Fri May 1
When specifying replacement sash windows for a period property, the margin for error is slim. Heritage aesthetics, building regulations, thermal performance and acoustic demands all need to align within a single joinery package.
For architects and specifiers working on listed buildings, conservation areas or high-end residential schemes, the detail in the specification is what separates a competent window replacement from one that genuinely stands the test of time.
This guide sets out the full technical specification of TRC Contracts’ replacement sash windows, covering glazing, acoustic performance, security and compliance. It is written to help decision-makers understand what a specification-led heritage window solution looks like in practice.
Why specification matters for sash window replacements
Period buildings were rarely designed with modern performance in mind. Original sash windows typically feature single glazing, minimal draught proofing and timber sections that have weathered decades of British weather. Replacing them demands a careful balance between historical accuracy and contemporary requirements for energy efficiency, acoustic comfort and security.
Historic England notes that around 20% of heat loss in a traditional home can occur through poorly performing windows. That figure alone underlines why specifiers are increasingly scrutinising the technical detail behind every sash window replacement.
A well-specified box sash window should:
- Replicate the proportions and sightlines of the original
- Meet current Part L thermal performance requirements
- Deliver measurable acoustic improvement
- Incorporate modern security features without compromising the aesthetic
- Offer long-term durability through quality timber and protective finishes
Timber: The foundation of a quality sash window
TRC Contracts manufactures timber sash windows using engineered, laminated redwood, hardwood or Accoya, selected for stability and longevity. Traditional timber has always been the material of choice for period homes, and engineered sections or Accoya now offer considerably better resistance to warping, twisting and movement than solid timber alone.
Frame construction
Our wooden sash windows are built around a traditional box sash window frame, with deep boxes that accept counterweights where specified, or spiral balances for tighter reveals. Frame sections are finished with factory-applied microporous paint systems, providing a durable surface coating that requires minimal maintenance for up to a decade.
Georgian bars and heritage detailing
For Georgian sash windows, authentic through-glazing bars can be specified, along with run-through sash horns, putty-line glazing beads and bespoke moulding profiles. These details matter in conservation areas, where planning conditions often require precise replication of the original sash windows.
Glazing options for modern performance
Glazing is where specifiers can unlock the biggest performance gains in a complete sash window replacement. TRC Contracts offers a full range of glazing options to suit every brief:
- Slim-profile double glazing – ideal for conservation areas, with overall unit thicknesses from 14mm
- Standard double glazing – delivering U-values as low as 1.2 W/m²K
- Acoustic double glazing – for projects on busy roads, railways or flight paths
- Vacuum glazing – ultra-slim 8.3mm vacuum glazing closely resembles single glass, with a U-Value of 1.2 – 1.1 W/m²K.
Each unit is specified against the project’s sash window requirements, with warm-edge spacer bars and argon filling available as standard. For listed buildings where single glazing must be retained, we can supply windows with traditional single-glazed sashes and integrated draught proofing.
Acoustic performance
Acoustic performance is increasingly a driver in urban residential schemes. Our acoustic glazing delivers reductions of up to 40dB, making them suitable for properties in the most challenging noise environments. This is achieved through a combination of laminated acoustic interlayers, asymmetric glass thicknesses and optimised cavity widths.
What is the typical U-value of a replacement sash window from TRC Contracts?
A typical TRC replacement sash window with standard double glazing achieves a whole-window U-value of around 1.4 W/m²K, while slim double-glazed units for conservation applications sit between 1.6 and 1.8 W/m²K, and vacuum glazed windows sit between 1.2 – 1.1 W/m²K. Both comfortably meet current building regulations for window replacements in period buildings.
Thermal performance and draught proofing
Thermal performance in a sash window is not just about the glass. The integrity of the frame, the tightness of the sashes and the quality of the draught proofing all contribute to overall efficiency.
Every TRC sash window includes:
- Brush-pile draught seals on all meeting points
- Compression seals at head and sill
- Weatherstripping within the parting bead and staff bead
- Insulated box sections
This combination reduces air infiltration significantly compared with original sash windows, addressing one of the primary causes of heat loss in period property.
Security features
Modern security features are now expected across all window replacements, including those in heritage settings. Our replacement sash windows are tested to PAS 24 where required, and can be specified with:
- Concealed restrictor hardware
- Key-operated sash stops
- Multi-point locking on larger sashes
- Laminated inner glazing panes for enhanced resistance
- Dual-screw night-vent locking
Ironmongery is available in a full range of heritage finishes, including polished brass, antique bronze and chrome, allowing security to be integrated without visual compromise.
Compliance with building regulations
All TRC replacement sash windows are designed to meet or exceed the relevant sections of the Building Regulations, including:
- Part L – conservation of fuel and power
- Part F – ventilation
- Part K – protection from falling
- Part Q – security in dwellings
- Part B – fire safety, including egress sashes
For conservation areas and listed buildings, we work collaboratively with planning officers and conservation consultants to agree departures from standard compliance where historical accuracy takes precedence.
Installation and project delivery
Specification is only one part of a successful project. Expert installation is equally critical, particularly when working within occupied properties or heritage fabric. Our expert team manages every stage of the process, from site survey through to commissioning, giving specifiers and clients peace of mind that every replacement sash window will deliver a perfect fit.
Speak to our expert team
For further information on specifying replacement sash windows on your next project, contact TRC Contracts to discuss your brief with our technical team. We can provide detailed drawings, NBS specifications and sample sections to support your tender and design work.
