The Problem With How Windows Are Sold
The window industry has a well-documented problem: high-pressure sales, opaque pricing, and a tendency to sell upgrades that do not deliver proportionate performance gains. The “triple glazing is always better” pitch, the fear-based energy efficiency claims, the inflated original price followed by a dramatic discount — these are tactics, not advice.
What actually matters: frame material, glazing performance (U-value), compliance with Part L, proper installation, and FENSA certification. Everything else is secondary. Here is how to evaluate each.
uPVC vs Aluminium vs Timber
Frame material is a decision that affects aesthetics, performance, maintenance and cost across a 20–40 year lifespan. There is no universally correct answer, but there is a right answer for your specific project.
uPVC
From £400/windowAdvantages
- Lowest upfront cost
- Zero maintenance — no painting
- Good thermal performance
- Widely available, easy to source replacement parts
- Compliant with Part L as standard
Limitations
- Limited colour options (white, cream, anthracite — but not truly custom)
- Bulkier frame profiles reduce glass area
- Lower perceived value — affects property saleability in premium markets
- Cannot match the sightlines of aluminium in contemporary designs
Aluminium (Thermally Broken)
From £800/windowAdvantages
- Slim profiles — more glass, better sightlines
- Extensive colour options via powder coating (RAL colours)
- Very long lifespan (40+ years)
- Better suited to large spans and bi-fold/sliding door systems
- Contemporary aesthetic that suits extensions and new build
Limitations
- Higher cost
- Must be thermally broken — non-thermally-broken aluminium is a cold bridge
- Scratch damage is harder to repair than uPVC
- Requires specialist installation for large format systems
Timber
From £1,200/windowAdvantages
- Natural material — preferred in conservation areas and period properties
- Can be painted or stained any colour
- Excellent thermal performance if well maintained
- Traditional aesthetic that suits older properties
- Repairable rather than replaceable
Limitations
- High maintenance — requires painting or staining every 3–5 years
- Susceptible to moisture damage if maintained poorly
- Higher cost
- Lead times can be longer than uPVC or aluminium
- Warping can affect operation and sealing over time
Double vs Triple Glazing: When Triple Is Worth It
Triple glazing adds a third pane of glass and an additional sealed air or argon gas cavity. The result is a lower U-value (better insulation), slightly better acoustic performance, and a heavier, more expensive unit. In Scandinavia and Northern Europe, triple glazing is standard. In the UK, the calculation is different.
Double glazing is sufficient when...
- You are replacing windows in an existing home
- Budget is a constraint and you want to spread investment across more windows
- The property is not aiming for Passivhaus or EnerPHit certification
- You are in a south-facing room with solar gain to consider
Triple glazing makes sense when...
- You are building to very high airtightness standards (Passivhaus)
- The property is exposed to significant road or aircraft noise
- You are targeting a high EPC rating for resale or rental compliance
- North-facing elevation with no solar gain and high heat loss risk
RCB Guidance
For most residential extensions in Greater London, a high-quality double-glazed unit with argon fill, low-E coating and warm-edge spacer bar in a thermally broken aluminium or uPVC frame will comfortably meet Part L and deliver strong thermal performance. The money saved on triple glazing is typically better invested in airtightness measures.
U-Values and Part L Building Regulations
A U-value measures the rate of heat loss through a building element (walls, floors, roofs, windows). Lower U-value = better insulation. For windows, the whole-window U-value is the critical number — not the centre-pane U-value that manufacturers sometimes quote because it looks better.
Part L requirement (replacement windows)
Whole-window U-value. England Building Regulations 2021.
Part L requirement (new extensions / new build)
Whole-window U-value. Applies to windows in new construction.
Typical quality double-glazed unit
With argon fill, low-E coating and thermally broken frame.
Typical triple-glazed unit
Significant improvement in raw thermal performance, with cost and weight increase.
What “Thermally Broken” Means and Why It Matters
Aluminium is an excellent conductor of heat. Without intervention, an aluminium window frame conducts heat from the warm interior to the cold exterior rapidly — a phenomenon called a cold bridge. This causes condensation on the interior face of the frame, increases heat loss dramatically, and can lead to mould growth at the frame edges.
A thermal break is a structural barrier of low-conductivity material — usually fibre-reinforced polyamide — inserted between the inner and outer aluminium sections during manufacture. It interrupts the conductive path and brings the thermal performance of aluminium frames close to that of uPVC.
Non-thermally-broken aluminium
- × Cold bridges from exterior to interior frame
- × Condensation on interior frame face
- × Poor overall U-value despite quality glazing
- × Mould risk at frame perimeter over time
- × Not compliant with Part L in most applications
Thermally broken aluminium
- Polyamide barrier interrupts conduction
- Warm interior frame surface
- U-values comparable to quality uPVC
- Slim profiles maintained for aesthetics
- Part L compliant as standard
Bi-Fold vs Sliding vs French Doors
For rear extensions connecting indoor to outdoor space, this decision is often more about lifestyle than specification. Each system has genuine advantages and genuine trade-offs.
Bi-Fold Doors
Best for: Maximum opening, indoor-outdoor living, wider openings
Advantages: Fully open aperture (panels stack to one side). Strong connection between interior and exterior. Popular and widely understood.
Limitations: Panels need clear space to fold out. More moving parts = more potential maintenance. At fully closed, sightlines can be busier than sliding systems.
From £1,500 for uPVC; £3,000+ for aluminium, depending on width and panel count.
Sliding Doors
Best for: Contemporary aesthetics, limited swing space, wider spans
Advantages: Slimmer sightlines when closed. No panel swing clearance needed. Increasingly preferred for a cleaner contemporary look. Better acoustic performance when closed.
Limitations: Maximum opening is typically 50% of total width (panels slide behind fixed panels). More complex installation. Higher cost at equivalent specification.
From £2,500 for basic systems; £5,000+ for slim-profile aluminium lift-and-slide.
French Doors
Best for: Smaller openings, traditional or period properties, budget
Advantages: Simple two-panel system. Lower cost. Works well for narrower openings (1.2–1.8m). Suits traditional and period property aesthetics.
Limitations: Limited maximum opening width. Swing clearance needed both inside and outside. Does not create the same indoor-outdoor flow as bi-fold or large sliding systems.
From £800 uPVC; from £1,800 aluminium.
FENSA Certification: Why It Matters for Your Sale
FENSA (Fenestration Self-Assessment Scheme) allows registered window installers to self-certify Building Regulations compliance on your behalf. When windows are replaced in an existing dwelling, the installation must comply with Part L of the Building Regulations. A FENSA-registered installer handles that notification and provides you with a certificate.
- When you sell your property, your solicitor will ask for FENSA certificates for any windows replaced since 2002. Missing certificates can delay or complicate conveyancing.
- Without a FENSA certificate, you need a Local Authority completion certificate from your Building Control department — which requires an application, inspection and fee.
- Retrospective FENSA certificates can sometimes be obtained, but it is harder and may require an independent inspection to confirm compliance.
- Always confirm your installer is FENSA-registered before work starts — not after. Check the FENSA register at fensa.org.uk.
- For windows in listed buildings or conservation areas, permitted development rights for window replacement may not apply — planning permission may be required. Confirm this before instructing any contractor.