How to Price Sash Window Draught-Proofing: Labour, Materials and Margin Guide

Summary

Draught-proofing an existing timber sash window is the single most cost-effective retrofit a homeowner can make to a period property's energy performance, and it is often more profitable per hour than full restoration because the material cost is negligible and the skill premium is in the fitting, not the parts. Unlike full sash restoration — which involves stripping paint, replacing rotten sections, and often re-glazing — draught-proofing can frequently be done in situ, with the sashes lifted out of the frame for the brush-pile or compression-seal channel to be routed and fitted, then rehung.

The pricing question tradespeople get wrong most often is treating draught-proofing as a single product. In practice there are two distinct jobs that get sold under the same name: a brush-pile seal retrofit (cutting a channel and fitting continuous pile seal around the sash perimeter, typically a half-day to a day per window) and a full draught-proofing overhaul that also renews worn sash cords or spiral balances, re-seats a dropped sash, and addresses the meeting rail gap — because sealing a window that doesn't close properly in the first place wastes the customer's money.

This guide also exists because draught-proofing is one of the few jobs where a listed building or conservation area restriction rarely applies — Listed Building Consent is generally not required for draught-proofing because it doesn't alter the external appearance or remove historic fabric, unlike full sash replacement. This makes it a strong upsell on heritage properties where double-glazing replacement is refused by planning.

Key Facts

• Brush-pile seal retrofit — £180-£320 per window supply-and-fit; 1-2 hours per window once set up; the seal sits in a routed channel around the sash perimeter and meeting rail
• Full draught-proofing overhaul — £350-£650 per window; includes brush-pile seal plus sash cord/spiral balance renewal, meeting rail adjustment, and re-hang
• Typical pile seal material — Mighton Mel or Schlegel brush pile, 4-6mm pile height, supplied in continuous coil; material cost £8-£18 per window
• Compression seal alternative — silicone or EPDM bulb seal fitted to the staff bead and parting bead; better draught performance than brush pile but slightly more visible; £15-£25 per window material
• Meeting rail interlock seal — addresses the horizontal gap where top and bottom sash meet; often the single largest source of draught on an otherwise sealed window; £10-£20 additional material
• Full house job (6-8 windows) — £1,400-£2,500 typical for brush-pile; £2,500-£4,500 where full overhaul with cord renewal is included
• Labour day rate — £180-£280 for a skilled joiner; a competent fitter can complete 4-6 brush-pile seals per day once set up, fewer on a full overhaul
• Listed Building Consent — not normally required for draught-proofing existing sashes since it doesn't alter external appearance or remove historic fabric; always confirm with the local conservation officer if in doubt, particularly on Grade I/II* buildings
• Part L1B (existing dwellings) — draught-proofing does not trigger a notifiable Part L requirement in isolation, since it's a retrofit improvement to an existing component, not a window replacement
• Energy saving claim — Energy Saving Trust figures indicate well-fitted draught-proofing on a typical semi can reduce heat loss through windows meaningfully and is one of the better pound-for-pound retrofit measures; always quote ranges, not guaranteed bill savings, to avoid misrepresentation under consumer protection law
• U-value impact — draught-proofing reduces air infiltration (measured separately from U-value) rather than improving the glazing U-value itself; it should not be marketed as equivalent to double glazing
• Secondary glazing combination — draught-proofing is frequently sold alongside secondary glazing for a bigger thermal and acoustic uplift without altering the historic sash — see secondary glazing
• BS 644:2012 — specification for timber windows; relevant where new or replacement components (sash cords, staff beads) are fitted as part of the overhaul
• Warranty expectation — brush-pile seals typically carry a 5-10 year manufacturer life expectancy; quote should state this as a wear item, not a permanent fix
• Common upsell — ironmongery servicing (sash fasteners, lifts) while the window is apart, typically £15-£40 additional per window
• VAT — standard rate 20% applies; draught-proofing does not qualify for reduced-rate energy-saving materials relief under current rules, unlike some insulation measures — confirm current HMRC guidance before quoting a reduced rate

Quick Reference Table

Detailed Guidance

Pricing structure — why two tiers matter

Selling a single flat price per window undercuts jobs where the sash doesn't close properly and overcharges jobs that genuinely only need a seal. On survey, check three things before quoting: whether the sash runs freely in the box frame, whether the meeting rail closes flush, and whether the existing sash cords or spiral balances are intact. If any of these fail, the customer needs the full overhaul price — fitting a brush-pile seal to a sash that doesn't close square will not stop the draught and creates a comeback.

Brush-pile seal installation process

1. Remove the sashes from the box frame (lift out, noting cord/balance condition).
2. Rout a channel into the sash stiles and top/bottom rails (or into the staff bead and parting bead, depending on seal type) using a purpose-made router cutter matched to the seal profile.
3. Fit the continuous brush-pile seal into the channel, mitred at corners.
4. Fit a meeting rail interlock seal strip to address the horizontal gap at the sash meeting point — this is the step most commonly skipped by cheaper fitters, and the gap it leaves is often the single biggest remaining draught path.
5. Rehang the sashes, check operation, and adjust fasteners for a snug close against the new seal thickness (seals add a small amount of bulk, so fasteners often need re-setting).

Full overhaul — when cords or balances need renewal

If sash cords are frayed, broken, or the spiral balances have lost tension (sash won't hold position, or drops when released), draught-proofing on its own doesn't fix the underlying problem and shouldn't be sold as the complete job. The overhaul adds:

• Removing staff beads and parting beads to access the sash box
• Renewing sash cords (typically 8mm pre-stretched cord) or spiral balances matched to sash weight
• Checking and lubricating the pulley wheels
• Re-hanging with correctly set cord length so the sash holds at any height
• Then proceeding with the brush-pile/compression seal fitting as above

This combined job takes 3-5 hours per window depending on condition, versus 1-2 hours for seal-only — see sash window restoration for the full cord/balance renewal procedure.

Quoting the whole-house job

Full-house draught-proofing is where the margin improves, because setup time (tooling, access, dust sheets) is amortised across multiple windows. Price per-window rates should step down modestly for jobs of 6+ windows — a common structure is full rate for the first 2-3 windows, then a 10-15% per-window discount beyond that, which still improves the overall day-rate return versus single-window callouts.

What NOT to oversell

Avoid presenting draught-proofing as a replacement for proper window repair or as equivalent to double-glazing's thermal performance — it addresses air infiltration, not conductive heat loss through the glass. Where a customer's real problem is a failed double-glazed unit or rotten timber, draught-proofing should be scoped as an add-on to that repair, not a substitute for it.

Material and tool costs for the tradesperson

• Brush-pile seal coil: £1.50-£3.00 per metre, roughly £8-£18 per average sash window
• Router cutter set for seal channels: £80-£180 one-off tooling cost, reusable across jobs
• Sash cord (if renewing): £2-£4 per metre, roughly £15-£25 per window
• Spiral balances (if renewing instead of cord): £20-£45 per balance, two per window

Frequently Asked Questions

Do I need planning permission or Listed Building Consent to draught-proof a sash window?

No, in almost all cases. Draught-proofing does not alter the external appearance of the window or remove historic fabric, so it falls outside both planning permission and Listed Building Consent requirements. The exception would be an unusual case where the conservation officer has specifically restricted even internal alterations on a Grade I building — always check with the local authority conservation team first if the property has any listing or Article 4 direction attached.

How much can draught-proofing actually reduce heating bills?

Figures vary by property, but well-fitted draught-proofing is consistently rated as one of the best-value retrofit measures per pound spent, because the material cost is low relative to the draught reduction achieved. Avoid quoting a specific percentage saving to the customer in writing — present it as a comfort and running-cost improvement rather than a guaranteed figure, to stay on the right side of consumer protection rules around energy-saving claims.

Can brush-pile seals be fitted to uPVC or aluminium windows too?

Yes, the same principle applies to any opening window with a perimeter gap, though uPVC windows typically already have integral gaskets from manufacture and rarely need retrofit draught-proofing unless the original seal has perished. This guide focuses on timber sash windows because that's where the retrofit trade is concentrated — older sashes were never built with a compression seal and benefit most.

What's the typical lifespan of a brush-pile seal before it needs replacing?

5-10 years under normal use, depending on how often the window is operated and UV/weather exposure for seals near an exposed meeting rail. Quote this as a wear item in the written scope so the customer isn't surprised by a future refresh, and consider offering a return visit service at a reduced rate rather than a full re-quote.

Regulations & Standards

• Building Regulations Part L1B — conservation of fuel and power in existing dwellings; draught-proofing of an existing window is a retrofit improvement and does not in isolation trigger a notifiable Part L requirement, unlike full window replacement

• Planning (Listed Buildings and Conservation Areas) Act 1990 — governs Listed Building Consent; draught-proofing without external alteration or fabric removal is not normally within scope, but always confirm locally

• BS 644:2012 — specification for timber windows; relevant to replacement components (cords, beads) used during an overhaul

• Consumer Protection from Unfair Trading Regulations 2008 — governs how energy-saving claims may be presented to customers in quotes and marketing material

• Energy Saving Trust — Draught-proofing — guidance on draught-proofing as a retrofit measure

• Historic England — Energy efficiency and historic buildings: draught-proofing — guidance on appropriate measures for traditional windows

• Planning Portal — Listed buildings — Listed Building Consent scope

• Approved Document L1B — existing dwellings conservation of fuel and power

• sash window restoration — full sash cord and spiral balance renewal procedure

• sash window restoration pricing guide — pricing for full restoration where draught-proofing alone isn't sufficient

• heritage window repair pricing guide — heritage-specific repair pricing including draught-proofing as a line item

• window types — window types and certification context