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Double Glazed Sash Windows

Double Glazed Sash Windows

If all you’re looking for is a price, just click this link to go to our sash window pricing tool. This page aims to provide a rundown of all the frequently asked questions and typical concerns that people have before buying their double glazed sash windows. If you can think of anything that hasn’t been included please feel free to let us know with an email or a phone call and we’ll make the FAQ is updated.

Who Are Core Sash Windows?

double glazed sash window installation company We are a London Sash Window Installation company, committed to providing high levels quality in service and manufacture. Our lines are open all day between 8:30am and 5:30pm and our staff are on hand to answer your questions. We are one of the top London Sash Window installation companies and have been around for many years. Feel free to check out our Google or Which? Trusted Trader reviews to get a feel for our previous customers.

Terms You’ll Need to Know When Buying Double Glazed Sash Windows

  • Double Glazing – Your windows will have two panes of glass in them, not one (like in single glazing)
  • Sash Window – A window that slides up and down instead of being mounted on a hinge. A window sash is the openable part of any window. On casement windows these would be mounted on hinges, but on sash windows they’re traditionally mounted on lead weights.
  • Box Frame – The frame that surrounds the two sliding sashes.
  • Lead Weights – Sash windows are commonly balanced on lead.
  • Spiral Balances – Spiral balanced springs that counter balance the window.
  • Chains – Sash windows are occasionally hung on chains. This is quite rare.
  • Staff & Parting Beads – Small strips of timber that hold the window in place.
  • Comprehensive / Basic Fitting – We offer two levels of fitting service. These are outlined below, and if you need more help to understand our services then our office is always available.
  • Architrave – A wooden trim that goes around the windows.
  • Bullnose – A wooden sill that sits just beneath the windows.
  • Timber Wrap – A wooden ‘wrapping’ that goes all the way around the windows.
  • Argon / Krypton – Inert gasses that fill the double glazed units. These gasses are the main reason that glazed units retain heat. They can escape over time if the unit isn’t properly cared for or if the rubber melt fails and allows the gas to escape.
  • Double Glazed Unit – The actual glazed unit itself. Most double glazing generally has an argon gas fill to increase heat retention, but we can also produce double glazing with a Krypton gas fill if required. Double glazing or triple glazing with Krypton is generally considered a bit overkill for temperatures in the UK.

Choosing the Right Double Glazed Sash Windows

double glazed sash windows house example

Your Home

Depending on where your home is located, there may be limits on the changes you can make to the glazing. If you live in a conservation area you’ll need to make sure that the glass and glazing conform to the guidelines set out by the local authority and also that the timber frames themselves match the neighbouring designs.

Your Style

Generally speaking, when you’re thinking of buying timber sash windows it’s all in the eye of the beholder. Some people like simple windows and others like more ornate windows. We offer a bespoke service and can match almost any design of existing sash, period property designs, new styles (in new windows) and supply all styles of box sash windows and doors.

Your Glazing

We have many different glass suppliers that can produce all kinds of glass from toughened to satin and even ornate stained glass. Just let us know what you like, and we’ll try to make it work. It’s your home, after all!

The Typical London Sash Window

Sash windows are the predominant type of window in central London due to historical reasons. The London Sash Window, in so far as there is a ‘London Sash Window’, would probably be either a ‘1 over 1’ or ‘6 over 6’ design, conforming to the main period architecture at the times these windows were most popular. There’s always been a lot of building happening in London, but particularly so during the industrial revolution, where London sash window manufacturers had an extremely large domestic demand and no international competition in the manufacturing sector.

Repair of Double Glazed Sash Windows

If you’d prefer not to replace your sash windows, we can always look into repairs instead. The great thing about timber sash windows is their versatility, durability and repairability, so why not take advantage?

Draught Proofing

Draught proofing is one of our cheapest procedures, partly because it can take advantage of a government energy saving scheme. As a result of that scheme we only need to charge 5% VAT so we can keep the costs of the repairs to your existing sash low. If you’re looking for a quick way to warm up your home without spending too much money, our draught proofing procedure might be good for you. Please note that no glass or double glazing is altered in this procedure unless you specifically request it to be – this makes it quite a useful procedure for a listed building or period property.

This procedure is carried out the following way:

  • Ensure the home is properly protected.
  • Remove sash windows.
  • Router the sash windows as appropriate for inserting the new draught proofing.
  • Fit newly draught proofed staff & parting beads.
  • Rebalance lead weights as appropriate.
  • Refit sash windows.

sash window repair


This is quite a comprehensive repair procedure. It’s one of the most labour intensive services we offer on sash windows, and will make your sash window look as good as new. Again a great procedure for a listed building as nothing is replaced unless you request it. Glazing will be left in place and the existing sash will not be altered.

The procedure will be carried out in the following way:

  • Ensure the home is properly protected.
  • Taking care not to damage and glass, remove the sash window.
  • Remove all paint from sash windows and frame.
  • Rebuild sash windows as frames as appropriate.
  • Fill all sash windows and frames.
  • Repaint all sash windows and frames.

Can Sash Windows be Double Glazed?

Double glazing sash windows is something that might be better left up to the manufacturer in most cases. A double glazed windows sash is specifically manufactured for double glazing, whereas a single glazed window isn’t. The procedure might go well if you’re lucky, but please bear in mind that there’s a chance that it will fail over time. Either way, you should make sure you have your guarantee. Essentially, making any single glazed sash window double glazed is something that should be approached with a great deal of caution. For these reasons, we do not offer this service.

Do you do UPVC sash windows?

Unfortunately UPVC sash is not something we offer. For environmental reasons as maintaining our company brand, we only timber sash windows. We find that the london sash window market is highly preferable to timber, so it’s these windows that we target the most. Although UPVC sash windows are popular in many places in London, we feel that it’s better to specialise in one particular area than be a jack of all trades!

Cost of Double Glazed Sash Windows

If you’re just looking for a quick rough costing then just click here to go to our sash window estimating tool. If you’re not a huge fan of online tools or if you’d just prefer to speak to a person we have several different options for you.

Arranging a Site Survey

The most accurate way to get a costing for your double glazed sash windows is to arrange for a site survey with one of our surveyors. All of our surveys are free and no-obligation so you don’t have to worry about paying anything on your side until you decide you want to pay a deposit. Essentially, this is an appointment to send down one of our experts who will talk your project over with you, measure your items then come back with an accurate costing. It might take around 24 hours to come back with your costing so you may need to be patient. Occasionally our accurate pricing procedure means we have to speak with the production team in the factory and that can take time.

Call Our Office

Calling our office is straightforward, and our staff are on hand to deal with your queries. Call us on 020 3302 2060 or contact us at [email protected] and one of our staff members would be happy to assist you. Please be aware that should you require in-depth technical information about our products, you may need to wait for the appropriate staff member to be available. In this case, our staff will likely advise you of the relevant person’s contact details. If all you need is someone to book a survey or give you some basic information, anyone in our office will be able to handle the request.

Using Our Measuring Guide

If you give our office a call someone will be able to provide the measuring guide to you. This is just a very simple diagram that explains how you need to measure the windows accurately. Once you’ve taken the measurements you can just send them to our office and we’ll be able to get you a costing. Please be aware that if you choose to order your sash windows based on the measurements you provide, you will be responsible for the final dimensions. That’s why we always advise a site survey with one of our experienced team. It’s always free and no-obligation, so unless you’re really, really strapped for time it would be a good idea to take advantage of it!

Using Our Online Estimating Tool

The quickest way to find out the cost of your double glazed sash windows, as mentioned above, would be to visit our sash window estimating tool. This tool asks for a few simple bits of information then returns a rough costing. You are not required to provide measurements here, so while it’s the quickest way of getting a costing it’s also one of the less accurate ways. The tool also works for casement windows and several types of doors. If you can’t find the exact product you’re looking for, you can choose something that looks close. After all, this isn’t supposed to be a super accurate costing.

sash window estimate

Fitting of Double Glazed Sash Windows

Another question we get asked a lot is how we actually fit the windows. Obviously this is a big concern for people when they’re about to let construction work of any kind start in their home, so please rest assured that our process has been carried out numerous times over the years and our guys really know what they’re doing. We have a huge emphasis on cleanliness and dust protection, which will always be the primary concern of our team going in. After we’ve completed the dust protection, we’ll carry out the fitting in the following manner.

Comprehensive Fit

Our comprehensive sash window fitting service covers all aspects of the installation, from start to to finish. You won’t need to bring in any other tradesmen to support our work as we take care of everything. That includes removal of the old windows, fitting, hand-painting on site and removal of the old boxes. The procedure is broken down into two parts – Sash Window & Box Frame, in which the entire sash window is replaced, and ‘Sash Window Only’, in which only the sliding sashes are replaced.

Sash Window & Box Frame

A sash window & box frame would be the entire sash window assembly, not just the sliding sashes.

Carpentry Phase
  • Measure the sash windows and check them against the old sash windows prior to any removal.
  • Remove the old sash windows from the wall. This is the messy and dirty part of the work. It can also be quite loud, so if you’re planning on escaping the property during the installation this might be the right time to do it!
  • Plane the sash windows to fit the openings if necessary
  • Insert the sash windows into their openings.
  • Hang the sash windows if the lead weight system is used. If not, the window should already be hung on spiral balances, so we can make minor adjustments to the springs instead of having to hang the lead weights.
  • Fit any timber accessories like architrave, window board or timber wrap.
Painting Phase
  • Fill all the surrounds as appropriate
  • Fill all the timber as appropriate
  • Sand all the surrounds to a flat edge
  • Repaint all the surrounds
  • Repaint all the timber
  • Fit any remaining furniture or ironmongery

Sash Window Only

A ‘sash window only’ procedure means that we only replace the sliding sashes, not the box frame. The box frame will be refurbished.

Carpentry Phase
  • Remove the old sash windows from the wall.
  • Begin preparation of the box frame for refurbishment. This is the messy and dusty part of the procedure, as we will need to do a lot of sanding or paint burning.
  • Fill the box frame as appropriate.
  • Plane or build the sash window as appropriate.
  • Insert the sash window and check operation.
Painting Phase
  • Paint the box frames after refurbishment.
  • Fill the sash window as appropriate after carpentry.
  • Paint the sash window after fitting.

Basic Fit

A basic fit procedure is our quickest and most minimal procedure, typically for homeowners or builders who have other work happening in the house at the time. This allows the trades to co-operate a bit better since they won’t be treading on each other’s toes. We don’t do any painting for our basic fit service, just the carpentry.

Sash Window & Box Frame

  • Measure the new sash windows and check them against the old sash windows prior to any removal.
  • Remove the old sash windows from the wall.
  • Plane the new sash windows to fit the openings if necessary
  • Insert the new sash windows into their openings.
  • Hang the new sash windows if the lead weight system is used. If not, the window should already be hung on spiral balances, so we can make minor adjustments to the springs instead of having to hang the lead weights.
  • Fit any timber accessories like architrave, window board or timber wrap. This step is only completed if requested by the customer.

Usage of Double Glazed Sash Windows

If you want your sash windows to last a life time, it’s important that you operate them properly. There are certain habits that might make them more prone to wear & tear, so it’s important that you avoid those kind of actions.


To open the sash window, you just need to grip the two sash lifts and then slide it upwards. It’s extremely important that you don’t overdo it at this point, since slamming the window upwards will impact the top of the box. There’s a furniture fitting on the top of the bottom sash, and should this be moved upward quickly and in an uncontrolled manner it’s almost certain to cause damage to the top. This damage would be easily identifiable as damage from improper use.


To close the sash window, simply grip the sash lifts or the bottom of the top sash and pull them down to the closed position. As ever, it’s important to move the window in a controlled manner to avoid damage to the box frame. Moving the window erratically or too fast may also result in the lead weights becoming tangled, which will mean your window won’t operate at all until someone comes around to fix it.


To properly secure a sash window, there are two options as standard. Firstly, the sash fastener can be locked in the closed position by a locking screw. This will hold the two sashes together. Secondly, there are restrictor locks which are placed on the face of the top sash. These two restrictor locks essentially just restrict the opening of the bottom sash by preventing it opening more than a couple of inches. If you ensure these are fitted at all times when you’re out of the property it will make it much more difficult for someone to force the window open.

Aftercare of Double Glazed Sash Windows


Over time, and especially in rooms with clothes dryers or in bathrooms, there’s a high risk of moisture damage. Since these sash windows are timber, great care must be taken to ensure that the window doesn’t become waterlogged or otherwise damaged by water ingress. You will need to wipe moisture off these windows on a daily basis, and if you have multiple tenants that use the shower many times a day you might need to wipe it more often than that.


Allowing the buildup of mould or damp on the window is going to lead to the window getting swollen and difficult to use. Further than that, an environment that allows such damp to occur is likely to be unhealthy and should be remedied as soon as possible. Should your room be excessively damp, we can fit trickle vents to the windows so that there’s always some flow of moisture.


Improper usage will lead to the sash window getting damaged or unusable. Please ensure that the window is always used correctly to prevent accidental damage that would void the warranty.


We offer a 10 year guarantee on new products and workmanship, and a 2 year guarantee against glazing unit misting. Call us and ask any of our staff for details regarding our guarantee or ask for a copy of the guarantee itself if you want to know more.

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Enjoy being warm and comfortable!

call us: 020 3302 2060 or email: [email protected]

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