Hi and welcome to our double glazed sash windows service. This page is crammed full of useful information about all the different types of wooden double glazed sash windows options available to you, so it’s been separated out with these handy anchor points:
To help you decide which service is within your budget or appealing please see a quick price list:
Double glazing existing sash windows – Cost £480 to £780.
Manufacture and install of wooden double glazed sash into original frame – Cost £620.00 – £980.00.
Full Replacement wooden double glazed sash windows – Cost £1280.00 – £1980.00.
Double glazing listed building sash windows – Cost £680.00 – £980.00. Please note these costs are for re-using existing joinery which is relevant to 99% of listed building sash window upgrades.
Our wooden double glazed sash windows service remains affordable and competitive. We always make use of your original wooden windows if possible, only replacing wooden double glazed sash windows where necessary. We can ascertain the viability of using existing wooden windows on site. As a general rule, as long as your wooden sash windows are one and a half inches thick, we can double glaze without replacing any joinery at all.
Double glazing existing original sash windows and draught proofing is by the far the best value for money pound for pound. The effectiveness of this service depends largely on the depth of your sash. There are theory examples below as well as a previous installs with undecorated examples of the unit rebated within the original sash to give you a clear idea of what to expect when we have finished the installation. Our vans are kitted out and perfectly tailored to this service, and a standard original sash window double glazed will range as low as £480 to £780 depending on client requirements, condition of sash, and size.
We firmly believe in clients being well informed and provided as much information prior to engaging us as possible, without bias, and therefore I’d like to share some facts.
Typically the largest unit we can install in an original existing sash window is 16mm in depth. We use Argon cavity filled, Low E, double glazed units.The unit is made up of 4mm Low E glass, with an 8mm Air Gap Argon Filled and then normally a 4mm Clear Float(4mm – 8mm – 4mm). A picture is going to help understand the unit much more clearly:
This unit is of exceptional quality and has not been bettered by Pilkington in around 8 years. In the event your sashes are thick enough to carry this unit the value for money is incredible.
Thinner sashes may require a double glazed unit as slim as 4mm Low E – 4mm Air Gap Argon Filled – 4mm Clear Float (4mm-4mm-4mm), which is not as effective for sound proofing or heat retention as a larger unit(4mm-8mm-4mm), however that said, a normal person will not be able to tell the difference in noise or heating between the two. If we were to use a fancy dB meter for volume then, the (4mm-4mm-4mm) would not be as technically effective as the larger unit(4mm-8mm-4mm), and although the difference would be noted on a U-Value meter for sure – the real heat loss would be unnoticeable on your gas bill. To summarise this paragraph – once you’ve got a double glazed sash window that is nicely draught proof, the additional glazing cavity spacer doesn’t make a significant difference. Double glazing is double glazing after all.
We can see a typical sash window profile 37mm pictured below, it’s about as small a sash as we can rebate under normal circumstances. This depth will allow a (4mm-6mm-4mm) or (4mm-8mm-4mm) double glazed unit. To install the double glazed unit neatly, we need to make the sash rebate deeper to hold the glass properly. Because we do not rebate the internal decorative profile, we do not disturb the mortice and tenon and therefore the structure of the sash remains good. By reducing the external glazing bead to a minimum depth we can minimise the impact structurally on the sash. The end result of double glazing the original existing sash windows is exactly the same as manufacturing sashes of equal depth, only with considerable saving your end and the luxury of retaining your original joinery. Here is a look at the cross section of the original sash:
To allow for the double glazed unit we must rebate the glazing profile more deeply using a specialised router cutter on a work bench.
Once we rebate the sash style it will look something more like pictured below:
We then install the double glazed unit into the deeper rebate and rebuild the window with a draught proofing system. Here is a look at the sash close up after being installed. It clearly needs a little tidying decoration wise but certainly no worse for wear. It is the perfect example of what you’ll get before decoration. To have any idea the sash window is double glazed we need to look inside the sash to see the white space edge of the double glazed unit. Looking at the sash window straight on you would not know this sash window has been double glazed. Pictured below is a 40mm sash with a (4-8-4) Argon filled unit. Once decorated you’ve got a fully uprated double glazed sash window for 70% less than an entire replacement cost.
Replacement double glazed sashes making use of the original frame is the second most best solution in terms of cost, and being sympathetic to your period property. We will measure sashes and take note of the original profiles as well as external horns. We will then manufacture wooden double glazed sash windows to match the originals. The service is almost identical to double glazing original existing sash windows with the exception we supply a new sash instead of making use of the original joinery. There are some added benefits of replacement to note. We can tinker with profile sizes and this allows a much larger sealed unit to be installed.
We offset the mortice and tenon to provide additional structural strength and then a 42mm sash is capable of holding a 4mm Low E – 12mm Argon filled air gap – 4mm clear float (4mm-12mm-4mm) should you require the extra cavity depth. The larger air cavity is more effective at heat and noise insulation, although once you have any of the following sizes;(4mm-4mm-4mm), (4mm-6mm-4mm), (4mm-8mm-4mm), or (4mm-12mm-4mm) it is not easy to tell apart as they are all wonderfully thermal and noise efficient. I certainly do not believe I can tell a unit depth by listening to a bus pass or someone shouting outside. It is fairly clear technically though, that a (4mm-12mm-4mm) unit will be more effective than a (4mm-4mm-4mm) as there is a large step up in air cavity.
Below is an example image of a replacement double glazed sash using existing joinery. As we can see, the slender profiles are retained and the window looks very traditional and impossible to tell the window has been double glazed at any distance at all – the picture is taken just a couple of metres away. In stark contrast, if you look at the right hand panel top pane – across the road from some 30 metres you can see the nasty result of UPVC which would of cost about the same as the service we provided, if not more!
We supply our double glazed sashes in softwood and hardwood. We can provide the sash finished bare timber, stained (the stain of your choice), primed, undercoated, or with a gloss finish. Hardwood sash will normally increase the price by around 20% of the overall cost.
Full Replacement wooden double glazed sash windows provide the most effective sound and heat insulation properties but do come with the added cost as we’ve mentioned previously. We provide replacement double glazed sash windows in softwood and hardwood. Full double glazed sash window replacement allows us to install our largest, most effective double glazed unit, 4mm Low E – 20mm Argon filled Cavity – 4mm clear float(4-20-4). This unit represents excellent U-Value ratings and is way beyond the standard requirement to obtain Building Approval (or a Fensa certificate if you prefer). Full window installation carries an insurance backed guarantee so you have full piece of mind for the 10 year warranty period. It is expensive and there is no real payback on heating as a result of the upfront costs but there are plenty of benefits should you prefer complete double glazed replacement.
They look amazing and work perfectly. They are extremely secure and will not require maintenance for many years to come. They will undoubtedly be marginally better than using original joinery at thermal and acoustic insulation. We are not against Replacement wooden double glazed sash windows, we just feel there are other more viable options to consider prior to this costly exercise.
Listed buildings require more sensitive restoration and upgrade, we fully understand what planning are looking for.
We will install slim sealed units into the original sash as to not effect the appearance at all. Please note! Not all sashes are created equal and thorough site assessment will be required to ensure the viability, the structural integrity, and the listed sash window aesthetics remain entirely as was. This is not an art, it’s simple mathematics from an experience carpenter/estimator and so we will show you exactly how the assessment is measured leaving you in control of the important decisions in your home.
There are two important factors in determining the viability of the install. Firstly the sash needs to be of sufficient depth on the original rebate. The next important factor to consider is the width of the rebate. That’s a little tricky to understand so I will provide an image of a typical installation.
This cross section makes clear the minimum depth and width of the rebate requirements. We can see from the example that the rebate needs to be 20mm deep, whilst the rebate width needs to be 6mm minimum, the examples shows 7mm and the extra mm is more comfortable, in fact 8mm-9mm is ideal for tolerances as the sash may not be perfectly square after all these years but as you can see with the small margins involved this really is a sensitive operation.
Im some circumstances we can however afford to slightly rebate a little deeper should we not have 20mm in depth or we can switch to a 10mm unit( 3mm Low E – 4mm Argon Cavity – 3mm Float glass.) and it’s all down to experience, knowing the correct approach to take in order that we maximise the efficiency of the installation.
A couple of mm on the depth will have absolutely no visual impact on the overall finish externally, and in most cases it cleans up the edge of the rebate. The preferred bedding is Butyl putty however another, more appropriate method is a low modulus silicone. There is too greater security risk with Butyl putty as a solid cure can take a month and the property is vulnerable to anyone using a knife to remove the glazing in that time. Silicone sets and holds the unit firmly in place overnight and therefore the suitable choice for bedding of the slimline unit.
This has no effect on conservation as the silicone is hidden internally once the window is painted but when a property is listed we must use Butyl putty externally rather than a glazing bead to ensure an exact like for like installation which the planning department will be delighted with
The performance of the slimmer sealed unit is not inline with a larger 12mm air cavity that we talked about with typical sash windows, however the results combined with a draught proofing system are very impressive considering this all fits inside a slender, original listed building sash window profile. When soundproofing is an issue double glazing listed sash windows using a slim unit is the only option a planning department will accept with open arms at the moment.
If we were to consider value for money on a listed building, I would cary out sash window repair and sash window draught proofing as this creates a reasonable payback in energy savings, but will not help to reduce noise pollution significantly like an Argon air cavity will.
Slim line units typically carry a five year guarantee. In practice the unit will far exceed the usage period. They are very well built indeed and the sash window will only require typical maintenance for decades to come.
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