A window subframe is a separate frame fixed into the wall opening before the window itself goes in; the window is then installed into the subframe rather than straight into the masonry. You need one when the window sits forward in the insulation layer, when the walls go up long before the windows arrive, or when the wall is too uneven or soft for fixings to hold. For an ordinary replacement in a solid brick wall, you usually don't need one.
What is a window subframe and when do you need it?
A subframe is made of timber, plastic or an insulating material, and your builder sets it in the opening in advance. It makes sense when:
- the window stands in the external insulation, in front of the masonry, and needs support plus a flat surface for sealing tapes;
- the walls are built well before the windows and the opening should have an exact, square size from day one (common practice in the Netherlands, where the subframe goes in while the brickwork is laid and the windows come last);
- the wall is uneven or crumbly and fixings won't hold reliably.
We don't sell subframes; your installer orders and fits them. For your window order this has one key consequence: you take the window size from the clear opening of the subframe, not from the bare wall.
Why the joint around the frame matters
A subframe only does its job with a properly sealed joint. In a warm installation, the gap between frame and wall is closed in three layers: airtight on the inside, insulation in the middle, vapour-permeable on the outside. Moisture from the house can't get into the foam, and any that does can dry out towards the outside. With foam alone, the inside lets vapour in and the outside is attacked by rain and sun, and damp foam insulates poorly. A window with a Uw from 0.73 (Kömmerling 88 MD) loses much of its benefit if heat escapes around the frame. How much heat goes through the window itself is covered in our article on the Uw value.
Fitting a window in a subframe, step by step
This is a general outline. Details depend on the profile system and the wall, so always check the manufacturer's instructions for the profile and the fitting materials.
1. Measure the opening, after the subframe is in. Measure width and height in three places (top, middle, bottom) and note the smallest value. 2. Prepare the reveals: remove dust and loose plaster, fill any gaps. Tapes only stick to a clean, dry surface. 3. Apply the tapes to the frame before it goes in: airtight on the inside, vapour-permeable on the outside. It's easier to lay them straight on a frame lying flat. 4. Set the window on packers, then level and plumb it. 5. Fix the window mechanically with frame anchors or frame fixings, roughly every 70 cm and at least 2 per side. Foam insulates; it doesn't hold the window. 6. Fill the gap with foam. As a rough guide, one can covers about 6 m of frame perimeter. Don't overfill: expanding foam can bow the frame. 7. Bond the tapes to the wall: fully airtight all round on the inside, and on the outside so that water runs onto the sill. 8. Fit the external sill and check that the drainage slots in the bottom of the frame are clear. Remove the protective film within 4 weeks of delivery.
Mistakes we see most often
We've been working with windows since 1997 and have more than 5,000 orders behind us. Corrections usually come down to the same few errors:
- The opening size given as the frame size. A window made to fit the opening exactly leaves no room for insulation and tapes. With every quote we check whether a size refers to the opening or the frame.
- Measuring before the subframe goes in. The subframe makes the opening smaller, so a window ordered to the masonry won't fit.
- Opening side described from outside. Always describe it looking from inside the room; that's how we read the order.
- A late decision on a top-mounted roller shutter. The box adds to the window's height, so the shutter has to be in the order before the final measurement and before the subframe is set.
Who does the fitting when you buy windows online?
We don't offer installation, measuring or site visits. You or your own fitter install the windows, and you pay for the fitting separately to whoever you choose. You get made-to-measure uPVC or aluminium windows with a declaration of performance and CE marking. Before production we send a drawing and specification for approval: the last chance to check the sizes against the subframe. The 2-year GALI warranty doesn't cover damage caused by incorrect installation, so careful fitting protects your warranty too. How an order runs from measurements to delivery is explained on how it works.
Once you have the sizes, build your windows in the window configurator. You see the price straight away, and the "Don't forget" step suggests fitting materials: foam, tapes, anchors and packers.
Frequently asked questions
How to install a window frame?
Fix the frame mechanically with anchors or frame fixings, then insulate the gap and seal it airtight inside and vapour-permeable outside. Exact spacing and materials depend on the profile system and the wall, so follow the manufacturer's instructions.
How to fit a window?
Measure the opening, prepare the reveals, set the window on packers, level it, fix it, foam the gap and finish with tapes and an external sill. Whether you need a subframe depends on where the window sits in the wall.