PVC or aluminium windows? Compare the window you will actually get

Tell the supplier where the discomfort occurs

Note which rooms overheat, where you feel draughts and when outside noise is most intrusive. Record orientation, exposure, existing shutters and how the windows are used. An exposed opening and a sheltered courtyard window can call for different decisions even within one flat.

If condensation is the main concern, have its causes considered alongside the window proposal. Glazing and frames are part of the picture, but indoor moisture and ventilation also matter. Tighter windows change air leakage; ask how the home will continue to be ventilated properly. A promise that replacement alone will solve every damp patch deserves closer examination.

Compare whole-window performance

Ask for the whole-window thermal transmittance, commonly written Uw. It describes how readily heat passes through the complete assembly. The glass-only figure, Ug, describes the glazing rather than the complete window, while Uf refers to the frame. Comparing one supplier's glass value with another's whole-window value gives a misleading result. The quoted declaration needs to match the dimensions and configuration being offered.

For aluminium, establish whether the specified system has a thermal break: a low-conductivity separation between inner and outer metal components. Then compare declared performance with the proposed PVC system. In Spain, relevant replacement-window requirements depend on the planned work and climate zone, so there is no single universal U-value target for every property. Ask the responsible professional to check what applies.

Official source: IDAE · Soluciones de acristalamiento y cerramiento acristalado · CTE · Documento Básico HE Ahorro de energía

Make the competing offers describe equivalent openings

Use a window schedule with each opening's location, dimensions and chosen operation. Keep those assumptions consistent across suppliers, then examine the glass, solar control, air permeability, water and wind performance and acoustic information relevant to your priorities. Ask for supporting declarations for the actual configuration, rather than accepting a headline about a product range.

Consider how the window will open around curtains, furniture and cleaning access. Shading and glass choices also affect summer conditions and daylight. The most useful specification balances those needs with the way you live in each room.

Ask how the frame will meet the building

A product declaration does not prove that the installed opening will perform as intended. Ask how the contractor will assess the existing opening and form the window-to-wall junction, including the necessary sealing and repairs. Where a shutter box is present, include its condition and performance in the proposal.

Confirm what will be removed and retained, how the fixing and finishing details will be resolved and who is responsible for the complete installation. Changes to the external appearance or common elements may need property-specific checks with the municipality and owners' community. Establish those requirements before committing to a colour or configuration that changes the façade.

Official source: IDAE · Soluciones de acristalamiento y cerramiento acristalado · BOE · Ley 49/1960 sobre propiedad horizontal

Compare the price of the completed replacement

Use the same window schedule to request itemised quotations. Ask suppliers to explain differences in their proposals so you can distinguish an optional upgrade from work needed for a suitable installation. Keep every total on the same basis, including applicable tax and any necessary access equipment.

• Identify each window's dimensions, exact frame system, glazing and opening type.
• Include shutters, shutter boxes, handles and other agreed accessories.
• Confirm removal, waste disposal, installation and making good around the opening.
• Request the relevant performance declarations and the proposed installation details.
• Read the product and installation warranty terms, including who handles a problem.

Choose the offer that addresses your priorities

Return to the room-by-room problems you recorded. If traffic noise was the main concern, look for a proposal that addresses acoustic performance and the full installed opening. If summer overheating dominates, discuss glazing, orientation and shading together. A material label cannot tell you which offer does that work most convincingly.

Do not translate a better thermal figure directly into a promised percentage reduction in household bills. Actual energy use depends on the home and its operation. If grants or Spanish tax relief form part of your plans, consult the relevant guidance and check the specific conditions separately; window replacement does not automatically establish eligibility.