Owners of listed buildings and buildings in Conservation Areas may not need to satisfy these requirements in all cases. However, it is still necessary to apply for consent either through the council or the FENSA scheme. Please remember that separate Planning Permission may be needed for these works.


Thermal insulation

The new windows will need to meet the new thermal insulation requirements of the regulations. If the replacement windows have wooden or plastic frames; then the glazing needs to achieve a maximum U-value of 2.0W/m2K while for metal frames a slightly higher U-value of 2.20W/m2K is allowable.

Please note that these values are very difficult to achieve and many double glazing units currently on sale will not meet these new standards. Take great care when ordering new windows that your supplier can prove the glazing units will satisfy this requirement. The Building Control surveyor will need to see proof that the windows meet these requirements before they issue a completion certificate. We recommend that you leave any labels on the glazing until the inspection has been carried out by the Building Control surveyor.

In some cases you may be able to use glazing units which do not meet the above specifications, but to do so you (or your supplier) would have to submit calculations to prove that the overall insulation requirements of the regulations would still be met. This may be possible if other insulation measures are undertaken at the same time as the window replacement.  For example; installing cavity wall insulation or 'topping up' loft insulation. The benefits gained by installing this extra insulation can be used to offset the higher losses through the glazing, but this should not be undertaken lightly. We would strongly advise you get any such calculations checked by the Building Control Surveyor well before the replacement windows are installed, so that expensive mistakes can be avoided.


Means of escape

All first floor windows in habitable rooms should have opening lights large enough to allow you to escape through them if you were trapped in the room by a fire. This also applies to rooms in bungalows which open into a hall (unless the hall itself has an external door through which you could escape) and all inner rooms. To meet this requirement all such windows should have an unobstructed openable area of at least 0.33m2 and be not less than 450mm high and 450mm wide (the route through the window may be at an angle rather than straight through). The bottom of the openable area should be less than 1100mm above the floor.

If your existing windows do not have opening lights which meet these requirements, we would strongly recommend for your own safety that you take the opportunity to provide them in the replacement windows. This is not, however, a requirement of the regulations, which just state that the new windows must be no worse than those they replace. Where the existing windows already having opening lights which are larger than the above requirements, those in the new windows can be reduced in size as long as they are mp smaller than the sizes given above.


Safety glazing

Low level glazing (glazed areas within 800mm of the floor) and glazing in doors within 1500mm of floor level should generally be of a type that will break safely. In practice this means the glazing should be either laminated or toughened. Ordinary glazing can still be used in small pane sizes however, provided the glass is strong enough to resist breakage. Approved Document N of the Building Regulations gives maximum sizes according to the thickness of glass - for example, in a single pane less than 1.1m square, 8mm glass would be satisfactory.


Structural safety

If the new windows are wider than those they replace, or involve the replacement of bay windows, then proper structural support is required above the window. In older buildings, the timber frame of the window was often strong enough to carry the load of a wall or roof above it without a lintel. Obviously in these cases, either a lintel needs to be installed when the window is replaced, or the new frame carefully reinforced to carry the load. Further advice on structural stability can be obtained from your local authority Building Control Section or from any member of the FENSA scheme.



It's important to make sure the building has adequate ventilation and this should be considered when you decide on the size of opening lights in new windows. For most rooms, one or more opening windows totalling 5% of the floor area, with background 'trickle' vents totalling 8000mm2 will be adequate. For kitchens, utility rooms and bathrooms 4000mm2will be enough. These rooms should have suitable mechanical ventilation (e.g. fans) to deal with condensation.

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