Define the problem before choosing
A technical proposal is more useful when it starts with what is currently happening at each opening. It is important to distinguish between draughts, cold internal surfaces, excessive solar gain, noise, condensation, difficult operation, water ingress and insufficient safety. Several problems may occur at the same time and they do not always have the same cause.
It also helps to record when the problem appears, which room is affected, the orientation and the weather conditions at the time. This information makes it easier to prioritise the work and avoids choosing a window only because of its material, one isolated figure or a generic sales description.
- Main problem and secondary problems.
- Rooms and times when the issue is noticed.
- Orientation, exposure and existing shade.
- Opening type and condition of the current window.
- Shutters, boxes, vents and other parts of the opening.
- Comfort, energy, acoustic, safety and usability objectives.
Assess the window as a complete system
The result does not depend on one component. Frame, sash, glazing, gaskets, hardware, shutter, shutter box and the joint with the wall must work together. Advanced glazing can lose much of its benefit when there are air leaks, uneven closing pressure or an incomplete perimeter seal.
When comparing proposals, keep the dimensions, opening method, sash layout and glazing composition equivalent. Only then is it possible to understand whether a difference really comes from the window system or from a different scope of work.
- Frame and sash profile geometry.
- Full glazing composition.
- Gaskets and closing pressure.
- Hardware, hinges, rollers and keeps.
- Shutter, box, guides and operating system.
- Fixings, insulation, sealing layers and finishes.
Technical figures should correspond to the actual configuration and to clearly identified dimensions. Ug describes the centre of the glazing, Uf refers to the frame and sash profiles, and Uw represents the complete window. In acoustics, a value for the glass alone is not necessarily the performance of the complete window.
Glazing can combine low-emissivity coatings, solar control, asymmetric pane thicknesses, laminated glass, acoustic interlayers, warm-edge spacers and gas-filled cavities. The composition should respond to climate, orientation, noise, safety and opening size, rather than simply to the number of panes.
- Thickness and purpose of every pane.
- Cavity widths and the gas used.
- Low-emissivity coating and solar factor.
- Visible light transmission.
- Acoustic or safety laminates.
- Spacer and edge-of-glass performance.
- Total weight and compatibility with profiles and hardware.
Opening method, gaskets and hardware
The opening method affects airtightness, ventilation, clear passage, cleaning and the space used by the sash. Casement and tilt-and-turn windows generally compress perimeter gaskets, while sliding windows use different tracks, meeting stiles and locking systems. Fixed panes remove moving joints but require another solution for ventilation and cleaning.
Dimensions and weight must be compatible with hinges, stays, rollers and locking points. An oversized sash may lose adjustment, rub or require excessive operating force. Performance should be checked for the specific mechanism rather than for a generic opening category.
- Space available for sash movement.
- Compression and continuity of the gaskets.
- Number and distribution of locking points.
- Maximum weight supported by the hardware.
- Handle height and ease of operation.
- Safe access for cleaning the outside face.
- Compatibility with shutters, insect screens and solar protection.
Orientation, climate and room use
A north-facing façade, a large west-facing opening and a bedroom beside a busy road do not necessarily need the same configuration. Solar radiation, shade, wind, occupancy times and glazed area change the priorities for each window.
In the northern hemisphere, façades with little direct sun usually need careful thermal insulation and good daylight. South-, south-west- and west-facing elevations may require solar control and external shading. Bedrooms, living rooms, kitchens, bathrooms and home offices also have different requirements for noise, privacy, ventilation and safety.
- Exact orientation and hours of direct sun.
- Climate, altitude and wind exposure.
- Glazed area and existing shade.
- Occupancy pattern and use of the room.
- Need for privacy, blackout or access outside.
- Possibility of cross-ventilation.
Safe use and protection of the opening
Safety should respond to floor level, outside accessibility, changes in level, dimensions and the people using the room. It is important to distinguish resistance to unauthorised access, impact safety, fall prevention and child opening control.
Glass, glazing beads, hardware, keeps, handles, restrictors and wall fixings form a chain. No component makes an opening invulnerable; a coherent configuration can make access more difficult, reduce opportunities and limit accidents.
- Laminated or toughened glass according to its function.
- Retention of the glazing and position of the glazing beads.
- Distributed locking points and securely fixed keeps.
- Locking handles or restrictors where required.
- Protection against falls and entrapment.
- Coordination with escape, accessibility and ventilation.
The window must be properly supported, level, plumb and fixed to a sound substrate. Perimeter insulation can fill and insulate the joint, but it does not replace structural fixings or the layers needed to control air and rain. A cover trim hides the joint; it does not close an open gap by itself.
In refurbishment, the project must decide whether the old frame is removed completely or partly retained. Keeping it can reduce disruption, but it can also reduce the visible glass area or preserve distortion, moisture, leakage and thermal bridges. Jambs, head, sill, weathering details and thresholds should be reviewed together with the window.
- Condition of the wall, subframe and old frame.
- Number, position and type of fixings.
- Supports and setting blocks suitable for the weight.
- Continuous perimeter insulation.
- Internal air-control layer.
- External protection against rain and weathering.
- Drainage, sills, thresholds and finishes.
Thermally broken aluminium and PVC: compare specific systems
Both thermally broken aluminium and PVC can form part of a suitable solution. Comparisons should use complete windows with the same dimensions, opening method and glazing. There is no universal superiority based only on the frame material.
For aluminium, geometry, system depth, thermal separation, gaskets, joints and structural capacity all matter. For PVC, profile depth, chamber arrangement, reinforcement, gaskets and hardware fixing are important. Polyamide depth or chamber count are partial data and do not replace Uw, airtightness, dimensional limits or installation quality.
- Performance of the complete window.
- Permitted dimensions and weight.
- Glazing capacity.
- Gaskets, hardware and reinforcement.
- Finish, colour and solar or coastal exposure.
- Maintenance and the ability to adjust or replace components.
What the proposal should specify
A useful quotation identifies every opening and explains the complete scope. Terms such as ‘double glazing’, ‘installation included’ or ‘acoustic window’ are not enough for comparison unless the composition, opening, removal, sealing and finishing work are described.
Initial dimensions and photographs are useful for a first assessment. Manufacturing dimensions should come from a final technical survey after the substrate, allowances, opening method, shutters, access and possible building work have been checked.
- Reference, room, dimensions and orientation of each opening.
- System, opening method, handing and sash layout.
- Full glazing composition and available performance data.
- Hardware, handles, shutters, boxes and accessories.
- Retention or removal of the old frame.
- Fixings, internal air seal and external weather protection.
- Finishes, building work, decoration, access equipment and waste.
- Included and excluded work.
Common mistakes to avoid
Many poor decisions result from comparing isolated components or leaving installation undefined. A favourable figure may be unrepresentative when it relates to another size, different glazing or only one part of the window.
Absolute expectations should also be avoided. Replacing windows can substantially improve comfort, but the final result is still related to the façade, roof, ventilation, building services and the way the home is used.
- Choosing only by material, chamber count or polyamide depth.
- Comparing quotations with different glazing and openings.
- Using the same glass composition on every orientation.
- Ignoring the shutter box and ventilation grilles.
- Retaining an old frame without inspecting it.
- Covering the perimeter without sealing the depth of the joint.
- Making sashes too large for comfortable daily use.
- Failing to define cleaning, ventilation, safety and maintenance.
Final checklist
Before accepting a proposal, check that the solution addresses the original problem and that every document describes the same configuration. A simple checklist helps reveal differences in scope before manufacturing begins.
- Has the main need for each opening been identified?
- Are dimensions and sash layout clear?
- Is the glazing described in full?
- Do the performance figures apply to that size and opening?
- Have orientation, noise, humidity and exposure been reviewed?
- Are the shutter and shutter box included in the assessment?
- Is the removal and fixing method explained?
- Are seals, sills, thresholds and finishes specified?
- Is the opening comfortable and safe for its users?
- Do the quotations being compared genuinely cover the same work?
