Why Replacement Accoya® Sashes Outperform Retrofitting Original Frames

When upgrading traditional sash windows for better thermal performance, homeowners generally face two paths: retrofitting double-glazed units into existing sashes or replacing the sashes entirely with new, slimline Accoya® versions.

While retrofitting might seem like a "quick fix," it often compromises the integrity of the window. At Revival Timber Windows, we advocate for the installation of new Accoya® slimline double-glazed sashes into your original frames. Here is why this approach is the superior long-term investment.

1. Material Superiority: The Accoya® Advantage

Original sashes are typically made of Victorian or Edwardian softwood, which is prone to swelling, shrinking, and rot. Accoya® is a "modified" timber that undergoes an acetylation process, making it virtually rot-proof and incredibly stable.

• Dimensional Stability: Accoya® does not shrink or swell significantly. This ensures your sashes glide smoothly year-round without sticking in the winter or rattling in the summer.

• Longevity: Accoya® carries a 50-year warranty above ground, far outlasting the remaining lifespan of a repaired softwood sash.

2. Structural Integrity vs. "Thinning Out"

Retrofitting double glazing into an original sash requires "rebating"—cutting a deeper channel into the wood to hold the thicker glass.

• The Risk of Retrofitting: Original sashes were designed for 3mm or 4mm single glazing. Removing significant amounts of timber to accommodate a 12mm or 16mm double-glazed unit structurally weakens the sash, often leading to joint failure or "dropping."

• The Replacement Solution: Our replacement sashes are purpose-built to hold slimline double glazing. They maintain full structural thickness and joinery strength while fitting perfectly within your original outer box frame.

3. Thermal Performance and "Sightlines"

Retrofitting often results in bulky, unattractive finishes because the original glazing bars (astragals) are usually too thin to support double glazing.

• Authentic Aesthetics: With replacement Accoya® sashes, we can replicate the exact profile of your original period moldings.

• Advanced Glazing: We use Slimline Double Glazing, which utilizes Xenon or Krypton gas. This allows for a very narrow cavity that mimics the look of single glazing from a distance but offers modern U-values (thermal efficiency).

4. Optimal Functionality: Weatherstripping and Balance

A window is only as good as its seal.

• Integrated Draught Proofing: When we manufacture new sashes, we integrate high-performance weatherstrips directly into the timber. Retrofitted units often rely on "stick-on" solutions that degrade quickly.

• Correct Weight Calibration: Double glazing is significantly heavier than single glazing. When replacing sashes, we fully recalibrate the internal sash weights and replace the cords/pulleys to ensure the window is perfectly balanced and effortless to operate.

Summary Comparison

Feature	Retrofitting Original Sashes	New Accoya® Sashes
Durability	Limited by existing rot/age	50-year timber warranty
Structural Integrity	Weakened by deeper rebates	Purpose-built for glass weight
Maintenance	High (softwood requires frequent painting)	Low (coatings last 2x longer on Accoya)
Thermal Seal	Often inconsistent	Fully integrated draught-proofing
Value Addition	Short-term improvement	Long-term heritage restoration

The Verdict

While the initial cost of new Accoya® sashes is higher than a retrofit, the lifecycle cost is drastically lower. You avoid the cycle of rot and repainting, achieve better energy ratings, and preserve the architectural beauty of your home with a material that is built to last a lifetime.