How to install a window frame from the inside, in short: fix it against the inner face of the wall so that it overlaps the edge of the opening, then seal and insulate the joint. In France this is called fitting "en applique" and it is the usual method where walls are insulated on the inside, because the frame ends up flush with the insulation layer. Below we compare it with the other two common methods, then go through the installation step by step and the mistakes that spoil it most often.
We do not offer fitting. We sell made-to-measure uPVC and aluminium windows, which you install yourself or with a fitter of your choice. This guide helps you order a window that suits the fitting method and check your installer's work.
Three ways to install a window frame
| Method | Where the frame sits | When it is used | Frame in the configurator | |---|---|---|---| | Against the inner wall (en applique) | inside, against the wall, overlapping the edge of the opening | new builds and renovations with internal insulation | standard | | In the reveal (en tunnel) | within the wall thickness, between the reveals | walls without internal insulation, often with external insulation | standard | | Over the old frame (en rénovation) | on the existing frame, which stays in the wall | replacement without breaking out plaster or reveals | renovation (depends on system) |
The method determines the window size. In the reveal, the frame is smaller than the opening by a fitting gap. Against the inner wall, the frame overlaps the masonry, so its size is worked out differently. Your fitter sets the overlap, because it depends on the wall and the insulation.
What to decide before ordering
On enquiries for France we check that the fitting method is stated. Without it there is no way to tell whether a size refers to the opening or the frame. Since 1997 the mistake we see most often is the opening size entered where the frame size should be. Before you order, agree with your fitter:
- the frame size, not just the opening, including the overlap onto the wall;
- the thickness of the internal insulation and how the joint with the frame will be finished;
- the opening side of each sash, always as seen from inside;
- roller shutters: a top-mounted box adds height to the window, so decide before entering the sizes.
Measure the opening in three places (top, middle, bottom) and take the smallest figure. If unsure, send us the raw sizes, photos of the opening and a description of the wall through the contact page, and we will check them when quoting.
How to install a window frame step by step
This is a general outline. Details depend on the system and the wall, so always follow the instructions from the window and fixings manufacturers.
1. Check the opening. The inner edge of the wall the frame will sit against must be flat, dry and sound. Remove dust and loose plaster. 2. Prepare the frame. With large, heavy windows it is easier to work with the sashes removed. Apply the tapes: vapour-tight on the room side, vapour-permeable on the outside. 3. Offer the frame up from inside. Set it on packers and wedges, level and plumb it, and check the diagonals. The frame must not be twisted. 4. Fix it mechanically. Use anchors or frame fixings roughly every 70 cm, at least 2 per side. Do not overtighten or you will distort the profile. 5. Test the sashes before foaming. Refit the sash and check it closes without force and presses evenly on the seal. Corrections are still easy at this point. 6. Fill the gap with foam. Foam insulates; it does not hold the window. As a guide, one can covers about 6 m of perimeter. 7. Close the tapes. Bond the vapour-tight tape to the wall inside and the permeable tape outside. This gives a three-layer seal: airtight inside, insulation in the middle, moisture let out to the exterior. 8. Fit the external sill. Do not cover the drainage slots at the bottom of the frame with the sill or with silicone. 9. Finish the joint inside. The internal insulation and its lining meet the frame. How depends on the wall and materials, so agree it with your builder.
After fitting, remove the protective film no later than 4 weeks after delivery. Adjust and service the hardware at least once a year; this is also a condition of the 2-year GALI warranty.
Common mistakes
- Size worked out as for a reveal fit. A frame ordered with a fitting gap will not cover the edge of the opening from inside.
- No vapour barrier on the inside. Moisture from the room gets into the foam, and damp foam insulates poorly.
- Foam instead of fixings. A window held only by foam shifts over time and stops closing properly.
- Blocked drainage. Water sits in the bottom of the frame instead of running outside.
- Shutter added after measuring. The top-mounted box raises the window, which then no longer fits the opening.
- Fitting that ignores the instructions. The warranty does not cover damage from incorrect installation, so follow the instructions and keep them with the window documents.
What you get from us
You can put together a window for this type of fitting in the configurator: choose the system, glazing and colour, and the price appears straight away. Choose between uPVC windows with 76–88 mm profiles and Aluprof aluminium. The configurator also offers external sills and fitting materials: foam, tapes, anchors and wedges. You get a quote including delivery within 3 working days, then a drawing of every window to approve before production. To compare the insulation of the whole window, read our article on the Uw value.