The effective U-value of the roof was reduced from 0.26 W/m²K for the insulation alone (not the build up of the entire roof) to 0.03 W/m²K due to inflow (hardly any heat flows out of the building through the insulation but in this case into the building).

Aerogels are a form of translucent insulation material which is located within a glazing sandwich. Aerogels are materials that are mostly air – usually around 99 % by volume – and can be fabricated from silica, metals and even rubber. They are extremely light. For example, a cubic metre of silica glass would weigh about 2000 kg. A silica aerogel block of the same dimensions would weigh 20 kg. Despite this, aerogels are relatively strong. Silica aerogels consist of tiny dense silica particles about 1 nanometre across which link up to form a gel.

Aerogels are excellent insulators, having about one hundredth the thermal conductivity of glass. Double glazing that replaced the air gap with an aerogel would improve the insulation value by a factor of three as against the very best current multiple glazing. It would be possible to achieve a 99% vacuum between the panes, since they are supported by a solid. However, even with a thin aerogel sandwich the window would have a slightly frosted appearance. The thermal properties of aerogels also make them ideal for harvesting solar heat. Flat plate solar panels collect heat then radiate it back into space.

Insulation can come in various forms.

Blanket or batt Insulation
This insulation is a flexible insulation usually made from mineral fibers such as rock wool, fiberglass or wool. Such insulation is usually manufactured in standard widths and must be cut for non-standard openings.

Rigid Insulation
Rigid insulation can consist either of fibrous materials bound together or plastic foams. These are produced in fixed sizes and can be cut for non-standard areas. Rigid insulation is often used where structural loads (such as in foundations) needs to be carried by the insulation.

Loose insulation
This insulation comes in loose from which is either poured or blown into a cavity or onto a space. Common types are cellulose, fiberglass, and perlite, foam glass or expanded clay granulates. One drawback of such loose insulation is that some forms can settle reducing the effectiveness of the insulation. To prevent this they are often blown/sprayed into the cavity with adhesive or foam to make them resistant to settling.

Foam Insulation
Foam insulation is an insulation which is sprayed into cavities. Two types of foams are generally used, an expanding and a non-expanding foam. Expanding foams are mixed in a two-component together when sprayed. One advantage of foam insulation is that it can effectively fill cavities with cracks which would influence the insulating properties.

Movable Insulation
Windows are generally on of the weakest points in the insulation of a building. Movable insulation can be used for example as shutters or roller shutters to better improve the insulation of for example a window.

In hot and humid climate zones, the priority lies on the reduction of the cooling load. Lightweight materials with low thermal mass should be used in hot and humid climates to avoid unnecessary heat build up and a rapid cooling at night. As the diurnal temperature range is relatively low mass materials should be used. In actively cooled buildings the focus should be on a good insulation.

In addition to this reflective insulation and vapour barriers are important in actively cooled buildings as these help to reduce the vapour into the building through the building façade. Insulation should be used that minimises heat gain during the day and maximises heat loss at night. Advanced reflective insulation systems and reflective air spaces can be effective.

In hot and arid regions thermal mass is of benefit for night cooling. External walls with high thermal mass are to advantage. These can be combined with insulation that reduces heat transfer to the thermal mass through the outer wall. Adding 50 mm of polystyrene insulation to the roof of a one-storey house*in Cyprus reduces the cooling load by 45%. Adding the same amount of insulation to the external walls, reduces the remaining cooling load by a further 10% (the heating load is reduced by 67% and a further 30% respectively). Even though, this lies in the temperate climate zones, the study still shows the effect of insulation on cooling. (Florides et al, 2002) For a one-storey building in Tehran, 100 mm of insulation on the walls and roof (U-value of 0.38 W/m²K) reduces the cooling load by 14 % (and the heating load by 55% (Safarzadeh and Bahadori, 2005)).

Insulating the walls of a typical high-rise residential building in Hong Kong, reduces annual and peak air conditioning energy use by about 15% if both external and internal walls are solid. Applying insulation to both sides of the external walls and the internal partitions reduces the annual cooling energy use by about 40%, though, while increasing peak energy use by 4%. This is a special case however, because in China, residents tend to air-condition only those rooms that are occupied at any one time. It is important to consider the occupants’ requirements and behaviour right from the start of the planning process.

The most important parts of the building’s envelope with regards to heat gain, are the roof and the glazed areas. The walls play a smaller role here but are also important (see pie chart below), especially during the morning and afternoon hours when the low altitude sunlight heats up the vertical walls. Insulation is therefore required on the roof as well as on the walls.

The minimum requirements for the overall U-value of roofs and walls should be 0.5 W/m²K for low-energy buildings which corresponds roughly to 50 mm of insulation depending on the construction (proof may be required here: Indian Green Building Council says ca. 20 mm on walls and 50 mm on the roof (U-value of 0.5 and 1.25 respectively); CSBE study for Jordan says U-value of 0.45 for roofs, 50mm insulation and 30 mm insulation to walls achieving U-values of 0.5-0.9). Green roofs or rooftop gardens can also substantially reduce roof temperatures as well as the ambient air, therefore reducing the cooling load of the building in two ways.

Care must be taken in closed buildings in hot and humid countries to prevent any condensation within the building envelope and thus condensation. This condensation is caused by warmer humid air entering the building uncontrolled through the building envelope. Here it is cooled and water can condense out. This uncontrolled infiltration can lead to higher energy consumption, water condensation and damage in the building envelope, as well as the possibility of mould.

Roof ponds make use of evaporative cooling and are therefore applicable to hot and dry regions. The cooling potential of roof ponds is also two-fold. They cool the structure of the roof itself, therefore reducing the heat transfer to the interior, as well as cooling the ambient air. Both green roofs as well as roof ponds will be dealt with in more detail in ‘natural ventilation and passive cooling’.

In hot-arid climates with high diurnal temperatures dense materials can be used to good effect. The high thermal capacity of dense materials allows for heat storage during the day and dissipation at night. In addition the thermal time lag can be used to good effect with walls where the time lag is greater than 9 to 12 hours.

In recent years attention has been focused on the use of very high levels of insulation within the building fabric in order to minimise the heat loss. Sometimes this is also referred to as superinsulation. Most buildings with superinsulation achieve U-values between 0.1 and 0.2 W/m²K in walls, roofs and floors. Compared to the current standard in many countries, this represents an enormous improvement. The higher the insulation levels in a building, the more attention needs to be paid to air tightness and ther-mal bridging (link to ‘thermal bridging and air tightness’).

However, insulation thickness is often constrained by common construction techniques or by a reduced floor area when the perimeter of the building is fixed. Vacuum insulation or nano fibre insulation may in future allow thinner walls for the same insulation value. Translucent insulation makes use of materials that enhance the solar heat gain whilst simultaneously reducing the heat loss from the interior. It makes this type of insulation attractive for cooler climates, where the additional heat source of the sun can be used to advantage. The insulation allows the incoming solar radiation to penetrate, but acts as a barrier to conductive and radiative heat loss from internal spaces.