Historic Windows Evaluation and Replacement

Assessing the Condition of Window Units for Repair or Replacement

Many refurbishments, extension or new build projects may need repairing, replacing or replicating historic windows. Assessing and dealing with works on historic buildings can be a complex process.

Conservation Area Window Replacement
Replacement of sliding sash windows in conservation area

The first step is to consider the architectural and historical importance of the windows with reference to local and national planning regulations. If the windows are regarded to be of historical significance, repair or rehabilitation is generally preferable to full replacement.

Historic Windows Assessment

You can evaluate the architectural integrity of existing windows by mapping out the condition of each unit, either with photographs, graphic drawings or listing the aspect of each element of the window. A survey should cover:

  • Window location
  • Condition of frame and sill
  • Condition of the sash (rails, stiles and muntins)
  • Condition of the paint
  • Problems with glazing
  • Condition of the hardware
  • Overall condition of the window (rate as excellent, fair or poor)
Historic Window Detail
Historic window detail assessment

Factors that can advance window damage include poor design, moisture, vandalism, insects and lack of maintenance.

Wooden Windows Evaluation

Moisture is the main cause of wooden windows decay. You should start checking the lower portions of the frame and sash, since rainwater and condensation can flow downward along the window, infiltrating and sitting at points where the flow is blocked. Inspect:

  • Edges of the window frame, joints or seams for caulking
  • If the glazing putty is cracked, loose or if any sections are missing, especially at the joints
  • Back putty on the interior of the pane, as this forms a seal that prevents condensation running down into the joinery

Make sure that the sill tilts downward away from the building and allows water to drain off – it may be recommended to cut a drip line along the underside so that water can run-off properly.

Conservation sliding sash windows

The condition of the paint and wood can be another clue of excessive moisture. Spots of blistering, cracking, flaking and peeling are common signs of water penetration, saturation, and possible deterioration.

You can test the wood beneath unsightly paint by jabbing a bradawl into a wetted surface at an angle to pry up a small section. Sound wood will separate in long fibrous splinters; decayed wood will lift up in short uneven pieces because the strength of the wood fibres has broken down.

Lastly, check the windows are operating smoothly and correctly, including the balance system, sash locks and other hardware, and weatherstripping.

Benefits of Window Replacement in Conservation Areas

Although conservation criteria dictate that retention of the original materials and characteristics is preferable to window replacement, the damage might be costly enough to justify partial or full replacement.

In a building with several windows, it can be more cost-effective to replace than to repair impaired units. It is recommended to match the look, if not the material of the original windows. The use of aluminium and other materials has become acceptable when replicating original profiles.

Listed Buildings and Conservation Windows
Full replacement of historic windows.

Replacement can also take advantage of upgrading in glazing, which makes windows more energy-efficient. This can allow for better weatherability. Architects that need to specify replacement windows should consider how the units reflect the period, region or style of the building and their addition to the appearance of the facade. Note the following:

  • Pattern and size of openings
  • Proportions of frame and sash
  • Configuration of window panes
  • Muntin profiles
  • Type of wood
  • Paint colour
  • Features of the glass
  • Related details, such as arched top, hoods, etc

Challenges to Replacing Historic Window Profiles

Some replacement windows may maintain the building’s architectural integrity, but fail to meet current performance standards, such as loss of daylight. Also, structural integrity can be challenging when replacing radius windows. The width of the profile can limit how tight a radius will be angled. To replicate rails and stiles, make sure they are wide enough to accommodate spacer bars when using insulating glass – usually no less than 40mm.

The fenestration industry has developed significantly in recent years, expanding the options for historic windows replacement and performance improvement. It may be beneficial to specify the use of prototypes in determining the solutions to meet the client’s original specifications.

Are you considering window replacement in listed buildings or conservation areas? Learn more about requirements on the link below:

Window Replacement in Conservation Areas

Cardinal News

Excellent quality and very reliable lead times at a fair price. Would highly recommend Cardinal Windows to anyone looking for traditional styled windows.

- Dermot Long - Dublin

I ordered 15 sliding sash windows for my home refurbishment project and had my builder install the windows for me. I think I spent hours playing around with the different design options on the window builder! I absolutely love my new windows!

- Rachel Chalke - London