A Victorian bay window is structurally separate from the main wall line. It projects outwards, exposing three sides to the cold. The bay floor is typically a suspended timber structure over an unheated void, which creates a significant cold bridge at floor level. Combined with single-glazed sash windows in all three elevations and timber frames that have dried and shrunk over 130 years, the bay window can account for 20 to 30 per cent of the heat loss of a typical Victorian ground floor.

The three main heat loss routes in a Victorian bay window are:

• Air infiltration through gaps in the sash window frames and sliding mechanism
• Conduction through single glazing (U-value 4.8 to 5.8 W/m2K)
• Cold bridge at the bay floor and soffit (unheated void below)

Draught proofing addresses the first of these three -- and it is the cheapest to fix. Addressing the glazing and the floor requires larger investment (secondary glazing or slim double glazing, and floor insulation). Draught proofing is always the right first step.

Before spending money on draught proofing, it is worth identifying where the draughts are worst. The main locations in a Victorian bay window:

• Sash meeting rail -- where the top and bottom sashes meet in the middle. This gap widens as timber dries and shrinks.
• Sash sides (pulley stiles) -- the channels where the sashes slide up and down. Gaps here allow cold air to enter via the weight boxes.
• Bottom rail to sill -- the joint between the lower sash and the window sill. Often very draughty in older windows.
• Bay return corners -- the junction between the bay window frame and the main wall. Gaps here are often filled only with old putty or paint.
• Bay floor perimeter -- the gap between the bay floor boards and the skirting, allowing cold air from the void below to enter.

A simple way to find draughts: on a cold windy day, hold a lit candle or incense stick near each of these locations. Flickering flame indicates draughts that are worth addressing.

DIY draught proofing is accessible to most homeowners with basic DIY skills and costs £50 to £150 for a full bay window. The main materials are:

Professional sash window draught proofing uses spring bronze strips or brush pile seals routed into the frame. This is a precision task that requires the sashes to be removed (or at least lowered fully) to access the pulley stiles, and a router to cut the seal channels. The result is a long-lasting, virtually invisible seal that still allows the windows to slide freely.

Professional draught proofing also includes:

• Overhauling the sash cords and pulleys (replacing broken cords)
• Re-painting or priming exposed timber
• Sealing the parting bead and staff bead with flexible sealant
• Fitting a bottom rail seal (brush pile or brush-on silicone)

Cost: £150 to £300 per bay window (3 sash windows in a typical bay). Lifespan: 10 to 15 years for spring bronze; 5 to 8 years for brush pile. Professional treatment is significantly more effective than DIY strips and is strongly recommended if the EPC improvement is the goal.

Draught proofing is recorded in the RdSAP EPC methodology under the air infiltration rate. An assessor scores the property as "draughty" or "not draughty" based on visible evidence of draught-proofing measures on windows, doors, and other openings. Moving from "draughty" to "not draughty" typically adds 2 to 4 EPC points for a Victorian terrace.

The assessor will specifically look for evidence of:

• Draught-proofed sash windows (visible seals or brush strips)
• Draught-proofed external doors (letterbox, keyhole, and frame seals)
• Chimney balloons or draught excluders in unused fireplaces

Note that assessors are not permitted to assess air tightness themselves -- they rely on visible evidence. Ensure that draught-proofing work is visible (or recorded) at the time of the EPC assessment. Professional installations with a job completion certificate make it easy to demonstrate the work to the assessor.

Draught proofing delivers the best EPC return when combined with the other low-cost measures on a Victorian terrace. Together, these four measures can add 10 to 17 EPC points for under £2,000 -- often enough to move a D-rated property into C: