9-Step Plan to Draught-Proof a Sash Window With Mighton Brush Seals for 20% Less Heat Loss

Measure each run before ordering

A box sash window moves vertically in its frame, and the draught path is spread across four places: both sides of each sash, the meeting rail, and the space under the bottom rail. Before buying carriers, push a feeler gauge or a folded strip of paper into each run. Victorian and Edwardian sashes often show 2mm to 4mm of working clearance. Once the clearance drops below 1.5mm, adding pile can make the sash bind. Mighton sells Aquamac and brush ranges with pile heights from 4.5mm up to around 12mm, so the measured gap sets the specification.

Log the four readings separately. A left stile may have 3mm clearance while the meeting rail has 5mm, and one pile height used everywhere can either grip too tightly or miss the largest gap. Pencil the numbers on the frame while the window is open. The Building Research Establishment has long treated uncontrolled air infiltration as a meaningful part of heat loss in older housing stock, and in a draughty sash the meeting rail is often the largest contributor, ahead of the perimeter many people seal first.

The 9-step order

The work needs to follow the strip-down sequence. Retrofitting often stalls when the parting bead is left in place until too late.

1. Lower both sashes fully and lift out the staff beads with a flat bar, starting at the mitre.
2. Remove the lower sash, then prise the parting beads from their groove.
3. Lift out the upper sash and put both sashes on a padded bench.
4. Scrape the runs clean, since old paint ridges and casein putty can keep the pile away from its seat.
5. Rout a 4mm groove into the staff bead edge, or use a Mighton self-adhesive carrier where routing cannot be done.
6. Press the brush carrier into place and trim the corners with a fine-tooth saw.
7. Seal the meeting rail with a parting-rail brush or a compression Aquamac strip, using the clearance already recorded.
8. Refit the upper sash, then the parting beads, then the lower sash.
9. Pin the staff beads back and check that the sash slides without squealing.

Keep the original beads when they are sound. Off-the-shelf softwood replacements usually alter the profile, which spoils the way the bead sits against the brush.

Why the meeting rail deserves attention first

The meeting rail is the horizontal joint where the two sashes overlap at mid-height. It is also the joint that many retrofits leave until last, with a poor result.

Take a 900mm wide sash with a 4mm gap on the left stile, the right stile, the bottom rail, and the meeting rail. The vertical runs have roughly equal lengths, yet the meeting rail lies in the moving air stream created by stack effect in a heated room. Warm air rises, leaves through the upper sash gap, and pulls colder air in at low level. The mid-height joint cuts across a fast part of that flow.

Mighton makes a parting-rail brush for this position. Its pile is angled so it wipes against the underside of the upper sash as the lower sash closes. Fit it to the top edge of the lower sash meeting rail.

The brush cannot compensate for bad alignment. If the two rails fail to meet flush, often because a sash has dropped on worn cords, the pile may touch on one side only. A sash hanging 3mm out of square needs its cords or balance corrected before sealing. Geometry comes first, then the pile can do its job.

On the side runs, continuous pile in the staff and parting beads closes the gap along the full height. The sash slides past these beads every time it is opened, so brush pile normally survives better than a compression strip that is repeatedly flattened over a few seasons. Aquamac fin seals suit a narrow, consistent gap. Where old timber rocks slightly in its runs, which is common in windows over fifty years old, brush pile copes better with the uneven movement. The four feeler-gauge readings taken at the start decide which condition you are dealing with.

The bottom rail has a different exposure because it bears on the cill, where water can sit. Brush pile lets moisture drain and dry. Closed-cell foam can trap damp against the timber, speeding rot at the point already most vulnerable. For that run, brush is usually the right specification across the range of gap widths.

The 20 percent figure in context

The headline figure applies to heat loss attributable to that window alone. It is not a whole-dwelling saving. Consider a single-glazed sash in an uninsulated solid-wall room. Whole-house air-change rates in draughty pre-1919 housing commonly sit well above the 0.5 to 1.0 air changes per hour targeted by modern guidance, and unsealed sashes and their boxes can supply a substantial share of the excess leakage.

If one window adds an air-leakage penalty equivalent to 0.3 air changes per hour for the room it serves, and brush seals close about two thirds of the perimeter and meeting-rail gap, the reduction in that window’s convective loss falls into the 15 to 25 percent band reported across draughtproofing field trials. That covers convective loss only. The glass still loses heat by radiation and conduction at the same rate, so the energy saving for the room is smaller than the headline suggests. Brush seals do not change the U-value of the glass.

Limits of sealing single glazing

Air infiltration and conductive loss are separate problems. A sealed single-glazed sash still loses heat through glass with a U-value near 4.8 to 5.0 W per square metre per kelvin, compared with roughly 1.2 for a modern double-glazed unit. The draught may close while the pane remains cold, and inside-face condensation can increase because warm room air that used to escape is now held against cold glass.

Tools, timber prep, and corners

The basic kit is a flat pry bar, a sash separator for beads that have been nailed and painted over, a scraper, and a router fitted with a 4mm or 5mm groove cutter matched to the Mighton carrier. A multi-tool with a flush-cut blade is useful where levering would split a mitre. For refitting the staff beads, keep a punch and 25mm panel pins nearby.

Surface preparation has a large effect on how long the seal stays put. Strip each run back to sound paint or bare wood. A carrier fixed onto flaking 1970s gloss can lift with the old coating inside a year. If the rebate is uneven, scraping it flat is worth more than upgrading to a dearer carrier. Mighton self-adhesive carriers need a clean, dry, dust-free surface, because the adhesive will not bridge a paint ridge.

Corners expose rushed fitting. When a brush pile is cut square at 90 degrees, a small triangular opening can remain at each mitre. Cut the carrier slightly long and press the filaments into the adjoining run so they overlap by a millimetre or two. At the meeting rail, let the parting-rail brush run a fraction beyond the stile pile so the two brushes wipe together as the sash closes. Close the sash on a strip of tissue paper at each corner. If the paper pulls out freely, the pile is still too short or set at the wrong angle.

The pulley stile can keep leaking

Brush seals close the visible sash gaps, yet they leave the cord pockets and the space around the pulley wheels connected to the unheated box. On a thorough retrofit, a brush or a small foam pad behind the pulley face reduces that route into the box. On some windows, that hidden path can carry as much cold air as the perimeter gaps combined.

A window with the box sealed and the sash gaps left untreated can behave differently from one treated the other way round, sometimes by more than the brush pile alone would suggest. After the meeting rail wipes cleanly and the side piles touch, the remaining cold trace may still be coming from behind the pulley face.