Is sash window replacement the right choice for my house?

Is sash window replacement the way forward? We investigated 4 different ways to improve on single glazing during the refurbishment of our Victorian house.

Heat loss though glazing typically accounts for approximately 10% of total heat losses from a domestic building. This means that improving the thermal performance of your windows has to be part of any eco refurbishment strategy (especially if you are aiming at reducing your carbon emissions by 60% or more as we did!).

Heat loss from a typical house includes 10% from windows - a possible motive for sash window replacement

Heat loss from a typical hous

Sash window replacement dilemma

As part of the refurbishment of our Victorian house, we investigated four different approaches to improving the thermal performance of our existing timber box sash windows. The approaches were:

  1. Sash window replacement with high performance triple glazed timber sliding sash windows
  2. Replacement with new timber box sash but with double glazing
  3. Secondary double glazing added to internal face of existing sash windows
  4. Window upgrade retaining existing sash but replacing single glass with slim double glazed units

These options were then assessed on their combined merits of U-Value, embodied energy, disruption to occupants during installation, visual change and cost. A summary table of these comparisons is shown below.

Sash Window Replacement Option Visual Change Cost/m2 Disruption during installation U-Value Embodied energy
Table 1: 4 options for improving the thermal performance of existing timber box sash windows
Replacement: high performance triple glazed, draught stripped windows Mid High High Low High
Replacement: new box sash windows to match existing but with double glazing (16mm air gap) and draught stripping None Mid Mid Low-Mid High
Secondary glazing: internal aluminium frames double glazing and draught stripping Mid Low Low Mid Mid
Window upgrade: to slim double glazing (4mm air gap) and add draught stripping None Low Mid Mid Low

Sash window replacement cost vs alternatives

But as with many people undertaking their own refurbishment the capital cost is the most important factor – because if we couldn’t afford it we couldn’t do it. The cost comparison below is based actual prices/quotes (June 2010 prices) and is shown below along side approximate U-Values. The existing single glazed box sash windows had a U-value of 5.1W/m2k

Glazing/Window Option U-Value W/m2k (approx) Cost estimate for 11 windows based on 2010 prices
Table 2: Comparative Actual Costs and U-Values of Glazing/Window Upgrades
New high performance timber triple glazed windows with draught stripping 0.9 £15,000
New box sash timber windows double glazed plus draught stripping 1.7 £12,000
Secondary glazing – fixed internally 1.5 £6,000
Re-glaze existing sash windows and add draught stripping 2.1 £5,000

In our situation the cost of either replacement window options was considered to be too expensive for our budget and too disruptive. Secondary glazing while relatively cost effective for the thermal performance and lack of disruptive was ruled out for aesthetic reasons.

Re-glazing with Slimlite double glazing

Re-glazing the existing windows with slim double glazed units was within our budget and had the lowest embodied energy of the all remedial measures as the existing timber frames would be retained and the only discarded material would be the existing single glazing sheets which could be recycled. Although the U-Value would be higher than the other measures considered it would still be significantly lower than the 5.1W/m2k of the existing single glazed windows.

Sash window refurbished with slim double glazed units

Sash window refurbished with slim double glazed units

The thickness of the Slimlite double glazed units are a total of 11 mm made up of 3:4:4 mm with the outer pane of clear float glass, cavity filled with a combination of krypton and xenon and the inner pane 4 mm low-e glass.

The existing glass was 4mm thick so the adjustment required to the sash frames was to rout out an extra 7-8 mm to the glazing rebate. This process required the individual sashes to be removed, adjusted to receive the wider double glazed glass unit and draught stripping and then replaced once re-glazed.

The work was undertaken by an experienced joiner/carpenter and was not too disruptive as it was done on a room-by-room basis. Once the re-glazed sashes were refitted additional works were then required to adjust the existing sash weights to take account of the extra weight of the glass.

However, we found that there are situations when replacement is appropriate – for example, in our house, 2 of the existing sash windows at the rear of the house were in need of substantial and costly repair work and so here we took the decision to replace the windows as the most cost effective option. We opted for high performance timber triple glazed windows from Vrogum  as opposed to replacement double glazed sash windows because this would give us the best thermal performance for the cost.

Pros and cons of 4 options for upgrading sash windows

In our opinion when refurbishing any house pragmatic decisions need to be made about all the elements of the building fabric based on a number of factors, not least cost and performance as every building is different. There are no hard and fast rules.

Below we have added what we consider to be the pros and cons of the 4 window options we considered which may help you decide the best option for you and your particular house.

1. Sash window replacement with High Performance Timber Triple Glazed Windows


  • very good thermal performance
  • very good air thightness
  • essential if you are aiming for Passivhaus standard or aiming to achieve 80+% carbon reduction


  • requires building work to infill wall pockets at either side of window jamb if you are replacing box sash windows and subsequent decorations around new window
  • is the most expensive option
  • will change the appearance of the house as the frames will be thicker – so not an option if you are in a conservation area or the building is Listed.

2. Sash window replacement with New Double/Triple Glazed Timber Box Sash Windows plus draught stripping


  • good thermal performance
  • can be made to match existing windows – so useful if you are in a Conservation Area (may not be acceptable though if the building is Listed – you will need to check with your Local Authority)


  • will fit into the existing opening but will require some internal redecorations around new window
  • new architraves, window board etc will need to be fitted around window as it is unlikely that the original ones will be removed in one piece.
  • some subtle visual change to the appearance of the house as mullions may be wider than the original in order to hold the wider glazing unit – so may not be accepted if building is Listed

3. Addition of Secondary Glazing to existing sash windows, generally with Double Glazing in Aluminium (powder coated) Frames


  • good thermal performance
  • existing windows not altered – so could be an option if you are in a Conservation Area or the building is Listed or have windows with intricate glazing patterns such as leaded lights/stained glass or a number of small window sections
  • fitted internally so generally no disruption


  • visual change to internal appearance of window
  • some restriction on opening of windows
Sash frame being adjusted to receive slim double glazed unit

Sash frame being adjusted to receive slim double glazed unit

4. Glazing Replacement with Slim Double Glazed units and Draught Stripping


  • good thermal performance
  • existing box sashes retained
  • no visual change to appearance of the house
  • very little wasted material and removed single glass can be recycled
  • as the cavity between glazing sheets is so narrow (4 mm) there is very little (if any) double imaging/reflection in the pane – unlike some standard 4:16:4 mm glazed units


  • adjustment of sash weights and new cords will be required to accommodate for the additional weight of the glass – it is important to check that there is depth in the sash pocket for the required extra weights before starting
  • some touch up re-decoration will be required to frames once they are re-glazed


If your existing sash windows are in good condition and the frames are deep enough to take the required rebate of 7-8 mm for the slim double glazed units then in our opinion this (along with the draught stripping) is by far them most cost effective option.

But, as we have mentioned above – do take an objective and pragmatic view based on what you want to achieve, your budget and the condition and location of you house and windows. And please don’t even think of using UPVC, at all, anywhere.

eco renovation - talk to homeowners

© Susan Venner of Venner:Lucas Architects Jan 2013.

Further information: You can see the results of Susan’s sash window upgrade at Susan’s SuperHome Open days.

Also see:
Slimlite double glazing review
Energy efficient windows
Secondary glazing