When you replace windows and doors in England and Wales, you are making a ‘material change’ to the property’s thermal performance and safety profile. As such, the work is subject to compliance with current Building Regulations, specifically Part L (Energy Efficiency) and Part F (Ventilation).
The purpose of FENSA (or equivalent schemes like Certass) is to provide a swift, efficient mechanism for confirming compliance without involving the often-slow local authority building control directly. This system protects the homeowner and ensures the work is carried out by a vetted, competent fitter. It is a cornerstone of consumer protection in the UK fenestration industry.
FENSA vs Certass: Competent person schemes (CPS)
FENSA (Fenestration Self-Assessment Scheme) and Certass are the two main government-authorised Competent Person Schemes (CPS) for the window industry in the UK. Both are equally valid ways to certify your window installation, offering the same level of legal protection.
| Scheme | Description | Key Homeowner Benefit |
| FENSA | The largest and most widely recognised CPS in the UK, established in 2002. | Provides a straightforward compliance route, widely understood by solicitors and mortgage lenders. |
| Certass | An alternative, equally government-approved CPS providing certification. | Functions identically to FENSA, ensuring the same legal compliance and consumer guarantees. |
Why the certificate is critical: The resale factor
The FENSA or Certass certificate is arguably the most important piece of paperwork you receive after the installation.
- Compliance Proof: It serves as official, documented proof that the installation meets the necessary thermal (Part L) and safety (Part B, relating to fire escape and safety glass) standards.
- Warranty: It provides a warranty directly from the installer and often includes an independent, third-party 10-year guarantee on the installation (covering failure due to workmanship or material defect).
- Resale Value: It is a vital document required by solicitors during conveyancing when you come to sell your house.
Do I need a FENSA certificate to sell my house? Yes. Without it, your solicitor may raise queries, potentially delaying the sale or forcing you to purchase an expensive indemnity insurance policy to cover the lack of evidence.
Building Regs Part L windows: Energy efficiency standards
The core regulation governing the thermal performance of window replacement is Part L, which aims to reduce carbon emissions from dwellings.
- U-Value Limits: The regulations set a maximum allowable U-value for replacement windows. Any unit installed must meet or exceed this target. This compliance is achieved by using modern, energy-efficient units.
- Key Components: Achieving the required thermal performance means the installer must use Low-Emissivity (Low-E) glass coatings (a thin, invisible layer that reflects heat back into the room) and inert gas fillings like Argon within the double-glazed unit.
- Part F (Ventilation): Alongside Part L, the regulations also mandate adequate ventilation (Part F). For replacement windows, this usually means incorporating trickle vents into the new frames to ensure continuous background ventilation, even when the window is closed, which is vital for reducing condensation and mould growth.
The pitfalls of non-certified installation
If you choose a non-registered fitter, you have two unappealing options:
- Building Control Application: You must inform your local authority’s Building Control before the work starts. You will have to pay a fee, and an inspector will need to visit the site to check if the work is compliant. This process can be lengthy and lead to costly remedial work if the windows do not meet the standard.
- Indemnity Insurance: If you don’t use Building Control and later discover you lack the necessary certification when selling the house, you will likely be forced to buy indemnity insurance. This policy insures against the potential financial risk of the local authority taking enforcement action (rare, but possible) but does not certify the quality of the windows themselves.
Hiring an installer registered with a scheme like FENSA or Certass is always the easiest, most reliable, and ultimately cheapest route for guaranteeing the quality and compliance of your windows.
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