High-Rise project >> Assessing the Situation > Current potential >

Throughout the quantitative assessment of the cost-effectiveness of energy efficiency measures, the rationale applied in terms of each measure’s thermal properties has been the so-called best available technology or BAT principle. “Available” is defined as what is widely commercially available, and is thus not restricted to prototypes or demonstration projects. BAT is not necessarily the most cost-effective at the present time, but justification for employing the BAT principle arises from the fact that minimum European energy efficiency standards are set to continually rise under the requirements of the European Energy Performance of Buildings Directive.

Building fabric

According to the results of the VROM survey of European housing ministries, almost 50% of European high-rise stock was built between 1960 and 1980. The construction techniques used for high-rise buildings in this era to a large degree determine what building fabric energy efficiency measures can be applied in the renovation cycle.

• Wall insulation

Three major wall construction techniques have been identified in the European high-rise stock. These are the use of in situ (cast on site) concrete, prefabricated concrete panels and load bearing brick. The latter is more common in pre-1960 high-rise buildings because it preceded the widespread application of methods using concrete, and because the limitations load bearing brick construction places on a tall building are more severe than concrete both technically and financially. Nevertheless, modern engineering has resulted in some load bearing brick high-rise construction up to 15-16 storeys high. In principle, cavity wall construction is possible with all three types, but has only been found to be a predominant type in the high-rise stock in conjunction with modern forms of bearing brick construction. Cavities do exist in concrete construction types with external (e.g. brick) cladding and render, but external wall insulation (applied when the cladding/render needs to be refurbished) rather than cavity wall insulation is considered in these cases due to the advantages in solving problems of thermal bridging in intermediate floors, balconies and access walkways.

in situ concrete	prefab concrete panels	load bearing brick

• Roof insulation

High-rise buildings can have either pitched or flat roofs, although the overwhelming majority of post-1960 buildings are likely to have been constructed with flat roofs. The immediate option, simultaneously the option considered quantitatively, is the application of warm deck insulation. However other, less direct possibilities for insulating high-rise roofs exist, including constructing a pitched roof with loft insulation (much higher investment, lower maintenance), or even constructing an additional storey with new apartments and a pitched roof. The latter can obviously generate additional returns through sale or lease of new dwellings.

application	with waterproof layer	new roof/storey

• Floor insulation

With respect to floors, there are two main possibilities for insulation considering that the majority of high-rise buildings’ lowest floors are of a solid concrete construction. If there is no basement or the ground floor is above a heated basement, the main option is to improve the thermal qualities of the floor by using an insulation/chipboard composite; however this may entail additional costs related to shortening doors and installing raised thresholds. If the ground floor is above an unheated basement, the best approach is to insulate the basement ceiling.

• Window replacement

Replacement windows can theoretically take any combination of double or triple, low-emissivity or gas-filled glazing and metal, plastic or wood frames – along with various g-values and associated U-values. G-values, elaborated upon more in the section below on external solar shading, are a measure of the solar energy arrested by the window – the more, the better in warm climates. Low-emissivity, or low-e, means that the glazing has had a special layer applied which in summer can absorb short-wave solar energy and reflect long-wave or infrared heat energy (reducing cooling demand) whilst letting in most natural light. In winter, low-e reflects internal long-wave heat energy inwards, reducing heating demand. Gas-filled double or triple-glazed windows – usually with argon or krypton – have reduced thermal conductivity. The windows considered as replacements are all low-e and gas-filled.

The replacement window types considered in the cost-effectiveness assessment have been selected with the assistance of EuroACE members to suit the regional climatic conditions and materials used, and act as suitable upgrades given the predominant existing window types. As an example, triple-glazed windows have been modelled as replacements in cold climate regions, but not at all in warm climate regions. Generally, single or secondary-glazed windows have not been considered as replacements.

Heating system

These measures, as for building fabric measures, have only been considered at the building level. Electric heating systems – storage heating or non-fixed appliances – are assumed to have 100% system efficiency for the calculation of useful energy demand, building and dwelling-based central heating systems are assumed to have a system efficiency of 75% and district heating networks an efficiency of 80% in terms of useful energy demand.

• Distribution

Measures considered quantitatively for heat distribution are thermostatic radiator valves (TRVs) and balancing valves. To illustrate the importance of this, dwellings closest to a centralised source of heat tend to suffer overheating whilst dwellings furthest away (highest up) from the heat source tend to be under-heated. Residents of upper floor flats may use additional heating appliances to meet their demand and occupants of lower floor dwellings may open windows to cool down, resulting in both increased energy consumption and wasted energy. Balancing valves are installed on the so-called ‘risers’, the vertically oriented heat pipes in the building, to ensure an even distribution of heat throughout the building. TRVs further enable the control of temperature on a radiator-by-radiator (room-by-room) basis, with substantial potential to save energy by reducing heating where it is not required. Savings for these measures are assumed to be 30% of the heating demand prior to their installation. There are two important points regarding TRVs and balancing valves. First, they save a larger amount of heating energy when installed in an uninsulated building compared to an otherwise refurbished building – 30% of heating demand in each case. Second, the contribution of TRVs to reducing heating demand is dependent on their proper use, so guidance for tenants is as important as the installation itself. Balancing valves and TRVs have not been applied to base buildings A and B as these are assumed to be heated by dwelling-based electric heating systems or appliances.

Both measures also increase comfort levels by ensuring consistent and controllable temperatures. Not part of the quantitative assessment, click here for a discussion of improved comfort and wellbeing benefits.

• Generation

Energy efficiency improvements in terms of heating generation have been considered quantitatively in the base buildings where on-site boiler replacements are possible. The best available condensing boiler technology has been considered. Off-site improvements, such as in the case of district heating generation, and replacements of the heating system infrastructure (e.g. replacing electric with gas central heating) have not been considered. In order to maintain comparability with base buildings where boiler replacements are not possible, additional savings due to improvements in domestic hot water generation efficiency have only been reported in individual base buildings’ subsections and not in the summary section.