How To Install Replacement Windows: The Ultimate Step-by-Step Guide

1. Remove the Existing Window Frame Safely

Begin by taking out the existing window carefully:

Congratulations, the hard part (messy part) is done – the old window is out! Take a breather, and get ready to install the new one.

2. Prepare the window opening

Now you have a bare opening, you need to do a few things to get it ready for the new frame:

• Test fit the new window: With a helper, dry-fit the new uPVC window into the window opening before doing anything else. Simply place it in the hole and see how it sits. This is a crucial sanity check. It should slot in with the expected small gap around (approximately 5mm each side if you deducted correctly). Check that the outer face of the window will sit flush or slightly set back as intended. In the UK, windows are often fitted so the frame's outer face is roughly flush with the outside brickwork (with perhaps a slight recess of 10mm or so). Ensure it's oriented correctly (we've seen people accidentally put windows upside down or back-to-front – make sure the drainage holes in the frame, if any, go at the bottom on the outside!). Once confirmed that it fits, carefully remove it again to continue prep.
• Attach the sill (if applicable): If your window came with a separate uPVC sill piece (cill), now is the time to attach it to the frame. Usually, the sill sits underneath the bottom of the frame or is screwed from underneath. Apply a line of silicone on the contact areas and fix the sill to the frame so it's one unit. The sill will project outwards; ensure it's oriented properly (the upstand on the sill should snug against the frame bottom).
• DPC check: Older windows might have had a thin strip of damp-proof course (DPC) membrane under the sill or around the opening. If there was one and it's damaged, slip a new piece of DPC or flashing tape along the bottom where the new window will sit, for extra moisture protection.
• Set up packers: Place a few plastic packers on the bottom ledge of the opening (on the brick or sill) before putting the window in. Typically, you put two packers about 100mm in from each corner to initially sit the window on. These support the frame bottom and also ensure a gap under the frame for sealant. You can also have a couple ready at the sides.
• Have fixings ready: Prep your frame fixing screws – often you'll need to drive screws through the sides of the uPVC frame into the wall. If the frame doesn't have pre-drilled holes, you might drill pilot holes now (commonly ~3 holes on each side for a small/medium window – one near the top, one near the bottom, one in the middle; avoid drilling where the glass will sit or where any hinges/locks are). It's easier to mark and pre-drill the uPVC when it's not in the wall yet. If using fixing lugs/brackets instead, fix those to the frame now (they usually slide into channels on the frame and screw in place).
• One last sweep: Ensure no debris is left that could obstruct the new frame. Also check the alignment of the opening – if one side has a big bulge of plaster or mortar, you might need to knock that off or be ready to pack that area. Now you're ready to install for real.

3. Insert the new uPVC frame

This is it – time to put the new window in place:

• Position the frame: With your helper, lift the new window (frame with sill attached, if applicable) into the opening from the outside if possible. If working from inside (e.g. for an upper floor where external access is difficult), you can lift it into place from inside – just be very careful not to push it outward too far. Guide the bottom onto the packers you set on the sill/brick. Center the frame in the opening.
• Support and level: Add additional plastic packers under the frame until the frame is perfectly level along the bottom. Use your spirit level along the bottom uPVC sill; adjust the packers as needed. Keep at least two packers under each jamb (side) right near the bottom corners – this prevents the frame from dropping over time. The window should now be roughly sitting in place, held by gravity on the bottom packers.
• Ensure even gaps: Check that the gap on the left and right sides between the frame and wall is roughly equal. You want the frame centered, not tight to one side. If one side has a much bigger gap, shift the frame a bit now. The small gaps will later be filled, so symmetry is more about ease of insulating and aesthetics.
• Plumb and square: Now use the spirit level vertically on both sides of the window frame (inside or outside, against a flat part of the frame). Shim with packers under the side or bottom as needed to get the frame plumb (perfectly vertical). Next, measure diagonally from the inside top-left corner of the frame to bottom-right, and top-right to bottom-left. These diagonal measurements of the frame itself (not the opening) should be identical if the frame is not twisted. If they differ, adjust the supports until the frame isn't skewed.
• Place side packers: Wedge a couple of packers between the frame and wall on the sides to hold the frame steady vertically. Typically, put one near the top on each side (between the frame and the wall) and one mid-way. These will later also serve to keep the frame from flexing when you tighten the screws.
• Check alignment with exterior: Go outside and ensure the window frame is positioned correctly relative to the outer wall. Commonly, you want the frame's external face to align where the old one was – either flush with the brick or slightly recessed. Also check that the sill (if present) is sitting correctly and evenly on the substrate.

At this stage, the window should be sitting in the opening, level and plumb, held loosely by packers. Take a moment to simulate operation: if the window has opening sashes already hung, try opening them to ensure they clear the reveal and operate smoothly. (If your window came with the opening sashes separate or you removed them, you will re-hang them later once the frame is fixed.)

4. Secure the frame in place

With the frame positioned correctly, it's time to fix it permanently to the wall:

• Drill fixing holes: If not pre-drilled, mark points on the frame where you will put screws – usually 150mm from the top and bottom of each side, and at roughly 600mm intervals in between (avoiding positions that align with hinge or lock hardware). Using an appropriate size masonry drill bit (often 6mm for frame fixings), drill through the uPVC frame and into the wall behind by at least 50mm deep. You might drill through the plastic first, then switch to hammer action for the masonry. Tip: Put a piece of tape on the drill bit to mark the required depth so you don't drill too far.
• Insert frame screws: Drive the special frame fixing screws through the frame and into the wall. These screws usually don't require wall plugs if sized correctly (they bite directly into brick/block). Tighten them gradually and evenly. A cordless impact driver or drill driver on a low torque setting works well. As you tighten each screw, watch the frame – it should pull up snug to the wall or to the packer, but do not overtighten. If you crank too hard, the uPVC can distort or bow inward. Tighten until the frame is secure and doesn't move when gently pushed, but the frame remains straight (check with your level after each screw).
• Alternate sides and check: It's best to fix one screw on one side, then a screw on the opposite side, alternating, to keep pressures even. After putting in a couple of screws, double-check level and plumb. The act of screwing can sometimes shift the frame slightly, especially if a packer falls out. If something shifts, back the screw out, reposition with packers, and screw again.
• Use packers behind screws: Ensure that wherever a screw is going to tighten the frame, there is a packer between the frame and wall at that point. This prevents the screw from bowing the frame. For example, if the gap is 10mm and you drive a screw without a packer, it will pull the frame outwards into the gap. So insert packers right next to each screw location (you can slide them in from either side of the screw) so the frame stays true when the screw is tightened.
• Fix top and bottom (if required): Depending on the size and type of window frame, you may also put fixings through the top of the frame (head) and the bottom (through the sill or bottom frame). Many standard windows rely on side fixings only, with foam supporting elsewhere, to avoid bowing the frame. Follow the window supplier's recommendations. If you do screw the top/bottom, take extra care not to overtighten and distort the frame.
• Using fixing lugs (if applicable): If instead of drilling through the frame you have metal fixing lugs attached, then swing those out and screw them into the surrounding wall with appropriate wall plugs. Usually you'd have pre-fixed these lugs at similar spacings (near corners and every ~600mm). Make sure those screws are solidly in the masonry.
• Final check: The window frame should now be firmly anchored. Try giving it a gentle shake – it should feel solid. Check again that it is plumb and level. Also confirm that the gap around the frame is still even and not excessively large on any side (indicating a possible misalignment). If all looks good, you're ready to add the glass back in.

5. Install the window glazing (glass panes)

With the frame secure, you will put the glass units (and any opening sashes) back in:

• Prepare glass units: If your window has separate double-glazed sealed units to fit into fixed portions or sashes, make sure you have the correct unit for each section. Clean the edges of the glass and the frame rebate (the channel where the glass sits) of any debris.
• Place setting blocks: These are small plastic blocks that support the glass inside the frame. Typically, two blocks are placed on the bottom of the opening (frame) about 50–100mm in from each corner, to support the weight of the glass. They also ensure the glass units don't sit directly on the window frame. Put these in place before dropping in the glass.

• Carefully fit the glass: With gloves on (and using suction cup handles if you have them for large sealed units), lift the double-glazed unit and gently set it into the frame opening from inside. Insert the bottom edge first, resting it on the setting blocks, then tilt the top in until the glass is fully in the frame rebate. Be very careful not to hit the glass against the frame edges.
• Center and pack the glass: The glass should have a uniform gap between its edges and the frame. Use plastic packers to achieve this. Typically, you place packers on the sides or top to wedge the glass so that it is evenly spaced and to prevent it from moving or "dropping" over time. For an opening sash (casement) that hinges on one side, you would "toe and heel" the glass: place packers at the bottom corner on the hinge side and the top corner on the opposite side. This counterbalances the glass and prevents the sash from sagging. Adjust the packers until the sash closes perfectly and the glass is snug.
• Refit glazing beads: Once the glass is positioned and packed, refit the uPVC glazing beads to secure it. Start typically with the bottom bead: align it and press it back into the groove in the frame until it snaps in. You may need to give it a firm tap with the palm of your hand or a non-marking mallet. Then fit the side beads, and finally the top bead (or follow the removal order in reverse). Ensure each bead clicks in fully along its length. This locks the glass units in place.
• Test operation: Now test any openers again. The sash should open and shut smoothly, and locks should engage properly. If the lock is slightly misaligned, slight adjustments of the striker plate or additional tweaking of glass unit packers (toe and heeling) might be needed. Take your time to get it right – the window should operate just as well as a professionally installed one, with no sticking or gaps.

At this point, your window is physically installed – congrats! It's in the wall, glazed, and should look like a window again rather than a big hole. The remaining steps are about sealing and finishing it off so it performs well.

6. Seal and insulate around the window

With the new window in place, there will be gaps around the window frame (the fitting tolerance we left). Now you'll fill and seal those gaps to weatherproof and draft-proof the installation:

• Apply expanding foam: Shake your can of low-expansion window foam insulation and attach the nozzle. Spray foam into the gaps between the frame and wall, primarily around the top and sides. Use sparingly – fill about 50% of the depth of the gap, and the foam will expand to fill the rest. Be especially cautious not to overfill the top gap, as expanding foam can bow the window frame if it exerts too much pressure. If the gap is very small (<5mm), you can skip foam and just rely on sealant. For larger gaps, a two-thirds fill with foam is good. (Make sure any packers remain in place and are not pushed out by foam.)
• Foam the bottom (if needed): The gap under the sill or bottom window frame can also be foamed, but many installers prefer to leave the bottom open for drainage and just seal with silicone. If you foam the bottom, do so very lightly, as this is where foam expansion could lift the frame. Alternatively, you can stuff some backer rod (foam rope) in the gap and later silicone over it from outside for a waterproof finish.
• Allow foam to cure: Follow the foam can instructions – usually it will become tack-free in 10 minutes and fully cure in a few hours. While it's curing, do not touch or press it. It will likely expand beyond the wall surface; that's normal.
• Trim excess foam: Once the foam is fully hardened (typically after an hour or more), use a sharp knife or small saw to cut off any foam protruding beyond the plane of the wall or window frame. Cut it flush or just recessed slightly so that there is room to apply sealant on top.
• Seal the exterior: Using exterior-grade silicone, run a continuous bead around the outside of the replacement window frame where the frame meets the surrounding wall. This is critical to prevent water ingress. Aim for a smooth bead covering any gap. It often helps to run masking tape on the wall and frame for a neat line: apply tape parallel to the gap, then caulk, smooth it with a wet finger or caulk tool, and remove the tape before the silicone skins over. Ensure no gaps or pinholes in this outer seal – it should be continuous around all four sides. If your window has an external sill, also seal the joint where the sill meets the masonry or any end caps.
• Seal the interior: Now apply sealant to the interior gap between the window frame and the wall (the reveal). You can use silicone here as well (clear or white), or use a paintable decorator's caulk if you plan to decorate over it. Fill all around where the frame meets plaster or wall. This inner seal stops drafts and gives a tidy finish. Smooth it out for a clean look.
• Fit trim (if needed): In many cases, the new window frame will cover the old footprint and you can simply caulk the edges. But if there are rough edges or slightly larger gaps, you can fit uPVC trim around the interior edges. These are flat plastic strips (often 20–50mm wide) that cover the gap between frame and wall. Cut them to length for each side and top/bottom as needed, and adhere with a thin line of silicone or a few dots of adhesive. Press them in place for a neat, clean border around your window. On the outside, if there's a gap or damaged render, you might use external-grade trim or mortar repair, but ideally the silicone seal on the outside is sufficient if the gap is small.
• Cover screw heads: If you used through-frame fixings, there may be little plastic caps provided to cover the screw heads inside the frame grooves. Pop those on now for a finished appearance and to protect the screw heads from moisture.
• Cleanup excess: Wipe away any extra silicone while it's still wet (use a damp cloth, but be careful not to smear it on the frame or wall). Once cured, silicone is hard to remove. Also check you didn't accidentally leave any foam on visible areas – it can be gently scraped off uPVC if needed (special foam solvent cleaner helps for any residue).

Your window is now installed, insulated, and sealed. It should be both air-tight and water-tight around the edges. The final step is to tidy up and do some quality checks.

7. Final touches and quality checks

With the window fully in place, take time to finalize the job:

• Hardware and fittings: Reattach or tighten any loose handles, locks, or hinge covers. Many new windows come with the handles not yet attached – follow the instructions to fit them (usually a couple of screws). If your window has trickle vent covers to install, screw those on now at the top of the window frame (if not pre-fitted).
• Test all functions: Open and close each opener several times. Lock and unlock it. It should operate smoothly and latch securely. If you notice any catching or misalignment, adjust it now. uPVC windows often have adjustable hinges and striker plates – a slight turn of a screw can raise/lower the sash or tighten the lock engagement. Also ensure keys (if any) operate the locks correctly.
• Clean up: Remove any protective film from the window frames (those thin plastic sheets that protect during transport). Don't leave them on too long in the sun or they can stick. Clean the glass inside and out for a clear view – use a glass cleaner or a vinegar-water solution and a lint-free cloth. Wipe the frames with warm soapy water to remove dust, fingerprints, and sealant smudges. A clean window will also let you inspect the sealant lines clearly.
• Dispose of old materials: Carefully dispose of the old window frame pieces and broken glass. Remember, old glass cannot be put in regular recycling bins – take it to a recycling centre if possible, or dispose of as advised by your council (some allow small amounts in general waste if double-bagged, but check local rules).
• Inspect the seals: Do a final check of all the sealing. On a dry day, you might not notice if there's a tiny gap, but on a windy or rainy day it will matter. One trick is to have someone shine a flashlight from outside around the frame at night – inside, check if any light peeks through the caulk line. Also gently feel for drafts on a breezy day. Seal any spots that need a touch-up.
• Multiple windows: If you have more windows to do, repeat the process one by one. It's generally best to not remove a second window until the first is fully installed and sealed (unless you have a team tackling multiple at once). This keeps your house protected from weather as much as possible.

Once all your windows are replaced, step back and admire the transformation. Not only do the new windows look great, but you've also significantly improved your home's comfort and energy efficiency – and you did it yourself! Just don't forget to arrange that building control inspection if required. The inspector will want to verify things like the presence of safety glass markings, the trickle vents, and may test that the windows open as required for fire escape. If you've followed this guide and installed high-quality windows, you should pass with flying colours and receive your compliance certificate.

Take a well-deserved moment to congratulate yourself – DIY window installation is a significant project, and you accomplished it! Next, we'll cover a few common mistakes to avoid (for future reference or for those reading ahead), and how to maintain your new windows so they last for decades.