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Published on January 26, 2024 3:06 PM
Fitting sash windows can enhance both the aesthetics and the energy efficiency of your home, combining the charm of traditional designs with modern advancements in window technology. Whether you opt for timber sash windows, uPVC sash windows, or aluminium sash windows the key to a successful installation lies in precise measurement and careful removal of old sash windows. With quality glazing, such as double-glazed units, your new windows not only improve the look of your property but also contribute to better insulation and increased value.

Preparing the window frame adequately ensures a snug and secure fit, preventing drafts and moisture ingress. Once the new sashes are in place, it’s essential to seal and finish the installation correctly to maintain the integrity of the window’s performance. With the right tools and attention to detail, you can achieve a professional-grade fit that you can enjoy for years. If, at any point, the process seems overwhelming or too technical, seek out the frequently asked questions section or consider consulting a professional.

One of our DIY customers with no previous experience has written a great article. Take a look here.

Key Takeaways

• Measuring accurately ensures a perfect fit for new sash windows.
• Proper preparation and removal of old windows precede successful installation.
• Sealing and finishing the installation are crucial for long-term performance.

Measure for Accuracy

When fitting sash windows, precise measurements are crucial. Using a tape measure, start by gauging the dimensions of the existing sash box or window aperture where your new window will fit.

Step 1: Measure the Frame Width

• Take three measurements at the top, middle, and bottom of the frame.
• Record the smallest measurement to ensure the new sash will fit the narrowest part of the frame.

Step 2: Measure the Frame Height

• Similarly, measure at the left, centre, and right of the frame.
• Again, use the smallest measurement.

Step 3: Check for Square

• Measure diagonally across the frame from corner to corner in both directions.
• The two measurements should be the same; a discrepancy indicates the frame is not square.

Step 4: Measure the Depth

• Measure the depth of the frame from the outside to the inside to ascertain the thickness required for your new sash window which is 128mm deep

Bear in mind that for accuracy, each measurement should be taken at least twice.

Additional Considerations:

• Use a level to ensure your frame is straight and true.
• It’s better to measure multiple times to confirm accuracy before ordering your sash windows.
Take a look at our measuring guide here

Achieving accuracy in your measurements is paramount for a successful installation. Careful attention to detail at this stage avoids complications during the fitting of your sash windows.

Remove Old Sash Windows

Removing old sash windows is a task that requires care and understanding of the window’s mechanism. Here’s a step-by-step guide to help you carefully extract your old sash windows:

1. Prepare Your Workspace:
• Lay down a dust sheet to collect debris and protect your floor.
• Wear safety goggles and gloves for protection.

2. Removing the Sashes:
• Gently pry off the beading or stops holding the lower sash in place using a crowbar, but be mindful not to damage the surrounding frame or wall.
• Lift out the lower sash. If resistant, check for hidden nails or screws securing it.

3. Dealing With Weights and Cords:
• Cut the sash cords on both sides of the lower sash if they are still attached, and allow the weights to fall into the weight pockets safely. Be diligent: these can be heavy and can cause injury or damage if not handled properly.

4. Accessing the Parting Bead:
• Remove the parting bead which separates the two sashes. This bead can be brittle with age, so proceed with caution.

5. Removing the Upper Sash:
• Lower the upper sash and sever its sash cords.
• Carefully remove the upper sash in the same manner as the lower one.

6. Extracting the Weights:
• Open the access panel or pocket door to remove the weights from their compartments.

Pry off beadsCrowbarProtect your eyes with safety glasses
Cut sash cordsUtility knifeDispose of cords safely
Remove weightsHandBeware of their weight, handle with care

Make sure to label or mark parts as you remove them if you plan to reuse any components or need to reference their positions for installing new windows. Additionally, keep the workspace clean and remove any debris or old hardware as you work to maintain a safe working environment.

Fitting Your New Sash Windows

It is vital when installing a sash window that you make sure it is fitted plumb and square. Ensure that the frame is not bowing inwards as this will make it impossible for the window to slide or tilt, likewise if the frame is bowing outwards then the window will be draughty.


It’s important to pack under the two corners of the cill directly below the jambs this then allows the dead load to be transferred directly to structure without bowing the cill.
Failure to do this could result in the cill bowing upwards not allowing the locks to engage correctly.


Fixing brackets are an optional extra. Most of our customers use low expansion foam. They are required if fitting new windows to a new house to comply with building regulations. Position the fixing brackets around the window no more than 150mm from the corners and no more than 600mm apart from each other.
Fit the brackets across the top of the window to enable getting a secure fixing in the head, although brackets are the recommended form of fixing, it is also possible to remove the cover from the head and direct fix throughout.


Lift the window into the opening making sure you get it plumb & square.
Fix through the brackets using suitable frame fixings using packers where required, this will stop the frame twisting.


After you have installed your sash windows, sealing and finishing are crucial steps for ensuring optimal performance and aesthetic appeal. Begin by checking the perimeter of the window frame to ensure there are no gaps or spaces.

1. Prepare: Ensure surfaces are clean and dry.
2. Apply Sealant: Using a caulk gun, apply a bead of high-quality sealant around the frame where it meets the wall. It’s essential to do this step meticulously to prevent drafts and water ingress.

1Cut the tip of the sealant tube.
2Fit the tube into the caulk gun.
3Apply sealant evenly, ensuring total coverage.

3. Smooth Out: Wet your finger or a caulking tool with soapy water and smooth the sealant bead to ensure a clean, professional finish.

You can also use a sand and cement mix if you have very uneven openings outside such as cobble walls.

If the linings inside the property have been removed then replace with new timber or uPVC linings and architraves or make a timber frame cover.


Regular Checks: Inspect the sealant seasonally for any signs of cracking or peeling.
Touch-Ups: Perform touch-ups as necessary to maintain the seal and finish.


Glazing Integrity: Examine the glazing for any signs of movement or leakage.
Function Test: Open and close the sashes to guarantee they move smoothly without sticking.

Regular maintenance will extend the life of your windows and retain their appearance. Remember, the quality of seals and finish will directly impact the window’s longevity and efficiency.

Enjoy Your Newly Fitted Sash Windows

Upon completion of your sash window installation, the immediate benefits can be felt in the comfort of your living room and throughout your home. Quality sash windows not only enhance the aesthetics but also improve the overall value of your property.

Enhanced thermal efficiency: Your new windows provide better insulation, keeping your home warm in winter and cool in summer.
Reduced noise pollution: The snug fit and solid construction of your sash windows can lessen outside noise, allowing for a more peaceful environment.
Ease of use: The modern mechanisms make opening and closing your sash windows effortless, promoting better ventilation when desired.

Initial Cost: Upgrading to new sash windows can be an investment, although the long-term energy savings and value addition often offset this.
Maintenance: While low maintenance, periodic cleaning and checks will ensure smooth operation over time.

When renovating, choosing new sash windows is a commitment to both upgrading the look and efficiency of your home. The installation of these windows is not merely a change but an upgrade to your daily living experience. Enjoy the fresh appearance, the sunlight filtering through clear panes, and the improved functionality that comes with your sash window renovation.

Remember to frequently clean the glass and frames to maintain their sheen and inspect the sealant and weather-stripping periodically to ensure optimal performance. Your investment in high-quality sash windows enriches your living experience and the quiet enjoyment of your space.

Frequently Asked Questions

When considering the installation or replacement of sash windows, you likely have a number of questions about costs, materials, and compatibility with your property. This section aims to address those key concerns directly and provide you with precise, helpful answers.

What is the cost implication of replacing old sash windows with double glazing?
Replacing old sash windows with double-glazed units often results in increased thermal efficiency and noise reduction in your home. The cost for this can vary widely based on the size of the windows, the type of glass used, and the frame materials, but you can typically expect the price to start from several hundred pounds per window.

How much does it typically cost to install new sash windows?
The installation cost for new sash windows generally averages around £1,000 per window but can increase depending on factors such as the window design and whether additional features like acoustic glass are required.

What are the benefits of using uPVC material for sash windows?
uPVC sash windows offer several advantages including lower maintenance, better energy efficiency, and affordability compared to traditional timber frames. They’re also resistant to rot, damp, and weathering, making them a durable option for many homes.

Can sash windows be installed in any style of property?
Sash windows are versatile and can be installed in diverse property styles, from historic Victorian and Georgian homes to contemporary buildings. The design can be tailored to complement the architectural period or aesthetic of your property.

What mechanisms enable sash windows to remain open?
Sash windows typically use a spring balance system, to keep the window open at your desired height. This allows for smooth and controlled operation when opening or closing the window.

Colin Greenslade
Written by Colin Greenslade
I'm the founder of Colin's Sash Windows. I disrupted the sash windows market in the UK in 2014 by introducing fixed prices for uPVC sash windows in the UK. Before this they were generally only available at very high prices through window installers. Today our business is one of the market leaders in supply only windows, doors and roofs in the UK.

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