CPD 23 2014: High-performance insulation for curtain wall facades

INTRODUCTION

Thermal performance is an increasingly important consideration for designers of commercial buildings. In the UK, Building Regulations are imposing progressively tougher targets, with the goal of achieving zero-carbon performance in new non-domestic buildings by 2019. Developers are becoming more aware of the potential impact of rising energy costs, and seeking to reduce demand for mechanical heating, ventilation and cooling. There is also a growing awareness of the risk of summer overheating due to climate change, and an accompanying drive to future-proof buildings so that they maintain their value.

New insulation technologies offer one solution for improving the thermal efficiency of commercial buildings, allowing designers to create energy-efficient building envelopes without compromising on aesthetics. This CPD module will examine the applications and benefits of two high-performance insulation technologies: vacuum-insulated panels and aerogel.

ENERGY EFFICIENCY IN COMMERCIAL BUILDING FACADES

The 2013 version of Part L2A, which came into force in April 2014, imposed a 9% carbon-reduction improvement on the 2010 standard across all building types. But it demands a 12% improvement in deep-plan office buildings with air-conditioning, and 13% for shallow-plan offices.

The facade of a commercial building is one of the key elements determining its thermal performance. Curtain walling is a popular choice among developers and tenants, who prize floor-to-ceiling windows and continuous expanses of glazing. However, large expanses of glazing are vulnerable to both solar gain and heat loss, with the result that facade design is often a trade-off between aesthetic and visual considerations and achieving the required thermal performance.

Glazing generally offers lower thermal performance than other materials, so designers must maximise facade performance wherever possible to help meet Building Regulations. Many buildings feature non-glazed spandrel panels which conceal the floor plates. These are typically composed of metal, stone and glass with insulation behind the face to improve the thermal performance of the facade. Spandrel panels can be used to create a uniform effect by blending very closely with the glass, or various colours and designs can be specified for additional visual interest. Traditionally, spandrel areas must be supplemented with an additional layer of thick mineral wool or similar conventional insulation to achieve the necessary thermal value.

As the targets on thermal performance become tougher, simply increasing the amount of conventional insulation used in a building is neither practical nor aesthetically pleasing. Increasing the insulation thickness can eat into the usable area of a building and damage its commercial viability, while reducing the area of glazing can also affect its desirability to potential tenants. High-performance insulation allows designers to achieve better thermal performance with much thinner profiles.

VACUUM-INSULATED PANELS

A vacuum-insulated panel (VIP) consists of a rigid core made of a highly porous insulation material, such as fumed silica, protected by a thin, gas-tight envelope. Air is evacuated from the panel to create a vacuum.

Heat transfer through a volume occurs by three modes: convection, conduction and radiation. Traditionally, insulators worked by reducing heat conduction and convection by creating a matrix of fibres or bubbles and filling the voids with air. The performance of an insulator is therefore determined by the thermal conductivity of the air within it, plus the transfer capability of the material itself. Replacing the air with a less conductive substance, or blowing agent, improves the overall performance of an insulating material. Removing the gas altogether to create a vacuum offers even better performance, as in a Thermos flask. Because both convection and conduction require the presence of gas molecules to transfer heat energy, neither can take place. VIPs may also contain opacifiers to minimise heat transfer by radiation.

VIPs therefore offer much greater thermal resistance than conventional insulation materials. In tests carried out at the Bavarian Centre for Applied Energy Research (ZAE Bayern) in Germany, VIPs achieved between five and 10 times the thermal resistance (expressed as an R-value) of conventional insulating materials of the same thickness. The researchers found that a typical 25mm VIP offered an R-value of 3.5W/m2K. To achieve the same R-value would require 154mm of mineral wool or 84mm of rigid polyurethane foam.

Dow Corning’s VIP offers U-values as low as 0.2W/m2K, compared to typical values of 1.6W/m2K for mineral wool, 1.4W/m2K for polystyrene, 1.2W/m2K for phenolic and 1.1W/m2K for polyurethane insulation at the same thickness.

In addition to thermal performance, VIPs offer other advantages, including a high degree of moisture resistance and fire resistance - the fumed silica core is an ash created by burning a silane, so effectively the material has already been burned.

However, as a pre-engineered product, a VIP must be customised by the manufacturer or packaged as part of a system. It cannot be cut to size on site because cutting or puncturing the material would damage the vacuum and result in a loss of thermal performance.

Architectural insulation modules

VIP technology can also be specified as part of an integrated facade module known as an architectural insulation module, which combines a VIP with a protective architectural finish.

Each module consists of a back pane of a rigid structural material, joined with a warm edge spacer (as used in the insulating glass industry) around the perimeter. The VIP is inserted into the space and covered with a finished panel on the front. Modules are available with various finishes, including opaque, metal, and glass with ceramic frit or ceramic frit patterns.

When tested to ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus, a 50mm-thick unit, measuring 1.5m x 1.5m, with a 6mm-thick piece of glass on each side of a 38mm-thick VIP, achieved an effective R-value of 3.4W/m2K.

In addition to offering superior thermal performance, the AIM is designed to meet the physical demands of commercial facade applications, such as withstanding typical windloads and meeting structural requirements. Constructed to standard or custom spandrel sizes, the modules require no special installation techniques, thereby eliminating the need for additional training.

AEROGEL

Like vacuum insulation technology, aerogel is not a new concept, but it has been optimised to help meet today’s building challenges. Aerogel has typically been used in applications such as aerospace, but over the last decade, its potential use in building insulation has been recognised.

Aerogel is a synthetic, porous material derived from a gel, in which the liquid component has been replaced with a gas. It is composed of 95-99% air, which makes it one of the lightest materials available. The pores within the material are extremely small, reducing the ability of air molecules to move through the structure, and therefore their ability to transfer heat. Aerogel can offer U-values as low as 0.1W/m2K.

An aerogel building insulation blanket is made from synthetically produced amorphous silica gel, offering a flexible product with very low thermal conductivity which can be cut to size on site and applied to reduce thermal bridging at specific locations in a building envelope assembly. Aerogel building insulation blankets are also highly resistant to flame.

Dow Corning’s HPI-1000 Building Insulation Blanket has achieved a U-value of 0.58W/m2K per 25mm thickness. When tested to ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, it achieved a Class A fire rating.

MINIMISING THERMAL BRIDGING

Minimising thermal bridging is vital to meeting Building Regulations on thermal performance. Heat losses typically occur at transitional areas, such as exposed slab edges, where glazing systems meet cavity wall components, where below-grade and above-grade systems meet and where parapets meet roofs. The availability of thin, flexible insulation materials can reduce the need to make trade-offs in design to meet regulations, and eliminate bulky or messy insulation in constrained areas.

In the US, three common construction details were modelled to demonstrate the effect of using aerogel building insulation blankets to minimise thermal bridging, based on ASHRAE Research Project (RP) 1365, Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings. These models were:

• Curtain wall-at-grade detail, with the aerogel building insulation blanket applied from the neck of the curtain wall to the below-grade rigid insulation. This resulted in a reduction in linear thermal transmittance of about 25%
• Curtain wall jamb at the exterior and interior insulated steel stud assembly, with the aerogel building insulation blanket applied around the adjacent steel stud and at the wall-to-curtain-wall transition. This resulted in a reduction in linear thermal transmittance of about 70%
• Rehabilitated window-wall system, with the aerogel building insulation blanket at the slab edge and around vertical and horizontal glazing mullions. This resulted in a reduction in linear thermal transmittance of about 53%.

In addition, two whole-building energy models were created to demonstrate the effect of using aerogel building insulation blanket with conventional and higher-performance assemblies to minimise thermal bridging, in the Chicago climate. The findings were as follows:

• For a building where the glazing system covered 100% of the facade area, addition of the aerogel blanket with conventional assemblies resulted in a 3.56% energy saving
• For a facade with curtain wall glazing and a steel stud wall assembly, addition of the aerogel blanket and higher-performing assemblies resulted in a 6.78% energy saving.