Click on the drop down links below to read a frequently asked question:
Timber shutters generally close tight to the outer window and unless the whole shutter assembly is moved forward to accommodate the secondary glazing they will cease to function. They should be sealed and fixed shut, using packing timbers and insulation material as necessary, allowing the secondary window to be fitted to a vertical frame member of the shutter. Avoid fixing across the panel.
The preferred location and operational style of the secondary glazing may affect the positioning of blinds and curtains which, if existing, may have to be relocated or even replaced. Some windows are fitted with blast curtains and in these cases the clients security adviser would have to be involved.
Establish type of control e.g. wand, cord, or turn knob and assess whether an enlarged timber ground is needed to house the control and allow access for operation.
Do these need to be relocated to allow fixing and subsequent operation of the secondary window? This will be of particular concern when door or window units need to be treated with inward opening secondary windows. Other obstructions include shelving, cupboards, cabinets, etc.
Double doors can be treated with either side-hung casement or horizontal sliding units subject to frame clearance at both soffit and cill. With horizontal sliding units the accessible opening may be quite narrow.
Single doors should be treated with side-hung casements. The standard frame section requires 55mm clearance but if only the threshold clearance is critical non standard sections can reduce this to 20mm. Ensure that all measurements take into account any proposed floor coverings.
The external door will not open to a full 90° with a secondary frame in the reveal. The actual angle will depend on the distance between the two doors.
Existing dry lined walls/reveals should be carefully examined to locate battens for key fixing points. Plasterboard fixings should be used at intermediate positions. Side hung units require particularly sound fixings at the hinge point and an enlarged structural timber sub frame may have to be considered to provide this strength.
The design of any new dry lining should take into account the fixing requirements of the secondary window frames. A suitable steel channel or timber section acting as a sub frame should be set behind the plaster board at the fixing point off the secondary glazing where load is to be transferred. This sub frame may have to be tied back to the structure which is particularly important if side-hung secondary windows are specified.
It is not advisable to lip the plasterboard face since this could lead to inadequate and unsafe fixing. It is also difficult to form a plasterboard opening which is fully squared and in one plane. The secondary window will accentuate any building tolerance.
Check whether a new partition layout is proposed which will split an existing external window and so affect the design of the secondary windows.
When treating existing ribbon windows long multi panelled secondary sliding units with the minimum number of mullion points will reduce initial costs but modifications to accommodate subsequent partition changes could be expensive.
Regular mullion points will increase partition flexibility. The mullions can be wide faced to accept a standard partition (see 'Coupling Details') or special closers can be introduced for narrow mullions, but the design must allow access to any handles or finger pulls. These closers are unlikely to be as acoustically efficient as the main partition.
If the head of the secondary window has to be fitted to a new suspended ceiling the bulkhead detail should be designed to give firm support and, if noise insulation is important, the bulkhead should be acoustically treated to avoid external noise travelling through the ceiling void.
If an existing bulkhead or suspended ceiling does not provide sufficient support or integrity, ties must be introduced between the head fixing point of the secondary glazing and the structure. The illustration shows a detail for a ceiling fixed across the face of a window reveal.
Check whether the outer window will open with the proposed secondary window fixed within the reveal. If not, a wall mounted timber sub frame will be required and the cill may have to be suitably modified.
Side hung casements: If the external windows open in pairs check clearance at the centre line since horizontal sliding units could restrict opening and double side-hung casements may then have to be considered.
The offset dimension for a Series 10 sliding unit is 26mm and a cavity of approximately 200mm is required for clearance. The Series 80 sliding unit has a minimum offset of 63mm and needs a cavity of about 350mm for clearance.
When the reveal depth is insufficient the frame can be face fixed (link to Minimal reveal) and offset relative to the central line of the external window. This will allow one window to open.
Vertical pivots: If cill clearance is the only problem consider the use of a horizontal sliding unit with an 18mm high slimline bottom track. Allowing for fixing tolerances this should accommodate clearances down to 25mm.
Horizontal pivots: If a horizontal sliding secondary window is specified the sashes may have to be removed to allow cleaning. Side-hung units can, in certain circumstances, allow full access to the pivoting window. Multiple sets of horizontally pivoting windows may present clearance problems at the mullion points.
Transoms: When a secondary transom has to be introduced it should be positioned to permit clearance for the external window or allow it to open in sufficiently for the required access.
Can these fully operate with the proposed secondary glazing in position or should they be removed and the external window fixed shut?
Are they in the preferred fixing position for the secondary window? In which case can they be relocated? This work must be carried out by a relevant specialist.
If secondary glazing is mounted on the room side of security gates check that the design allows key access to any locks.
If cleaners eye bolts are mounted behind side-hung secondary windows it is better for them to be adjacent to the opening side for ease of access.
Are they in the preferred fixing position for the secondary window? In which case can they be removed completely? If not, consider:
Adapting the fan to fit within the secondary glazing.
Providing a new fan which can be fitted within the secondary glazing.
Designing the secondary glazing to fit around a boxed-in fan.
Relocate the fan through a wall.
1. Retention of a fan will reduce the potential for noise insulation.
2. When the fan is fitted within the secondary glazing it is recommended that the glass is toughened.
Part F1 of the Building Regulations 1995 recommends controllable trickle ventilation for all new buildings and refurbishments requiring planning permission in order to combat condensation.
The recommendations are:
Domestic Building: Habitable Room - 8000mm2; Other Rooms - 4000mm2.
Non Domestic Building (without mechanical ventilation): Occupiable Rooms and Rest Rooms - 4000mm2/10M2; Common Spaces - No Requirement; Other Rooms - refer to regulations but secondary glazing not normally supplied.
Trickle ventilators can be fitted within an enlarged timber sub frame (typically 45mm-50mm depth) and are normally fitted at the head of the window.
Ventilators will reduce the level of noise insulation and where high levels of noise exist wall mounted acoustic ventilators may have to be considered.
Under no circumstances should secondary glazing block permanent ventilation provided for gas appliances, refer to Gas Safety (installation and use) Regulations 1994.
Check whether the secondary frame can be set to clear. If not can a sub frame detail provide a deeper cavity or should the window furniture be changed?
Check whether handles or catches will be accessible. This is particularly relevant to sash windows treated with a matching secondary vertical sliding window since stop positions on the secondary balances will limit access to the external meeting rail. If these are fitted with a Chubb dual screw, check that there is sufficient access to insert the key.
An odd leg frame allows a degree of tolerance of up to 12mm, without recourse to shaped timbers, through the use of packers and an optional cavity side timber fillet. Above 12mm enlarged scribed timbers should be specified to square the opening and so ensure optimum sealing efficiency. The exposed timber can be finished to match the surround or the aluminium framework.
The cill should support both the weight of the secondary glazing and of window cleaners without deflection. Large sliding panels could weigh more than 30 kg per meter run.
Any movement of the cill will reduce the sealing efficiency of sliding panes and at worst allow a panel to fall out of its frame.
Deep cills or cill casings having cut outs for air grilles must have sufficient supports or inherent strength to accept these weights without deflection. Air grilles may also have to be repositioned to avoid them venting into the cavity formed by the secondary window.
Cill height needs to be assessed taking into account any proposals to raise the floor level and safety glass must be specified in accordance with BS6262:1994:Part 4.
This affects glass less than 800mm from finished floor level (FFL) and glass in doors and adjacent windows within 300mm of doors to a height of 1500mm above FFL.
Check for loose or live plasterwork. Align fixing point for secondary window to be behind the corner bead to avoid the plaster being damaged at the edge. The face of the secondary window should generally be 12-20mm back from the wall face.
It is not advisable to lip a plaster reveal with the secondary frame since the fixings will be insecure and the plaster is likely to be damaged. Also since the walls are unlikely to be plumb or in alignment there will be sealing difficulties and deviations in building tolerance will be exaggerated.
Provide a shaped timber sub frame to 'square' the opening. Visible faces of the timber should be painted to match the aluminium frame.
A new timber sub frame, securely fixed back to the structure, is needed to form a suitable reveal. The timber size will depend on the required cavity and the type of secondary window proposed. A bead of non setting mastic must be applied between the new timber frame and the receiving structure to effect a seal.