Buying Double Glazing Sealed Units Online The Complete UK Guide

Buying double glazing sealed units online has become one of the most convenient and cost-effective ways for UK homeowners, landlords, and tradespeople to source replacement glass. Instead of waiting for a sales representative to visit, taking time off work for a home survey, or being locked into a single local supplier's pricing, ordering online lets you compare specifications, request quotes at any hour, and have units manufactured to your exact measurements and delivered directly to your door. The online glazing market in the UK has grown substantially over the past decade, driven by improvements in courier logistics, better packaging technology, and increasing consumer confidence in buying building materials remotely.

However, buying sealed units online also demands a higher degree of buyer knowledge than walking into a local glazier's showroom. You are responsible for taking accurate measurements, choosing the right glass specification, understanding the difference between gas fill options, and inspecting the units on arrival to ensure they meet the standard you've paid for. Get any of these steps wrong and you could end up with units that don't fit, perform poorly, or fail prematurely. This guide takes you through the entire process – from measuring and ordering to delivery, inspection, and fitting – so you can buy sealed units online UK-wide with confidence and avoid the most common pitfalls.

How to Order Sealed Units Online

Ordering double glazing sealed units online follows a straightforward process, but each step matters. Rushing through or skipping a stage can result in costly errors – particularly with measurements, which are the single most common cause of problems when buying glazing remotely.

Step 1: Measure your existing units. Before you do anything else, you need accurate measurements of the sealed units you're replacing. This is the most critical part of the entire process, and we've dedicated a full section to it below. For detailed instructions, follow our measuring guide, which walks you through the three-point measuring technique, identifies common mistakes, and explains how to measure overall thickness as well as width and height. If you're ordering online for the first time, take your time with this step – every millimetre counts.

Step 2: Choose your glass type. Once you have your measurements, you need to decide on the glass specification. The choice you make here directly affects thermal performance, acoustic insulation, safety compliance, and cost. Standard clear float glass is the most affordable option, but most replacements today use Low-E (low emissivity) glass as a minimum, which dramatically improves energy efficiency. If the window is in a bathroom, you may want obscure glass for privacy. If it's in a door, a low-level window, or near a staircase, Building Regulations may require toughened or laminated safety glass. We cover all the glass options available in detail on our products page, but Section 4 of this guide also provides a useful overview.

Step 3: Select your gas fill. The gas that fills the cavity between the two panes of glass is a major factor in thermal and acoustic performance. Argon is the industry standard and is included in the price by most reputable suppliers. Krypton and xenon offer superior performance but at a higher cost. We explain the differences further down this page, and our dedicated gas fills guide goes into greater detail.

Step 4: Specify your spacer bar. The spacer bar sits around the perimeter of the unit between the two panes of glass and maintains the correct cavity width. Warm-edge spacer bars – made from materials with low thermal conductivity such as composite polymers or stainless steel – significantly reduce heat loss at the edge of the glass and help prevent condensation forming on the inside of the window near the frame. Aluminium spacer bars are cheaper but conduct heat readily, creating a thermal bridge. For the best performance, always choose a warm-edge spacer bar.

Step 5: Submit your order or quote request. Most online glazing suppliers offer either a direct ordering system where you enter your measurements and specifications into a configurator, or a quote request form where you submit your requirements and receive a priced quote by email – usually within 24 hours. If you're placing a large order, it's worth requesting quotes from two or three suppliers to compare pricing on a like-for-like basis. Check that the quote includes delivery, as some suppliers charge separately. Once you're satisfied with the price and specification, confirm your order and arrange payment. Most suppliers accept debit and credit cards, and some offer bank transfer for trade accounts. To get an idea of typical costs before you start, take a look at our cost guide.

Measuring for Online Orders

When you buy from a local glazier, they often send a fitter to take the measurements for you. When you order double glazing online, that responsibility falls entirely on you. Inaccurate measurements are by far the most common reason for problems with online orders – and because bespoke units are manufactured to your exact dimensions, a unit that's wrong usually can't be returned.

The three-point method. For each window opening, measure the width at three points: top, middle, and bottom. Then measure the height at three points: left, centre, and right. Use the smallest measurement for both width and height. Window openings are rarely perfectly square, especially in older properties, and using the smallest dimension ensures the unit will fit into the frame rebate without forcing. Record all measurements in millimetres – never round up.

Measuring overall thickness. You also need to measure the total thickness of the existing sealed unit. This includes both panes of glass and the cavity between them. The most common configurations are 24 mm (two 4 mm panes with a 16 mm cavity) and 28 mm (two 4 mm panes with a 20 mm cavity). Some older units are 20 mm overall. The simplest way to measure this is to remove the glazing beads from one side of the window and measure the unit edge-on with a tape measure or vernier caliper. If you cannot remove the beads, a glazing thickness gauge – available cheaply online – can measure through the glass.

Common mistakes when ordering online. The first and most frequent mistake is measuring the frame opening rather than the glass unit. The sealed unit sits inside a rebate in the frame, and the visible glass area is smaller than the unit itself. You need to measure the full unit, including the portion hidden behind the glazing beads. The second common error is failing to check whether toughened glass is required – Building Regulations Document N and Document K specify that any glazing within 800 mm of floor level, or within 300 mm of a door, must be safety glass. If you order standard annealed glass where toughened is required, you'll need to reorder. The third mistake is forgetting to specify the spacer bar width. If you order a 16 mm cavity unit for a frame designed for a 20 mm cavity, the unit will be too thin and the glazing beads won't grip it securely.

Choosing the Right Glass Type

The glass specification you choose determines how well your sealed unit performs thermally, acoustically, and in terms of safety. Online suppliers typically offer a wide range of options, and understanding the differences ensures you get the right product for each window in your property.

Standard clear float glass. This is the most basic and affordable option – two panes of clear annealed glass with no special coatings. It provides reasonable thermal performance in a double glazed configuration and is suitable for ground-floor windows that do not require safety glass. However, it is increasingly being replaced by Low-E glass as the default option.

Low-E glass. Low emissivity glass has a microscopic metallic coating – usually applied to surface 3 (the inner face of the outer pane) or surface 2 – that reflects heat back into the room while still allowing sunlight to pass through. A standard double glazed unit with Low-E glass and argon gas fill can achieve a centre-pane U-value of around 1.1 W/m²K, compared to approximately 2.8 W/m²K for clear float glass. If thermal performance and energy efficiency are priorities – and they should be – Low-E glass is the minimum specification you should consider.

Toughened glass. Toughened (also called tempered) glass is heat-treated to be approximately four to five times stronger than standard annealed glass. When it does break, it shatters into small, relatively harmless granules rather than dangerous shards. It is required by Building Regulations in critical locations: windows below 800 mm from floor level, glazing within 300 mm of a door, and all glass in doors themselves. When ordering online, you must identify which windows require toughened glass and specify it accordingly – the supplier cannot assess this for you.

Acoustic glass. If noise reduction is a priority – for instance in properties near busy roads, airports, or railway lines – acoustic laminated glass is significantly more effective than standard double glazing. Acoustic units use a laminated pane with a PVB (polyvinyl butyral) interlayer that dampens sound vibrations. Some configurations use asymmetric glass thicknesses (for example, 6 mm outer pane with a 4 mm inner pane) to disrupt sound frequencies more effectively. Acoustic glass can reduce external noise by 35–40 dB compared to around 28–30 dB for a standard sealed unit.

Obscure glass. Obscure or patterned glass is used where privacy is needed – typically in bathrooms, cloakrooms, and ground-floor side windows. The level of obscurity is graded from 1 (lightly textured, some visibility) to 5 (heavily obscured, no visibility). When ordering online, specify the obscurity level you require. The most common choice for bathrooms is level 4 or 5.

Triple glazing. Triple glazed units use three panes of glass with two cavities, offering superior thermal and acoustic performance. While they are more expensive and heavier than double glazed units, they are becoming increasingly popular in new-build properties and energy-conscious retrofits. A triple glazed unit with two Low-E coatings and argon fill can achieve U-values below 0.7 W/m²K. However, not all frames are designed to accommodate the extra thickness and weight, so check compatibility before ordering. See our full range of glass options available for detailed specifications.

Gas Fill Options

The gas that fills the cavity between the panes of glass in a sealed unit plays a crucial role in both thermal insulation and acoustic performance. Denser, less conductive gases slow down heat transfer and dampen sound more effectively than air. There are three gases commonly used in sealed glazing units, each with distinct performance characteristics and cost implications.

Argon. Argon is by far the most widely used gas fill in the UK double glazing industry. It is approximately 34% less thermally conductive than air, which translates into a meaningful improvement in U-value. Argon-filled units are the standard offering from most online suppliers and represent the best balance of performance and cost. In a typical unit with 4 mm Low-E glass and a 16 mm cavity, switching from air to argon fill improves the centre-pane U-value from around 1.4 W/m²K to approximately 1.1 W/m²K.

Krypton. Krypton is a rarer and denser gas that offers even better thermal performance than argon. It is most commonly used in units with narrower cavities (10–12 mm), where its superior insulating properties compensate for the reduced cavity width. Krypton-filled units are more expensive than argon, but are an excellent choice where frame depth is limited – for example, in period properties with slim timber frames that cannot accommodate a standard 24 mm or 28 mm unit.

Xenon. Xenon provides the highest thermal performance of the three common fill gases but is significantly more expensive and is rarely used in residential applications. It is typically reserved for high-performance commercial glazing or specialist projects where the absolute best U-value is required.

For a comprehensive comparison of thermal conductivity values, costs, and recommendations for different property types, visit our gas fills guide.

Delivery and Packaging

One of the most common concerns about buying double glazing sealed units online is whether the units will survive transit. Glass is heavy, fragile, and expensive to replace if damaged – so understanding how reputable suppliers package and deliver their products is essential before you place an order.

How sealed units are packaged. Professional glass suppliers use purpose-built packaging designed to protect sealed units during road transport. Individual units are separated by foam spacers or interleaving material – typically closed-cell polyethylene foam or corrugated cardboard – to prevent glass-to-glass contact. The edges, which are the most vulnerable part of any sealed unit, are protected with foam edge strips or rubber corner guards. Multiple units are then stacked vertically in timber or heavy-duty cardboard crates, secured with strapping, and placed on A-frame stillages for transportation. Reputable online suppliers use specialist glass courier services rather than general parcel carriers, because glass requires careful handling and appropriate vehicle equipment that standard couriers are not set up to provide.

Delivery timeframes. Standard delivery for made-to-measure sealed units is typically 5 to 7 working days from the date your order is confirmed and measurements are finalised. Some suppliers offer express manufacturing and delivery within 3 working days for a surcharge. Bespoke shapes, triple glazed units, and units with specialist coatings may take 10 to 14 working days. Most suppliers will give you a confirmed delivery date at the point of order and offer tracking so you can plan accordingly. Delivery is usually to the ground floor – courier drivers are not expected to carry glass upstairs – so ensure you have adequate space to receive and store the units on arrival. We deliver across London, Manchester, Birmingham, Bristol, and areas throughout the UK.

Inspection on arrival. This is a critical step that too many buyers skip or rush. Before signing the delivery note, inspect every unit carefully. Open the packaging and check each unit individually – do not simply glance at the outside of the crate and assume everything is fine. Specifically, you should look for the following:

• BS EN 1279 marking on the spacer bar. Every sealed unit manufactured to the European standard should have the manufacturer's identification, the standard number, and the date of manufacture printed or etched on the spacer bar. This marking is your confirmation that the unit has been manufactured in a facility operating under quality-controlled conditions.
• Argon fill valve. If you've ordered argon-filled units, check for the small gas fill hole on the spacer bar (usually sealed with a plug or sealant). This is the point through which the gas was injected during manufacture. Its presence confirms the unit has been gas-filled rather than simply air-filled.
• No cracks or chips. Inspect both faces of every pane for cracks, chips, scratches, or impact marks. Even a hairline crack will compromise the unit's seal over time and should be rejected.
• No moisture inside the unit. If you can see condensation, misting, or water droplets between the panes immediately upon delivery, the unit's seal has already failed. Reject it immediately.
• Correct dimensions. Use a tape measure to confirm that the width, height, and overall thickness of each unit match your order. Even small discrepancies can cause problems during fitting.

If any unit is damaged or does not match the specification you ordered, note this clearly on the delivery paperwork, photograph the issue, and contact the supplier immediately. Do not sign for the delivery as "received in good condition" if it isn't – doing so can complicate any subsequent claim.

DIY Fitting vs Professional Installation

Once your sealed units have arrived and passed inspection, the next decision is whether to fit them yourself or hire a professional. Both approaches have their place, and the right choice depends on your experience, the type of unit, and the regulatory requirements that apply to your property.

When DIY is appropriate. If you are an experienced DIYer who has fitted sealed units before – or you're a tradesperson such as a carpenter or builder who regularly works with window frames – replacing a standard sealed unit in a uPVC or timber frame is a fairly straightforward job. The basic process involves removing the internal glazing beads, lifting out the old unit, cleaning the frame rebate, positioning the new unit with packers to ensure correct alignment, and refitting the beads. With the right tools (a glazing shovel or stiff putty knife, a rubber mallet, and appropriate packers), a single unit can be replaced in 30 to 60 minutes. DIY fitting saves the cost of a professional installer, which can range from £40 to £80 per unit depending on size and location. For full details of our supply-only and installation options, see our services page.

When professional installation is needed. There are several situations where you should seriously consider using a professional rather than fitting the unit yourself. If the window requires toughened or laminated safety glass under Building Regulations (Document N or Document K), a professional installer will ensure compliance and provide the necessary certification. If your property is a listed building or in a conservation area, there may be additional planning requirements that a professional installer will be familiar with. If the frame itself is damaged, rotten, or misaligned, simply replacing the sealed unit won't solve the problem – a professional can assess whether the frame needs repair or replacement before the new glass goes in. Finally, if the unit is large and heavy (triple glazed units can weigh upwards of 40 kg per square metre), it may be physically impractical to handle without specialist equipment and a second pair of hands.

Warranty considerations. Some sealed unit manufacturers specify that their warranty is only valid if the unit is installed by a professional or qualified installer. If you fit the unit yourself and it subsequently fails, the manufacturer could argue that improper installation caused the failure and decline the warranty claim. Check the warranty terms before deciding whether to DIY.

Quality Checks on Arrival

Beyond the immediate visual inspection described in the delivery section above, there are several additional quality checks you can carry out to verify that the sealed units you've received meet the specification you ordered and have been manufactured to a professional standard.

BS EN 1279 marking. We've already mentioned this, but it bears repeating because it's the single most important quality indicator. The spacer bar should be printed with the manufacturer's name or code, the number BS EN 1279, and the date of manufacture. If this marking is absent, the unit may not have been manufactured to the required standard. Some lower-cost suppliers skip certification to reduce costs – but units without BS EN 1279 compliance may have higher rates of seal failure, gas leakage, and moisture penetration over time.

Spacer bar information. Examine the spacer bar itself. A warm-edge spacer bar – which is what you should have specified – is typically black or grey in colour and made from a composite material (such as Thermobar, SuperSpacer, or Swisspacer). If the spacer bar is shiny metallic silver, it is an aluminium spacer bar, which conducts heat far more readily and contributes to condensation at the window edge. If you ordered warm-edge and received aluminium, raise this with the supplier immediately.

Edge seal integrity. Run your finger carefully along the edge seal of each unit. The secondary seal (the outer black sealant, typically polysulphide or silicone) should be smooth, continuous, and free from gaps, bubbles, or voids. Any discontinuity in the edge seal is a potential point of future failure – moisture will eventually find its way in through the gap and the unit will mist up. If you spot a visible defect in the seal, reject the unit.

Dimensions check. Measure the width, height, and thickness of each unit with a tape measure and compare against your order confirmation. The width and height should be accurate to within 1 mm of your specified dimensions. The overall thickness should match exactly – if you ordered a 28 mm unit and received a 24 mm unit, it will not sit correctly in the frame and the glazing beads may not secure it properly.

Gas fill verification. Verifying that a unit has been filled with argon rather than air is difficult without specialist equipment. However, one practical check is to weigh the unit on a set of bathroom scales and compare the weight against what you'd expect. Argon is denser than air (1.784 kg/m³ versus 1.225 kg/m³), so an argon-filled unit will be marginally heavier than an identical air-filled unit. The difference is small – typically 50–100 g for a standard-sized unit – but it can be detected with an accurate scale. A more reliable method is to use a Sparklike handheld gas analyser, although these are specialist devices more commonly used by trade installers and quality inspectors than by homeowners.

Returns and Warranty

Understanding the returns policy and warranty terms before you place an order is essential when buying double glazing sealed units online. Because sealed units are often manufactured bespoke to your measurements, the rules around returns are slightly different from off-the-shelf products.

Standard warranty terms. A reputable sealed unit supplier should offer a minimum guarantee of 10 years against seal failure, misting, and gas leakage, with many of the better manufacturers offering 15 or even 20 years. The warranty should be provided in writing, clearly stating the duration, what is covered (seal failure, gas escape, manufacturing defects), and the process for making a claim. Check whether the warranty covers the unit only or also includes the cost of removal and refitting. An insurance-backed guarantee provides additional security – if the manufacturer ceases trading during the warranty period, the insurance underwriter will honour the claim. Always keep your order confirmation, delivery paperwork, and warranty documentation in a safe place.

When returns are accepted. Most online sealed unit suppliers will accept returns in the following circumstances: the unit arrives damaged during transit; the unit does not match the specification you ordered (wrong dimensions, wrong glass type, wrong spacer bar); or the unit has a manufacturing defect (seal failure, moisture inside, defective coating). In all of these cases, the supplier should arrange collection and provide a replacement or full refund at no cost to you. If the error is on the supplier's side – a manufacturing mistake or a picking error – they are responsible for the cost of return shipping.

Consumer Contracts Regulations 2013. Under these regulations, which apply to all goods purchased online or by telephone in the UK, you have a statutory 14-day cooling-off period. This begins the day after you receive the goods, and during this window you can cancel the contract and return the goods for a full refund without needing to give a reason. However – and this is the critical point for sealed unit buyers – goods that have been made to your bespoke specifications or clearly personalised are exempt from this right. Because most sealed units are manufactured to your specific measurements, they are generally classed as bespoke goods and cannot be returned under the cooling-off provisions unless they are faulty or not as described. Standard stock sizes, if the supplier offers them, would typically be returnable within the 14-day window provided they are unused and in original packaging. Always check the supplier's specific returns policy before ordering, and be aware of this distinction between bespoke and stock items.

Consumer Rights Act 2015. Separate from the cooling-off period, this act gives you the right to receive goods that are of satisfactory quality, fit for purpose, and as described. If a sealed unit arrives damaged, fails prematurely, or does not meet the specification you ordered, you have statutory rights to a repair, replacement, or refund – regardless of whether the unit was bespoke. The 30-day short-term right to reject applies from delivery, and the six-month burden-of-proof reversal means the supplier must demonstrate the fault wasn't present at delivery rather than you having to prove it was.

Frequently Asked Questions