How to specify repairs and improvements to heritage timber windows
If you are an architect or interior designer, and you have found yourself working on a heritage building, you are likely to run into some old timber windows. These windows are most commonly:
- Double-hung sash windows
- Casement windows
- Yorkshire slider windows
- Fixed windows
They may be beautiful, but they are also likely to be:
- Non-compliant with current building code (you might not have to make them compliant, but you might want to for safety reasons)
- Glazed with low-performance glass
When specifying improvements to these windows, most people want a combination of:
- Preserving the heritage aesthetic the windows
- Conserving the built heritage of the windows
- Dealing with repair issues
- Improving performance
- Simplicity of project management
This page provides:
- A pdf download of a template specification
- A discussion of relevant considerations against each specification
- Possible alternative specifications taking into account these considerations
Template Technical Specification
Download a link to a pdf here, or copy the text below.
- Sashes (double hung, casement, awning and Yorkshire)
– Are to be removed from frames, and retained for reuse.
– The linings, sills and sashes are to be checked for repairs and rot.
– Existing glass is to be carefully removed where necessary to enable full repair
- Damaged or rotten timber is to be removed and treated with fungicidal wood stabiliser and replaced with like-for-like timber and/or flexible epoxy, with profile and dimension to match existing
- Pulleys, spiral balances or hinges are to be serviced or replaced as necessary with like components, rated for the finished weight of the sashes (including any new glass and counterweight if applicable).
- Sash cord is to be replaced with long-life waxed cotton cord with a poly core rated for the finished weight of the sashes (including any new glass and counterweight if applicable).
- Glazing is according to glazing specification (see important discussion below)
- Counterweights (where relevant) are to be adjusted so that the window is properly balanced. Lead counterweights should be used as necessary to ensure full travel distance of all sashes.
- Parting bead is to be fitted with twin-fin brush seals.
- Bottom sash meeting rail is to be fitted with twin-fin brush seals.
- Staff bead is to be fitted with twin-fin brush seals.
- Test the windows for range of movement and ensure effective contact between seals, sashes, beads and frame. Top and bottom sashes are to be operable.
It is usually, but not always, possible to reuse staff beads and parting beads. Unless the windows are of high heritage value, you might prefer to specify all new staff beads and parting beads. The cost of replacement is similar to the cost of re-use. Painting to an even finish will sometimes be easier on fresh bead. If lead paint is present, it can be safer to dispose of the beads responsibly rather than reuse, because reuse involves more disturbance of the old paint.
Other possible specifications
Carefully remove and retain staff beads, bottom sashes, parting beads, and top sashes for re-use. Provide replacement parting beads and staff beads to match original where necessary.
Replace all staff beads and parting beads with new. Carefully remove and retain bottom sashes and top sashes for re-use.
When re-glazing old double-hung windows, you need to take into account the thickness and weight of the new glass.
Thicker glass has the following implications for old windows:
- Some interior decorative detail may be lost when the sashes are machined to deepen the glazing rebate
- The integrity of joinery can be compromised, leading to medium-term maintenance problems with sashes falling apart.
- Generally glass up to 6.5mm thick can be accommodated without problems
Heavier glass has the following implications for old windows:
- Counterweights will need to be adjusted. Because there is limited space available in the weight pockets, the function of the window can be compromised when weight is increased past a certain point. To address this, lead counterweights can be used instead of cast iron. These are more expensive, so many contractors will not do this. It is therefore important to ensure that you specify that windows are to remain operable, and that the travel distance of the sashes should not be compromised.
- Original pulleys will deteriorate quickly under the increased weight of the sashes and counterweights. If you are increasing the thickness of the existing glass by any more than 1mm, you should specify new pulleys.
- Traditional all-cotton sash cord will have a short life with the increased weight of the sashes and counterweights. When it breaks, the window will be very heavy to operate, and top sashes may suddenly fall, leading to damage. Cord should be specified accordingly. In cases with very large windows with heavy glass, such as in public institutional buildings, you may wish to specify stainless steel cable.
- Pulleys should be specified for replacement if you are significantly increasing the weight of the glass and counterweights
- Good quality pulleys on bearings will be rated to a minimum of 75kg each.
Suggested Additional Specification
- Replace pulleys with new pulleys rated to minimum 75kg each.
- Painting a window is in one sense no different from painting anything else. But from a project management standpoint, the way you paint the windows can make a big difference to the budget and quality of outcome.
- It is possible to paint a window when disassembled. Some minor touch up painting is still required after the window is assembled.
- It is possible to do this safely at height from the inside of the building, which can sometimes avoid the cost of expensive scaffolding.
- Painting when disassembled ensures that windows remain operable, and that draught seals are protected from paint. It also gives painters the best chance to do a high quality job on paint preparations by giving them access to parts of the window that are otherwise difficult to sand properly.
- It is less efficient on site to do things this way, but delivers a better result. It may come out considerably cheaper if it can avoid the need for special access equipment for painters.
- Despite the advantages, many contractors will not like to do this because it is unfamiliar. This approach will need to be specified by the customer in order to ensure it happens.
- Paint as per painting specification. Paint after repairs are complete, but before window is reassembled. Touch up paint after window is reassembled.
For improved thermal performance, the above draught sealing specification should be adequate. However, if your client is concerned about acoustics, further improvements can be made by installing a second row of draught seals wherever possible. This second row of draught seals provides a surprising improvement to overall noise reduction at almost no extra cost.
Windows are to be fitted with secondary draught seals wherever possible. In double-hung windows this means in the bottom of the bottom rail, the top of the top rail, and (if possible) a second seal in the meeting rail. In casement and awning windows, this means a second seal around the perimeter of each sash OR a secondary seal in a bead mounted on the interior face of the window frame.