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!).
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:
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 |
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 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.
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.
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.
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.
© 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