What to Expect

This section also covers draught-proofing work expectations, ventilation in your sash windows, painting and future maintenance of your sash and case windows.

Below are the various works we undertake and what they will mean for you once your project is complete.

  • Refurbishment of Existing Windows
  • Double Glazed Upgrade (new sashes to existing casements)
  • Complete Replacement of Sash and Case
  • Draught-Proofing and Ventilation
  • Window Construction
  • Double Glazed Units and Astragal Width
  • Painting works
  • Shutters

Double Glazed Upgrade (new sashes to existing casement)

Our most cost-effective solution if you would like double glazing installed. We take measurements of your windows in situ and manufacture like-for-like replacements pre-fitted with double glazed units (IGU’s). We then renovate the existing window case in situ, undertake repairs and replace the original sand mastic and ironmongery.

Although we make every effort to retain your original window case and adapt the new sashes to suit, issues sometimes crop up during works. Occasionally, once we remove the original windows and begin refurbishment, the need for extensive repair work is uncovered. It then becomes more economical to install a replacement. 

Due to age, if your existing case is affected by subsidence, bowed or twisted, it can be hard to achieve an exact fit when installing new sashes. Allowance for a slight amount of lateral movement is required to ensure proper operation though this can reduce the effectiveness of draught-proofing. We expect to achieve an efficiency of around 80% on a majority of windows.

Full New Sash and Casement Replacement

A new sash AND case is a complete window strip out and replacement. Due to cost implications, we typically undertake these works only if your existing case is beyond economical repair though there are benefits to replacing a window in its entirety. A new case is custom-designed to house the sashes allowing for a close fit and a good seal for draught-proofing. When fitting new cases, there is more scope and flexibility to offset problems like subsidence and various other issues that can affect the appearance and operation of windows. Final draught-proofing will be in the region of 90% effective.

Replacing a window case can be quite an invasive procedure and involves removing shutters and panelwork for access. When removing an existing case, dust and debris from cavities, built up over the years, is now exposed. We do our best to minimise any impact on decor close to the windows though a level of redecoration work is usually required.

Draught-proofing and ventilation

Draught-proofing is an integral part of the above three services and its effectiveness can vary depending on the works undertaken and from window to window. All our window works involve installing new baton rods and parting beads, pre-grooved to house brush strips. We also machine a groove into the meeting rails of sashes and fit a small “eke” moulding in front of the inside bottom rail.

Sash and case windows facilitate, by design, the circulation of air throughout properties. When opened, cooler air enters under the bottom sash and warmer air expels from the top. There is also a slight amount of airflow, even while the windows are closed, from the meeting rail cuts and as such, 100% draught-proofing cannot be achieved.

Trickle vents, often specified by architects, allow airflow but most sash and case windows, due to their design, allow more than enough air circulation without their installation. PVC windows, hermetically sealed with rubber gaskets, require trickle vents to facilitate airflow and reduce condensation.

Due to expansion of timbers over the years, a 3mm tolerance between the sashes and case is left to allow for expansion of timber. This does not affect the draught-proofing of the windows. 

Window Construction

All our windows are traditionally constructed with haunched mortice and tenon joinery in the same manner as the originals. There are more easily machinable ways of joining timbers, such as finger joints, but we prefer traditional, hand-finished methods for longevity.

Moisture is the main culprit of joint weakness and will always penetrate timber windows to a degree and cause movement. This movement will have no short term effect on mortise and tenon joints, but other joinery methods that rely on mechanical fixings like glue are more susceptible to deterioration. Windows produced in this manner have a far shorter life span, and we sometimes find ourselves replacing windows that are 5 -10 years old.

Please note that timber is a natural product and is subject to slight expansion, shrinkage and occasional twists. There are other imperfections such as knots and checks, and while we discard the more egregious examples, it’s unrealistic to avoid their inclusion entirely. None of these things will adversely affect the overall operation of your windows.

Double Glazed Units and Astragal Width

All our windows, including those with multiple panes, are made with traditional mortise and tenon joinery and “through” astragals. As an example, our six-over-six style sash and case windows contain twelve individual double glazed units. Non-traditional and most PVC windows have a single pane with astragals (glazing bars) “planted on” the face. They may look similar in outward appearance but are unsuitable for use in historic buildings and conservation zones.

When replacing your windows, we match the originals as closely as possible. There are, however, certain compromises necessary where double glazing is involved. As a general rule, the width of the astragal (glazing bar) limits the possible dimensions of glass units. On historic, single glazed windows, the astragals are usually very narrow and leave very little space for double glazing. Astragals on our new windows are slightly wider than originals to allow more sealant, achieving a more durable and longer-lasting unit.

Our replacements have a standard size of 21mm astragals, allowing for a unit depth of 14mm and a sightline (sealant + spacer) width of 8mm. While it’s possible to manufacture thinner units, the dimensions mentioned here are what we consider the minimum for unit longevity. On windows with wider astragals or single-panes, there is more scope for deeper units with wider sightlines. A typical one-over-one (1/1) style sash and case window can house unit depths of 18mm with an 11mm sightline.

When describing unit sizes, these are abbreviated to, for example, 4/6/4. This figure shows a unit size of 14mm made up of 4mm glass + 6mm spacer bar + 4mm glass. An 18mm unit would be 4/10/4. Black spacer bar is our preferred choice for unit manufacture, although there are other colours and variations. The spacer governs internal cavity size, so a 6mm spacer means a 6mm cavity.
When viewed from inside, the moulding on a window covers the spacer leaving only a portion visible around the perimeter. External painting requires a slight overlap onto the glass, creating a shadow effect that helps disguise the spacer further.

With multiple astragal windows, where the panes are small, DG units are manufactured by hand as they are too small to be machined, so a few millimetres variation in spacer bar sightlines is inevitable.

When painting the exterior of traditional, putty finished windows, a slight overlap onto the glass is required to seal against water penetration. This overlap typically conceals most of the spacer bar.

Painting works

Painting for sash windows will usually involve 2 or 3 coats of paint. Primer, undercoat and top coat.

Unless specified old paint is not stripped back to bare wood, but sanded back to provide a key for the next coat.

When the exterior is painted, there should always be a small amount of overpaint between the putty and the glass to stop water from penetrating behind the putty.

Due to the way sash windows operate, not all of the timber is painted as the paint build up  would interfere with the operation of the windows. When a window is ‘finished’ you will see bare timber on the sides of the sashes and also round the inside of the case – visible only when the sashes are open.

The paint finish will be best on all new joinery compared to the finish on renovated joinery.

Fineo Vaccuum Glass

Fineo vaccuum glass has a u value of 0.7.  Fineo also offers better sound reduction than triple glazing and significantly reduces noise compared to standard IGU’s. Please see glazing industry figures for noise reduction below: 

  • Single glazing reduces sound by 29DB
  • Double glazing (up to 28mm) reduces sound by 31DB
  • Triple Glazing reduce sound by 32DB
  • Fineo vaccuum units reduce sound by 35DB.


Shutter refurbs and new shutters. Shutters are never completely black out in effect. The joiner has numerous issues to contend with: buildings being subsided, windows being off the level and timber movement/warping. As the shutters are mounted directly onto the window there is very little adjustment that can be made if shutters are warped.

On larger bay windows, quite often the weight of the shutters bends the window case when they are opened, so they may need ‘sprung’ back into place.

The joiner will aim for the best fit he can achieve but the end result will always be fully operational shutters.