A frame infill panel is the board or panel inside a window or door frame — including the solid area of a spandrel between the floor and the bottom of the window. In other words, it is whatever fills the opening in the frame and does not have to be glass.
Types of infill
- Glass — by far the most common: double glazing with a low-e coating, or triple glazing. Strictly speaking this is an infill too.
- Solid panel — an insulated sandwich panel, usually two facing sheets with an insulating core (PIR, PUR or EPS). This is the standard infill under a window or beside a front door.
- Solid board — plywood, hardwood, HPL or a fibre cement sheet, with or without a separate layer of insulation behind it.
The material of the infill usually matches the frame material, both in how it moves and in appearance.
Where it goes wrong
Infill panels are a classic weak spot in the building envelope:
- Poor thermal performance. An old uninsulated panel of 6 mm plywood barely insulates at all. A modern insulated panel of 28 to 40 mm reaches a considerably better U-value than the glass next to it — it pays to replace both at the same time.
- Moisture in the core. Once water gets behind the facing, the insulating core saturates and the panel bows or delaminates. Visible bowing or a soft spot almost always means moisture.
- No room to move. Like glass, a panel must sit on setting blocks in the rebate and keep a gap all round. Fixed tight, it bows in the heat.
- Glazing rules ignored. A solid panel is set into the rebate exactly as glass is: blocked up, with ventilation and drainage at the bottom of the rebate.
When replacing one
Measure the rebate size and the thickness, and allow for the weight — an insulated 40 mm panel is considerably heavier than the sheet that was there. In a spandrel, check whether the panel has to act as a barrier against falling; if so, extra requirements apply to its strength and its fixings.
Related terms
- Window frame
- Window frame finishing
- Spandrel
- Glazing
- Rebate
- U-value
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