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How much do U-value glazing?

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technical & practice

Part L increases the energy efficiency of the building fabric, but how much glazing can architects specify?

The new Building Regulations Approved Document Part L (Parts L1 and L2) comes into effect on 1 April (see AJ 3.5.01). However, it is important to realise that the new regulations will apply to your current design proposals unless:

lthere has been a substantial start on the erection of the building prior to 1 April 2002; or lfull unconditional building regulation approval has been obtained prior to 1 April 2002.

A 'substantial start' on building erection is considered to be the excavation or digging for foundations.

General site servicing works (for example, roadways and drainage) or removal of soil does not constitute a start to the erection of a building. In effect, if you start designing a building now, you have 10 weeks to get on site with a building notice and start digging, or you have, say, six weeks to complete sufficiently detailed drawing information to obtain an unconditional building regulations' approval after an additional four weeks' consideration. It is tight. The regulations do have transitional provisions, but the DTLR offers the following scenario:

'If (for example) only 10 out of 20 houses on an estate had been started before 1 April 2002, the other 10 houses would be subject to the amended requirements - unless plans of all the houses had been fully approved before 1 April 2002.' This is going to be a shock to architects, builders and clients. On the bright side, not many building inspectors are up to speed either, so there may be scope for relaxations.

The end of glass boxes?

The new Part L provides a methodology of reducing the energy consumption of new and refurbished buildings and this means many factors have to be considered. Many of the revisions are directly relevant to those specifying highly glazed buildings, and have implications at various stages of the architectural process.

The regulations require:

Design stage lAccountability of the energy efficiency benefits of assessing the integration of facade and services.

The improvement of facade thermal performance (see table 1).

The introduction of solar overheating restrictions.

Better detailing of insulation continuity and air leakage.

Improvement of services efficiency.

Greater flexibility in compliance methods requiring more design time.

Construction stage lAir tightness standards on buildings of more than 1,000m 2to be tested on completion.


The Building Standards (Scotland) Regulations Part J of the technical standards will be implemented on 4 March 2002 lDemonstration of insulation continuity.

Occupation stage lImproved handover procedures;

Introduction of log book requirements.

More energy metering.

Work on existing buildings lReplacement windows, walls and heating systems will have to comply.

Many architects have understood the regulations to mean the death of the glazed facade. Below we examine the regulations for glazing and the various methods of compliance.

So how much glass?

Basically, with three different types of compliance equations, it is important to choose the one which, for the given conditions, best suits one's needs. Different calculation methods, using the same data, can provide different results. Failing with a given window area in one calculation method need not imply that the percentage of glazing is wrong. By changing the type of glazing, or by shifting to another calculation method, the same area of glazing might comply.

To illustrate the different results obtained with alternative facade types and the compliance method used, a hypothetical building was tested. The building was created to the BCO standard with a floor plate of 2,000m 2gross, including a centralised core. The building was a simple box with a 4m floor-to-floor structure and a 2.8m ceiling height. A variety of calculations were made using different glass types and solar shading techniques, and, using the hypothetical model, the three compliance methods were used to calculate the percentage of glazing achievable.

1.Elemental method The revised elemental method combines the current elemental and SAP methods. Trade-offs between elements is limited; for example, only 50 per cent of the rooflight allowance can be converted to window area.

The main two elements of this method relating to the glazed area are:

the percentage of allowable glazing and the thermal performance requirement; and lthe restriction of glazing under the solar overheating requirement to limit summer thermal discomfort.

There are two methods of complying with the solar overheating requirement. The first is to ensure that the area of glass is limited to those shown in table 1. The alternative is to prove that the solar gain to the space does not exceed, on average,25W/m 2(using the calculation method in appendix H of the document) or that the space would not overheat according to CIBSE A, chapter 5. (This is an arbitrary method that takes into account any shading devices. ) This is the most straightforward route to compliance. In the case of an office building only, substantial input will be required from the mechanical and electrical (M&E) consultant to calculate the carbon performance rating (CPR) of the air conditioning system. The carbon performance rating in the elemental method compares the air conditioning system to benchmark efficiency standards; for example, for a new office building, the air conditioning system should not exceed 10.3 kgC/m 2.Whole building method At present, the whole building method can only be used for offices, schools and hospitals.

This method sets a carbon performance benchmark for the whole building. In addition to the air conditioning performance in the elemental method, the heating and lighting systems are also assessed.

This provides a slightly more flexible approach in that energy efficiency measures such as heat exchangers, high-efficiency boilers and lighting controls can be included to allow increases in glazed areas.

The flexibility of this method could justify a 20 per cent increase in permissible glazed area above that allowed by the elemental method.

3.Carbon emissions method This method is the most complex but offers the greatest flexibility in design.The annual carbon emissions of the designed building are compared with the annual carbon emissions of a 'notional building'. The proposed building complies if the carbon emissions are lower than those of the notional building.

The notional building is a building of the same size as the proposed building but with facade and services that pass the elemental method.

To calculate the carbon emissions, a thermal model is built incorporating a standard fancoil system with central fresh air plant. The areas of glazing for each facade type (see tables 3-5) are then increased or decreased to increase or reduce the carbon emissions to that of the notional building.

By comparing the performance of a standard fancoil system to that of a more efficient air conditioning system, such as chilled ceilings, trade-offs can be made between the efficiencies within the building services and the facade itself. A specialist simulation engineer would be required for the computer modelling. Table 2 shows how the complexity and flexibility of the compliance methods can increase the amount of glazing possible in the hypothetical building.

Double-glazed Low-E A poor solar control performance permits only small areas of glass, no matter which method is used.

Double-glazed Low-E plus external shading The inclusion of external shading helps to increase the allowable areas.

Using the carbon emissions method the allowable glazed area can be increased to 70 per cent on the east and west elevations. However, coupling this system with a low-energy system, such as chilled ceilings/beams, could increase this to 100 per cent.

High-performance double-glazed Low-E Replacing the shading with a high-performing solar control glass can increase the allowable area.However, body-tinted glass types not only reduce light transmittance but can also create a more uncomfortable thermal environment through absorbed and re-radiated solar energy.

Double-skin facade This system ultimately provides the greatest flexibility and 100 per cent glazing can be achieved on all elevations with the whole building method.

Double skin facades can improve not only the energy efficiency of the building but also the year-round thermal comfort and daylighting levels.

The additional cost of this type of facade system can in part be offset by potential elimination of perimeter heating and additional capital allowances.

Simon Wild is senior environmental engineer and Alan Fogarty is a director at Genesys Environmental, part of Cundall Johnston and Partners.For more information, contact 020 7776 5000 or visit www. g-e-n-e-s-y-s. com

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