Sustainability: Thermal insulation

01 — Introduction

Thermal insulation is specified to control three components of heat transmittance through the building fabric:

• Conduction of heat through building fabric
• Convection via air movement
• Radiant transmission, typically through glass but also through other elements of building fabric.

High levels of insulation and air-tightness are an absolutely critical element of the low-energy, “Passivhaus” concept, but elsewhere, enhanced insulation is not necessarily seen as being a key element of a sustainability strategy.

However, as most thermal insulation is an integral part of the building fabric, specification decisions will have a long-term impact on carbon emissions that is difficult to reverse and, accordingly, should be taken with some care.

There are a wide range of factors that could be considered when determining the appropriate insulation solution. While the target for an absolute reduction in energy consumption and carbon emissions should be the main driver, there are other factors which should be considered. These include:

• Effect on building design, such as the impact of external wall thickness on layouts, net floor area and light penetration through window reveals.
• A balance between heavyweight and lightweight construction, including considerations related to exposed thermal mass.
• Performance in use and longevity.
• Buildability and the risk of on-site work not meeting the required design standards.
• Sustainability implications of the production process including sourcing of raw materials, ozone depletion, embodied energy and eventual disposal.

Accounting for these issues in the round can result in opportunities for some quite interesting “green trade-offs” focusing attention on the client’s specific sustainability priorities.

02 — Building fabric performance standards and insulation strategies

The use of thermal insulation in building fabric is a relatively recent development.

While thermal mass and solar control have been exploited for centuries, 85% of housing built before 1965 in the 15 core EU states has no thermal insulation. Presently, only about 50% of dwellings in these countries have any form of thermal insulation. In reality, present-day minimum insulation standards represent only an intermediate step towards truly low-carbon buildings.

The specification of the insulation performance standard and the design solutions adopted are decisions that will have a long-term impact on the performance of a building. But this could be significantly degraded by the quality of the installation or by the long-term performance of the material in situ.

The UK’s current thermal transmittance targets are set out in the 2006 Building Regulations. The overall approach to the reduction of carbon emissions adopted in the regulations is much less prescriptive than in previous editions. As a result, the minimum insulation standards have remained unchanged, while the overall level of permissible carbon emissions have been substantially reduced.

Designers therefore have the discretion to achieve carbon reduction by other means, such as increased services efficiency, as well as by improvements to envelope performance. While balancing heat retention and cooling requirements in public and commercial buildings is often an appropriate approach due to the need to control cooling loads, for residential buildings, higher insulation standards can have a direct impact on the carbon emissions associated with heating and should be encouraged. The current U-value standards are set out in the table at the bottom.

By contrast to the standards which underpin the current Part L, an alternative standard, the Passivhaus concept of low zero-carbon housing, is driven by the concept of super insulation, minimal air infiltration, carefully controlled solar gain and use of high levels of energy recovery to minimise requirements for space heating.

While developments designed to follow the Passivhaus principles are rare in the UK, they illustrate the impact that high levels of insulation can have on carbon emissions. For example, it is calculated that 65-90% less energy is needed to heat a space designed to a low-energy standard than one designed to meet Part L. To achieve this performance, U-values need to be much higher – typically 0.15 W/m²K for walls, with triple-glazed, krypton-filled windows needing to achieve mid-pane values of about 0.8 W/m²K.

The key to devising an economical, low-energy design solution based on a passive model is in providing sufficient insulation and thermal mass. This eliminates the requirement for a heating system, offsetting the costs of additional insulation against the savings in building services. Passivhaus design standards are not directly equivalent to Part L and the table at the bottom provides a range of equivalent performance standards.

03 — Criteria for specifying thermal insulation

This section sets out the criteria against which insulation systems should be selected.

Performance in use and longevity

• Stability and expected effective life.
• U-values achieved in practice, including basis of calculation of the design performance; for example, does the design performance assume some degree of initial loss?
• Vulnerability to factors affecting performance including moisture, movement and compression of fill, and vermin attack.
• Durability of treatments such as flame retardants.

Balance between thermal mass and lightweight construction

• Availability of composite products such as SIPs (structural insulated panels).
• Location of insulation – there is an opportunity to exploit thermal mass of building fabric if insulation is placed on the outside face of solid construction.

Impact on building design

• Thickness of the insulation material and effect on floors and external wall thickness.
• Opportunities to incorporate the insulation into other aspects of building function; for example, creating falls to flat roofs using tapered products, or the use of a roof void as habitable space.
• The effect of retro-fit insulation solutions on a selection of facade options; for example, insulated render as opposed to rain screen panels.
• Ease of future upgrades.
• Initial capital cost.

Buildability

• Detailing required to achieve thermal integrity, including avoidance of cold bridges, air-flow paths, opportunities for settlement or compression of fill and other movement during and after construction.
• Requirements for vapour and radiant barriers to maintain performance.
• Ease of forming to shape and size, and availability of pre-cut materials.
• Ease of testing and inspection.
• Ability to undertake remedial work without significant disturbance to other work.
• Health and safety considerations for installers.

Manufacture and disposal

• Sourcing of raw materials.
• Impact of manufacture on environment; for example, the use of hydrochlorofluorocarbons (HCFCs) in producing plastics-based material.
• Embodied energy in the production process.
• Ability to reuse or recycle at end of life.
• Constraints on the disposal of materials.

The implications of these selection criteria are that specification decisions made on the basis of one or two initial criteria – say, cost and embodied energy – might not place sufficient weight upon long-term performance issues that will determine the overall contribution of the chosen insulation products to carbon reduction.

The selection of recently developed “green” insulation products could, for example, involve long-term durability and performance risk that could be mitigated by using other products. The clear ranking of the client’s priorities with respect to specification is therefore important, although long-term thermal performance should be the priority.

04 — Building fabric insulation products – alternatives compared

There are three primary families of insulating products distinguished by the sourcing of their raw materials:

Mineral fibre-based products

• The product family includes mineral fibre and glass fibre.
• Mineral-based products are available in batts, rolls and loose. This makes them suitable for most applications in off-site and in situ construction.
• Mineral-based products have an open structure and are air and vapour permeable. As a result, site detailing of vapour and radiant barriers, such as foil backing, are important in ensuring long-term performance.
• Mineral products typically have a higher rate of thermal conductivity than plastics- based products, requiring typically 50% thicker insulation to achieve the equivalent performance.
• Thermal conductivity can be increased by either compaction or wetting. Consequently, the long-term performance of the insulation layer depends on the quality of workmanship and the integrity of the building fabric.
• U-values of 0.2W/m²K can be achieved with a mineral wool thickness of 125mm in a standard cavity wall and 165mm in a timber frame. Reducing U-values to 0.1W/m²K requires between 300 and 350mm of insulation overall – equivalent to a total section thickness of just over 500mm – a significant potential loss of floor area.
• Mineral and glass wool products are sourced from recycled waste, albeit that chemical binding agents are required to form rigid sheets and insulation batts. Mineral fibre insulation can be reused if removed in a suitable condition. There are no health issues associated with the disposal of mineral fibre by either incineration or landfill.

Mineral cellular products

• Cellular glass is available in blankets, batts and boards.
• Cellular glass is dimensionally stable and impervious to air and water vapour movement, making it suitable for use in exposed conditions. Thermal conductivity is stable in the long term.
• Vermiculite can be used as loose fill.
• Cellular glass and vermiculite have a higher thermal conductivity than mineral fibre products, requiring an additional 30% thickness to achieve an equivalent insulation performance.
• Cellular products are formed using recycled waste product such as glass, together with an aerating agent.
• Mineral cellular products can be recycled as building aggregates.

Plastic cellular products

• Plastics products range from extruded and expanded polystyrene to phenolic foamboards.
• Plastics products are available as foam, rigid sheet and loose fill.
• The thermal conductivity of foam-based products is very low, enabling high levels of performance to be achieved using a significantly thinner section. The differential increases as required U-values get lower, making plastics products particularly suitable for super-insulation applications.
• Plastic products are dimensionally stable and are not affected by water ingress, rot or vermin attack.
• Plastics products are create mostly from oil-based raw materials. The production of plastic insulation has been associated with the use of ozone-depleting agents such as HCFCs. Hydrofluorocarbons (HFCs) are now used for production in Europe. They have no effect on ozone, but are still greenhouse gases. Over time, production is switching to the use of neutral hydrocarbons or CO2 as blowing agents. Specifiers need to take care when sourcing insulants from outside the EU to ensure the production process has not involved ozone depleting substances.
• Some plastic insulants can be difficult to recycle and dispose of (see below).

Plant and animal-sourced cellular and fibrous products

• Plant and animal products are sourced from renewable raw materials. As a result, their production typically has a low embodied energy and low impact on the environment.
• Cellulose and wool-based insulants require chemical treatment to protect them from fire, rot and vermin infestation. The long-term performance of the chemical treatments is potentially vulnerable to degradation because of the presence of moisture. The thermal performance of wool is unaffected by moisture.
• These products are available as fibre, batts or compressed board. Some, such as cellulose, have relatively low compressive strength.
• The thermal conductivity of wool and cellulose products is similar to mineral wool.
• End-of-life recycling or disposal is easy and without harmful by-products.

06 — Recycling and disposal of plastics-based insulation

Changes to the landfill regulations introduced in 2005 have made it difficult to dispose of plastics-based insulation containing CFCs and HCFCs. Until 2000, polyurethane (PUR) and polyisocyanurate (PIR) was manufactured in the EU using CFCs and HCFCs as blowing agents, but these are now classified as hazardous waste.

It is a particular problem for PUR-cored sandwich panels, and with nowhere to safely dispose of insulated panels more than five years old, the industry faces its equivalent of the “fridge mountain” crisis.

Owners must ensure that new panels using PUR and PIR insulation are clearly documented as not being manufactured using ozone-depleting substances, as otherwise they will be forced to use the hazardous waste route, irrespective of the presence or otherwise of CFCs/HCFCs.

07 — Thermal performance of glass

Natural light is an important component of the internal environment and modern design has evolved to exploit the architectural and environmental qualities of glass. While target U-values have not changed in the Building Regulations, requirements to control solar gain and overheating have placed the spotlight on glass specification and performance.

The thermal transmittance of glass can be reduced in several ways including the use of low-e coatings or inert gases such as argon or krypton within the glazing cavity. Such enhancements make it possible to reduce mid-pane U-values to 0.85 W/m2K for triple-glazed units.

The performance of the glass is, however, only part of the problem and the frame and associated edge condition is usually the weak point for thermal transmittance, particularly for curtain wall. Clients and project teams must be careful not to confuse mid-pane U values with the overall U-value for a facade, which will always be significantly lower.

Insulation within the frame sections, thermal breaks and so on mitigate the problem, but there is no escaping the fact that the introduction of more framing members, around opening lights or solid insulated panels for example, has a detrimental impact on the overall thermal performance of the walling system.

The contribution by glazing systems to other aspects of thermal performance are as follows:

• Air infiltration rates as low as 3m³/m²/hr @ 50Pa can be achieved using unitised curtain wall – well above the revised requirement in revised Part L of 7m³/m²/hr. With windows, sealing within the units is typically very good, but overall airtightness is reliant on on-site installation and supervision. Improvements in the airtightness of window units have necessitated the development of more sophisticated trickle ventilation systems, often at a premium cost.
• Glazing systems can be used to mitigate solar gain through either the incorporation of solar shading or the specification of solar-control coatings, which are applied to the inner face of the outer pane of double-glazed units. Low-tint, solar-control coatings known as “super neutrals” cut out up to 63% of solar gain, but cannot be bent or curved due to the soft coating used. Where bent or curved panels are required, hard coatings with a deeper tint need to be specified.

The table below sets out indicative extra-over costs of treatments to glass units.

08 — Performance and comparable costs

The table below (Comparative capital cost and performance of thermal insulants) sets out indicative costs and performance criteria for a range of insulants. Due to the variation in thermal transmittance and, as a result, the variation in thickness required to achieve a desired

U-value, it is difficult to achieve a direct comparison. The table also points to a wider range of sustainability issues that need to be considered when specifying.

From a cost point of view, mineral fibre products are the most competitive, but are associated with buildability issues. On the basis of thermal transmittance, plastics products have a clear advantage, while the polystyrene products are quite cost effective.

As plastics are rigid and stable, they also offer reliable long-term performance. However, despite strenuous efforts by manufacturers to update their processes and to educate specifiers, doubts with regard to sustainability and the disposal of plastics products continue to be raised. Naturally sourced materials have clear advantages from a materials source, embodied energy and disposal viewpoint but there are issues over long-term performance.

It is left to the specifier to determine whether lifetime carbon reduction should take precedence over the wider environmental implications of the selection of an insulant.