With the introduction of the new Building Regulations in April 2002, it has become compulsory that all replacement and 'new build' windows should achieve a 'u' value (measurement of thermal efficiency) of 2.00 W/mK or under for replacement windows and 1.8 W/mK for new build.

At the Sash Restoration Co. (Hereford) Ltd, we have been developing our double glazed timber box sash window for over fifteen years. The emphasis of our approach has been to produce a traditional box sash window which offers high levels of thermal and acoustic efficiency, whilst retaining the refined characteristics of a period window.

Our windows are quite unique in terms of double glazing as our designs are all based on fully constructed 'through glazing bars' (22mm) with individual glazed units. The majority of double glazed timber windows produced in this country are manufactured using "applied glazing bars" (small timber beads stuck onto one large double glazed unit to give the impression of individual units). There are many weaknesses to this idea, the main ones being that the stuck-on bars can easily be pulled off and of course if the double glazed unit were to fail for any reason, the complete frame would have to be virtually replaced.

Our double glazed windows have been independantly tested by the Building Research Establishment Ltd. (BRE) to assess their thermal performance. Finite element thermal modelling simulations were carried out using Physibel Bisco software to accurately determine the exact U-value of our windows. The modelling was carried out using the methods in BS EN ISO 10077-2. The results given by BRE confirm that our windows have a whole window U-value of 1.8 W/mK, thus giving them compliance with current building regulations for replacement windows and new build projects.

In older properties we have developed a unit made with handmade glass to the exterior, to give the correct visual feel to the property. This technique was used extensively in the 'Castle House Hotel' development in Hereford; removing the need for secondary glazing whilst affording the sound proofing required to achieve 4 star rating in this grade II listed building.

The scaled illustrations below show various constructional details of our standard Georgian "document L" double glazed box sash windows. All images can be enlarged by clicking.

A contract we recently completed was to replace 37 box sash windows in the Cirencester Council Offices. The windows we replaced were themselves replacement windows. Nothing too unusual about this, apart from the fact that they were just 15 years old. Poor construction and inferior materials, plus a large helping of English weather and what seemed like some rather shrewd economics, has in the long term turned into a very costly and inconvenient exercise.

Our standard sash box, frames and beads are manufactured using kiln dried Canadian Douglas Fir for its stability and durability. All box sills are in hardwood. For our "Finished as Furniture" option the sash frames and beads are constructed using kiln dried European Oak.

The boxes and frames are constructed using through mortice and tenon joints, giving the strength and rigidity required to maintain long term weather tightness and ease of use.