Why is secondary glazing needed in a period property?
Buildings account for 40% of the UK’s total carbon emissions with many buildings using nearly half their energy supplies for heating. However it is possible to raise the energy efficiency of existing buildings with payback being made from reduced fuel bills, the value of an enhanced Energy Performance Certificate and a visibly proactive policy on the Environment.
Single glazed windows are a major factor in energy loss as glass is a poor insulator and also because windows can be ill fitting and draughty. A simple and cost effective solution is to introduce secondary glazing - which is a reversible intervention approved by heritage bodies, including English Heritage - therefore appropriate for Listed buildings or those within Conservation Areas.
Secondary glazing works by trapping an insulating layer of air between the windows and efficient seals will exclude draughts over the entire frame. The use of low emissivity glass will allow the U-value (which is a measure of the rate of heat loss through the window), to fall below 2.0 – the typical rate for a single glazed window is 5.7.
Noise is also a major problem and, whether it is containment or exclusion, levels need to be carefully managed to provide an acceptable and comfortable environment. Sound is measured as a pressure and is expressed in decibels. The scale is logarithmic and an increase in Sound Pressure Level (SPL) of 10dB approximates to a doubling of loudness, whereas a change of 3dB is just about noticeable. Heavy street traffic will register 80dB whereas a reasonable internal level for a meeting room during the day is 35-40dB.
Most existing single glazed windows will reduce noise levels by 25-30dB and modern double glazed windows will normally achieve 30-35dB - often not enough to create a comfortable environment. However, secondary windows can reduce noise levels by 45-50dB and really make a considerable difference. The internal window is set with a minimum air gap of 100mm and this decouples the two glass panes and reduces noise transmission. The greater the cavity the better the dampening effect.
A further benefit of secondary glazing is increased security:
Physical Attack - “Secured by Design” accreditation and testing in accordance with standard BS7950 guarantees resistance against stealth attack on lower risk premises. Where the risk is greater and the glass element is likely to be forcibly attacked products should be tested to the Loss Prevention Certification Board’s standard LPS1175. There are a number of certified levels measured in terms of the attack tools and the time available to the attacker. Glazed products tested up to Level 3 will offer a secure option to heavy duty bars and grilles without the visual intrusion.
Blast Protection - A car bomb can shatter annealed glass more than 100m away and flying shards of glass can lead to many casualties. Windows can be tested using a 100Kg explosive charge to standard ISO 16933 which replicates the effect of a vehicle bomb. Secondary windows glazed with suitable laminated glass can be specified to safely resist a blast at distances from 45M down to 15M.
Fire Rating - Secondary windows may be needed for vulnerable areas such as windows adjacent to fire escapes. Fixed frame aluminium systems are available tested to 30 minute integrity.
Secondary glazing is available in a wide range of styles including horizontal sliding, balanced vertical sliding, hinged and fixed. The best material is aluminium, a strong versatile metal that can be extruded into complex shapes and readily formed to deal with curved and shaped windows often found in traditional and period buildings. Framing sections are powder coated to provide a permanent finish and almost any colour can be accommodated to match the internal decor.
As with all elements of a refurbishment, a successful outcome needs careful up-front design and close project management. Secondary glazed windows are purpose made to precisely fit each window opening and fully fabricated and glazed off-site, allowing rapid installation with minimal disruption to the building or its occupants. Just what owners of period properties would expect with a fully bespoke service.
Article courtesy of Selectaglaze Ltd. More information and literature is available on: 01727 837271; e-mail: email@example.com or visit: www.selectaglaze.co.uk