CPD: Panels in the frame for thermal efficiency

Structural insulated panels’ ability to deliver airtight buildings offers key benefits to designers. Pete Blunt of the UK SIP Association reports

Structural Insulated Panel (SIP) building systems meet the challenges of today’s construction industry by delivering thermally efficient and airtight buildings.

SIPs utilise composite panel technology to provide load-bearing construction elements that combine structural and thermal properties in one holistic solution.

Typically, a SIP consists of two layers of oriented strand board (OSB) bonded to an insulation core, usually made from expanded polystyrene (EPS) or polyurethane (PU). The composite structure is much stronger than the constituent parts, with the insulation core stabilising the OSB and preventing deflection under loading.

SIPs were first used in the US in the 1930s and more recently have been adopted in the UK and continental Europe. They have been used mainly in domestic low-rise buildings to date, but there is increasing demand for them in commercial, health and educational applications.

SIPs and thermal losses

Thermal losses are dictated by the hygrothermal performance – in other words, the transportation of heat, air and moisture through the building envelope. This exchange of energy and moisture is due to differences between indoor and outdoor temperatures, pressure and humidity. Appropriate specification of materials combined with accurate design detailing at critical junctions can ensure heat loss is kept to a minimum.

The composite nature of SIP systems provides an efficient solution to reducing thermal losses and therefore improving energy conservation.

Hygrothermal performance can be split into the sections listed below for analytical assessment. Each area involves a set of issues that require trade-offs and compromises to reach the optimal solution.

Thermal resistance (U-values)

The thermal resistance of a material indicates its ability to transfer heat. In solid materials, this is directly proportional to the material’s thickness since heat is transferred via conduction. To improve the thermal resistance and reduce the U-value, an increase in thickness of building elements is required.

However, since the thermal resistance of a building element is only one part of its performance requirement, increasing the thickness of insulation can lead to uneconomically thick elements. SIPs offer the efficiency of structural and thermal performance within a single product. U-values as low as 0.11W/m²K can be achieved while also limiting the increase in wall thickness.

With the advent of the Code for Sustainable Homes, the Energy Saving Trust has published ‘backstop’, or minimum levels it believes are needed to achieve the relevant code requirements (see table below). These are based on an integrated approach so that insulation, heating and ventilation systems work together to maximise cost effectiveness in construction and minimise the occupant’s fuel costs.

The main benefit of SIPs over other panelised forms of construction is the limited quantity of thermal bridges needed, such as studs and noggins. When calculating the U-value of building components to BS EN ISO 8990, 94% of the panel area is counted as insulation.

Further gains in thermal resistance can be achieved when assessing the surrounding build-up, such as the cavity in a cavity wall. Providing a low-emissivity surface, such as aluminium foil, on the face of the SIP reduces the radiation transfer across the cavity, so that the airspace has a higher thermal resistance and an associated reduction in U-value.

Care needs to be taken when applying this type of product since the majority require unventilated air space. However, this needs to be balanced against the associated risk of interstitial condensation. In general, a minimum width of 25mm for the air space is required before the low emissivity surface provides any thermal resistance gains.

Thermal bridging (Y-values)

Although repeating thermal bridges are accounted for in the U-value calculation, non-repeating thermal bridges require special consideration. Up to 15% of heat loss from the building envelope, combined with the associated issues of localised condensation and mould, can be attributed to localised cold bridges, which typically occur around openings and junctions.

Under Part L 2006, heat loss calculations must take account of these cold bridges when calculating the dwelling emissions rate (DER). The imminent Part L 2010 will put increased focus on this area.

Accredited Construction Details have been introduced to complement Part L and, more recently, Enhanced Construction Details have been developed. Provided these details are adhered to, designers can use the improved heat loss percentages within the SAP and SBEM calculations shown in the table opposite.

Although SIPs are not specifically mentioned within these details, it has been accepted through third party accreditation processes (such as British Board of Agrément) that the inherent benefits of SIPs exceed Accredited Construction Details requirements, and therefore the 0.08 Y-value can be used as a minimum. Upgrading to meet Enhanced Construction Detail requirements is possible using SIPs through modelling
and assessment of specific junction details at project level.

Air tightness

The flow of air through a building is either controlled (through ventilation), or uncontrolled (through air leakage). Air leakage is created by gaps and cracks in the fabric and leads to heat loss, discomfort, interstitial condensation, increased sound transmission and increased energy costs.

Air leakage (or air permeability) is the rate of leakage, in cubic metres, of air per hour per square metre of envelope area, at a reference pressure difference of 50 Pascals. Part L 2006 sets a maximum air permeability rate of 10 (cubic metres of air etc). The expected figure under Part L 2010 is 5, while the Energy Savings Trust best practice target is 3.

SIP systems are capable of achieving excellent air tightness figures without the need for additional measures. The use of two layers of OSB and a central insulation core provide a multi-layered air barrier combining low permeability materials. Large-format panels and manufacturing tolerances limit the air leakage at connections and junctions. This can assist in providing an airtight construction as low as 1m³/hour/m² at 50Pa.

The effectiveness of the air barrier is also based on the quality of the workmanship on site. However, a SIP system reduces this reliance on onsite workmanship. Offsite manufacturing tolerances facilitate excellent air tightness values that are more difficult to achieve with other building methods.

Thermal storage and thermal gains

Lightweight construction techniques such as SIPs, timber frame and light steel frame have been criticised due to their lack of density and associated low thermal mass. However, the use of low-density structural materials does not automatically prevent the provision of thermal storage within a building and vice versa.

The thermal mass (or admittance) of a material indicates how quickly a building element can absorb thermal gains. It is reliant on:

 •           
Material density. In general only the first 100mm depth will absorb heat from the air.

 •           
Surface area. Increasing surface area allows heat to be absorbed more rapidly.

 •           
Coupling. The dense material needs to be in direct contact with the air.

The use of dry lining means that the air space behind the plasterboard has effectively insulated the room air from the structural envelope, limiting the latter’s ability to absorb heat and provide thermal mass benefits. Utilising wet plaster and dense finishing materials such as ceramic tiling on a SIP system provides a relatively high thermal mass within a well-insulated envelope, giving an adequate balance between thermal resistance and storage.

This is highlighted in the Energy Saving Trust’s paper Reducing overheating – a designer’s guide, which provides admittance values for typical construction types. It shows that a timber-based external wall panel with a plasterboard and wet plaster finish has a better thermal mass performance than a standard aircrete block with taped and jointed plasterboard finish.

Site management and cranes

SIP building systems are normally erected as part of the supply contract, ensuring that the quality and safety standards and method statements set by suppliers will be met at all times.

When using large-format SIPs, cranes will be required to lift the product in place, and the SIP supplier will generally specify the crane based on lifting reach, access and ground conditions. Where a crane is installed by the principal contractor for the duration of the project, it is wise to consult with the SIP provider on their specific requirements to avoid unnecessary duplication of lifting equipment.

Groundworks heavily impact on the efficient assembly of SIPs buildings. Suppliers will provide foundation specifications, in most cases exceeding those that one might normally expect. A tolerance of +/-5mm on corner-to-corner levels, and a similar tolerance on squareness and dimensional accuracy, can be expected.

Scaffolding requirements vary slightly and project managers should consult with the specified suppliers before scaffolding specification is established. It is common to see some scaffolding being moved during the course of assembly to allow erection to take place inside the scaffold. For a typical installation see the diagram on page 39.

The onsite assembly of SIP buildings needs careful planning. A full working programme should be established to allow project managers to plan each phase of the build.

As with other timber-based products, it is not advisable to leave them unprotected on site for long periods. The supplier will provide guidelines, but three weeks is often considered appropriate. Many products leave the factory with breathable membranes attached, which offer weather protection.

Summary

The flexibility of the system permits a wide and varied building type and style to be built with very little restriction on size, shape and form. With the government’s commitment to lowering C0₂ emissions in construction, and announcements that all new homes will be zero carbon by 2020, thermal insulation and lower air leakage requirements within dwellings will increase dramatically. By adopting SIPs, these new requirements are achievable, especially if the SIP provider is integrated early enough in the design process.

Pete Blunt is a member of the technical committee of the UK SIP Association. Its website is at www.uksips.org

SIPs in short

> The UK SIP Association was formed in 2009 to represent the SIP industry, respond
to technical issues and provide information on SIP systems to the wider construction sector. 

> Members include the principal SIP manufacturers, industry suppliers and professionals involved with design and construction using
SIP technology.

> SIPs have been in use in North America for more than 50 years, and have been extensively used in Antarctica for constructing
research stations.

> SIPs can be used as wall or cladding infill with timber-, concrete- or steel-framed buildings.

> By reducing energy consumption through improved building structures, such as those provided by SIPs, the need for additional micro-generation facilities can be reduced or eliminated.

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