How To Fit DIY Double Glazing

If you want double glazing but fear the cost of professional installation, why not fit your own? There are plenty of DIY double glazing kits on the market, to suit nearly all types of windows.

Properly installed double glazing is a sure way to cut down heat loss and eliminate draughts through your windows. In certain cases, it can also help soundproof your home against exterior noise.

The chief advantage of DIY double glazing systems, however, is cost — at least half that of professionally installed unit. But you also have flexibility — you can double glaze your windows as and when you feel like it. Naturally, as ‘bolt-on’ accessories rather than custom-made windows, DIY systems don’t give you quite such a high standard of finish. But that’s not to say that with a little care and patience you can’t create perfectly respectable double glazed windows that may well add to the value bf your home.

What’s available

The more substantial DIY double glazing kits fall into two groups: sealed unit panes and secondary glazing systems. The first consists of two sheets of glass (normally 3mm) sandwiched together with a sealed air gap of 5mm-6mm in between. Providing your window casement has rebates wide enough to accommodate the overall 1 lmm12mm thickness of this replacement ‘pane’, fitting should present no problems.

DIY Double Glazing

Secondary glazing simply means fitting a second window inside the original, but within its own subframe. Fixed, sliding (horizontally or vertically) and hinged versions are available, to suit most types of windows.

Sealed unit panes have the best insulating properties, so they’re the ones to go for if you have a large area of heat-conducting glass within an otherwise reasonably draughtproof frame. They are also the most efficient at cutting down condensation. On the other hand, secondary glazing comes into its own if your window has many small panes or if the frame itself is ill-fitting and admits a lot of draughts. The larger air gap which normally results from using this method means slightly reduced insulating efficiency (20mm is a practical maximum gap), but better soundproofing. In fact, with an air gap of 150mm you should notice a significant reduction in the level of outside noise finding its way into your home.

With secondary glazing kits, the important considerations are safety, convenience and the subframe material (which has a large bearing on the look of the finished job).

As regards safety, there is one point that cannot be overstressed.

You must make sure that the system of your choice can be removed easily and quickly — or at least opened sufficiently wide — to provide an escape route in the event of a fire. Carefully examine the range available until you’re satisfied that there’s one to safely suit your original window.

When it comes to convenience, you will naturally want to ensure that you can still open the window as and when you want to. The cheaper and simpler kits have fixed panes, so if you take the easy way out you may well find yourself with a poorly ventilated — and possibly unsafe — home.

The most common subframe materials are white plastic (uPVC), and aluminium (in plain or brushed form). The aluminium types are more durable and tend to look better, but of course they are more expensive. The uPVC type can be painted, to blend unobtrusively with the surrounding window frame.

Most secondary systems are sold in the form of individual ‘packs’ corresponding to different heights and widths of windows, and there are also extension packs for splitting larger panes into more manageable sections. In each pack you can expect to get the appropriate subframe sections, plus the corresponding slide sections. Fixings, handles, hinges and other accessories may also be included, though in some kits they come as another separate pack.

What you don’t get is the glass: this must be ordered separately.

With some fixed and all sliding kits, you measure up for and order the glass once you’ve installed the subframe; with other fixed kits and all hinged systems, calculate the glass area at the time you measure up the existing window opening, after making the allowances specified in the kit instructions.

Surveying your windows

The first thing to ask yourself before double glazing a window is whether it’s worthwhile. If the frame itself is badly warped or rotten, then it should be replaced entirely, in which case you have the option of fitting what is known as a complete, double glazed replacement window.

Next, decide whether your priorities are better insulation against heat loss and condensation, or improved draughtproofing and soundproofing; this could determine whether you choose sealed units or secondary glazing, though the size and type of window also has to be taken into consideration.

Sealed units: If you go for these, check that the rebate in the window casement is deep enough (at least 11mm) to take what is effectively a thicker pane of glass. If the existing pane is glazed with putty, the rebate width should be immediately apparent, but you may not be able to calculate the depth until you’ve removed the glass. You may need to increase the depth with a chisel and plane.

Measure up the pane from the outside edges of the rebates. Start with the diagonals: if these are equal you know the window is truly square. Then measure the height and width at three points across the pane and deduct 3mm from each figure to allow for expansion and contraction of the frame.

When you come to order, it is customary to give the height before the width. If you need a special types of glass — safety, or one with a pattern for instance — ask your supplier if these options are available.

Apart from the new pane, you’ll need a supply of glazing putty and sprigs (wooden frames) or steel clips (steel frames). For the job itself make sure you have some stout working gloves, a putty knife, pin hammer and an old wood chisel for gouging out the existing putty.

Secondary systems: First of all decide whether you want fixed, hinged, or sliding double glazing, then choose a kit system and obtain an instruction leaflet telling you from which points to measure the window. (Even though most subframes are fixed either to the window frame surround or to the reveal and sill, kits do vary in details.) At the same time it’s worth measuring the diagonals of your window to check whether it is out of square. If it is, and your subframe fits inside the reveal, you’ll need to pack the frame sections to bring it back to shape. Start by adding small packing pieces to bring the frame level and plumb, then fill in with other pieces to support it along its length.

On the other hand, if the kit of your choice is attached to the surround, you should have enough room for manoeuvre to be able to square it up as you fit it.

Having measured the window, these are the points to consider: Number of frames — most manufacturers recommend that no single frame should cover more than 1.85 sq. m (20 sq. ft), that the width of the frame should be no more than three times the height, and that no frame should be more than 1.5m high.

Fitting room — depending on the kit make and type, a minimum width of window surround or reveal will be needed on which to fit the subframe. This should be given in the manufacturer’s leaflet, so make sure you check it at this stage.

If you can’t meet the width requirement, you can either choose a different type of kit or system, or modify the surround/reveal with extra woodwork. Unless you are a proficient carpenter, the latter option could look messy and is probably more trouble than it’s worth.

Existing window stays and handles —depending on how the kit of your choice fits and the resulting air gap, these may protrude too far inwards. Either replace them with smaller fittings or attach packing battens to the surround to enlarge the gap between the old and new glazing.

Air gap — where thermal insulation or condensation are the overriding factors, the air gap should be as small as is practical unless specifically stated otherwise in the instructions. For soundproofing purposes the ideal air gap is 150mm. To achieve this you will probably have to fit packing battens to the window surround or reveal (depending on the kit). In this case mitre the ends for a neat finish and make sure that all the wood is thoroughly primed and painted before you fit it — the packing will be highly susceptible to rot, particularly if condensation is present.

Selecting height and width packs — both types come in a range of sizes to suit standard window measurements: select the nearest sizes above the dimensions of your window. The same applies to extension packs, should you need them.

Tools and materials — most kits supply all the materials and hardware you need in one or more packs, with the possible exception of plastic wallplugs for fixing into masonry.

As regards tools, you will almost certainly need a junior hacksaw, electric drill, screwdrivers, steel rule, metal file, hammer and wood block (or mallet), spirit level, try square, and masonry and twist drill bits.


Providing the rebates in your window casements are wide enough, this job is perfectly straightforward — the same, in fact, as re glazing a broken window.

If the casement is hinged, unscrew the hinges at the frame and remove it. Lay it on a thick blanket of similar, soft material.

On all windows, carefully gouge out the old facing putty around the pane using an old wood chisel. Then, if you’ve removed the casement, wrap the blanket over the pane and smash the glass with a hammer. TIP: If the pane is fixed, you can prepare the glass for breaking by applying lengths of masking tape criss cross fashion over the whole of one side. An even quicker alternative is to tape pieces of thick blanket securely to both sides. This done, break the glass with one or two firm, but controlled, blows with a heavy object.

From now on, wear a pair of stout working gloves — to protect your hands from barely visible splinters of glass.

Pull the broken glass out of the frame with great care and dispose of it immediately in a safe place. Follow by brushing away the splinters, then take your old chisel and trim away the old bedding putty from the rebate.

When the rebate is completely clean, thumb a layer of fresh bedding putty into the corner on all four sides: aim for a thickness of about 3mm. Then offer up or lay the new sealed unit on top of the putty and press it firmly home, ensuring that it is the same distance inside the rebate all round.

On a wooden window, tap glazing sprigs into the rebate at 300mm intervals to hold the glass into position. Hold the flatsided sprigs against the glass as you tap them home —slide the hammer across the glass to avoid cracking it and to keep the sprig parallel.

On a steel window, steel clips hold the glass. You’ll find pre drilled holes in the frame for them to clip into — space them at 600mm intervals.

You can now thumb the fresh facing putty right around the rebate to cover the sprigs/ clips and seal the pane. When you’ve got.

More or less an even thickness all round, wet the blade of your putty knife and strike off any surplus so that the surface is flat and angled away from the pane. Follow by trimming off any excess bedding putty.

For a neat finish, return to the facing side and go over the putty again so that the top edge is level with the bedding putty on the inside — run your knife firmly across the putty using the point of the blade to trim off level.

Allow two to four weeks for the putty to harden (it does so faster in warm, dry weather) before you paint it. When you do, overlap the paint about 2mm onto the glass to provide a fully weatherproof seal.


Many fixed pane secondary systems and all sliding systems employ a subframe into which the framed glass is fitted. You install the subframe before you measure up and order the glass itself.

Start by laying out the subframe sections on the floor, in front of the window, exactly as they will be fitted. Then measure up the window surround or recess exactly as specified in the manufacturer’s instructions. Measure the diagonals too, to see if the window is square.

Before you mark the sections for cutting, be sure to deduct the specified amount for clearance: then mark each section individually using a try square and felt tip pen. Both uPVC and aluminium frames are cut using a junior hacksaw. However the sections join together — they may butt, interlock or use corner pieces — it is essential that they are cut square. You can ensure this by laying each one on a firm, flat surface (or against a bench hook) and then wrapping a piece of stiff cardboard around it to act as a saw guide.

When you’ve cut each section, take a metal file and remove the burrs around the cut to leave a smooth, clean edge.

At this stage make sure you know what preparatory work must be done on the sections before you screw them in place. Aluminium ones usually require screw fixing holes to be drilled — not less than 50mm from the end and then at 125mm150mm intervals — and with some kits you have to insert the fibre slide seals and then trim them to length.

On kits where the sections butt or interlock, you generally fit the side sections first, then saw small cutouts in the ends of the top sections before you fit these. Exact guidance will be given on this point in the instructions.

On kits employing corner pieces, you assemble the subframe on the floor and then fix it as one complete unit.

In either case, if you are fixing to masonry. Be sure to hold a spirit level against the section/frame as you mark the fixing holes.

If you’re dealing with a complete frame, get an assistant to hold the frame in position with a try square in one corner while you mark the fixing holes. Recheck for square after each mark.

After marking the wall, drill holes with a No.8 masonry drill bit to take standard plastic wallplugs.

If you are fixing to wood, simply locate the section/frame, check for level/plumb with your spirit level, then make pilot holes through the fixing holes in the frame sections.

Screw the sections/frame in place in the order given in the kit instructions and then check yet again that it is square. Minor discrepancies can be dealt with by packing behind the sections.

Note that on horizontal sliding systems it is most important that the top and bottom sections are parallel along their entire length. On vertical systems the same applies to the side tracks.


Depending on the kit, you either measure up for the glass against the subframe or direct against the window surround. Be sure to do it strictly according to the manufacturer’s instructions.

Having noted the dimensions. Order the glass and ask your glass merchant to remove the sharp edges. When you take delivery. Make sure that this has been done and check that the pane has been cut to within 1.5mm of the correct size.

On all kits the glass is framed with uPVC or aluminium sections, these being cut and assembled in much the same way as subframes.

Measure and mark them for cutting strictly according to the kit instructions, then saw them to length with a junior hacksaw using a piece of stiff card wrapped around the cutting line to guide you. Remove all burrs with a file.

UPVC fixed pane with subframe: Stand the glass upright on edge and rest it on an old blanket (to act as a cushion). Take the top frame section, clip one end over the edge of the glass, centre it, then take a hammer and wood block and tap it gently home over the glass, working from one end to the other.

Now take the bottom frame section, make sure its seal faces the same way as that of the top. And tap it home as described above. The two side sections may have to be notched before fitting to clear the top and bottom sections. Do this with a junior hacksaw, remove the burrs, then fit them as before —making sure their seals face the right way.

UPVC sliding pane: In this case the glass is framed in the same way as a fixed pane kit, but take particular care to ensure that the seals face the right way round. On horizontal sliding systems you need to distinguish between front and back panes: on vertical systems it’s between upper and lower.

Selfadhesive handles are usually provided for fitting to horizontal sliding panes: with vertical systems, don’t forget to clip the ratchet catches — that engage in the side tracks of the subframe — into the frame sections. Aluminium sliding systems (and fixed pane systems with subframe): Most kits of this type have individual frame sections which you cut to length according to the kit instructions and then assemble around the frame with screwon corner pieces. The sections are tapped onto the edge of the glass using a hammer and wood block, as with uPVC systems, but in this case they go over a PVC glazing gasket.

Make sure you cut the sections squarely and deburr them afterwards. Fit the sections in the order given. With the glass resting on edge on a blanket.

Lay the gasket in place. Then lay the section on top and tap home.

Some kits have slide rollers which slip in to a channel in the bottom section and are held by setscrews. On others, the handles must be added before you fit the relevant frame section to the glass.

The corner pieces are normally loose fitted as you go: then, once ‘the frame is complete, you tighten their securing screws.

As with uPVC sliding systems, make sure that the seals on each section face the same way and that you have distinguished between front and back panes.

Hinged systems: These are always aluminium, and there is no sub frame — the frame around the glass holds the hinge posts and the seals locate against the window surround.

Assembly of the frame is the same as for other aluminium types.

When complete, the hinge posts can be slid into channels in the side sections and secured with setscrews.

Some aluminium fixed pane kits also do without a subframe. In this case locking catches simply replace the hinge posts.


This is usually the simplest part of the operation, but before you go ahead check that all the subframe and frame parts are cor rectly butted and that, where fitted, the seals between panes are the right way round.

Sliding systems: If the system is uPVC, simply lift the back pane into the subframe and up into the top track: then lower into the bottom track. Check that it slides freely, then repeat for the front pane.

Use the same procedure on aluminium systems, but check first that all the correct seals are in position — with some kits you fit them separately to the completed pate. Also, if your system employs PTFE runners rather than rollers, fit these to the corner pieces on the bottom frame section at this stage.

On vertical sliding systems, start by fitting lengths of ratchet sash guide into the right hand track. They are a simple press fit, but you may need to cut the final length to size. Follow by fitting just one length of sash guide into the left hand tracks.

Now insert the window side pane into its tracks with the left hand ratchet catch above the guide: operating both catches. Gently lower it to engage in the guides on both sides. Repeat this procedure for the room side pane, then fit the remaining lengths of ratchet guide on the left hand tracks. Finally, measure and cut the top track insert to fit and press it in position on the inside of the top track. Hinged and fixed systems: With hinged systems there is no subframe — installation is simply a matter of hanging the hinges, then fitting stays and catches.

The best way of marking the hinge socket/post positions is to tape them

temporarily to their corresponding halves on the pane frame. Offer the pane up to the window, get an assistant to check it for plumb and level, follow any specific instructions on clearance, then mark the fixing holes with a bradawl and remove.

Separate the hinges and screw the posts/ sockets to the window surround. Hang the frame, then fit stays/catches where indicated in the instructions.

On fixed pane systems employing a subframe, you fit the pane inside as for a sliding system. If the system doesn’t have a sub frame, fit the locking turncatches as for a hinged pane.

Finishing off

On some systems, the look of the subframe can be improved by pinning and gluing lengths of wooden moulding against the surround in front of it.

Check carefully that the completed installation contains no air gaps between parts —particularly if the room is subject to heavy condensation. Such gaps can be filled by piping in a thin bead of silicone sealant.

If the subframe is uPVC and you want to paint it, rub over the surface first with fine abrasive paper to provide a good key.

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