Applying XPS (Styrodur) to Window Reveals Around Fitted Windows

XPS (extruded polystyrene), often called Styrodur on site after a well-known product, is a rigid board with closed cells. Compared with the white expanded polystyrene (EPS) used on walls, it absorbs far less water, takes more pressure and keeps its shape better. At the same thickness it usually insulates slightly better too. That matters on a reveal, because space is tight and the area around a window tends to be damper, for example from rain running off the sill or from condensation.

Boards of roughly 20 to 50 mm are usual for reveals. How much fits depends mainly on how much of the window frame is left visible. The insulation must not cover the hardware or stop the opening sash from moving freely. Boards with a roughened or grooved surface work best, because adhesive and basecoat grip poorly on a smooth board. If you only have smooth boards, roughen them before fixing.

First the background is cleaned. Loose render, dust and old paint have to come off, and absorbent surfaces such as aerated concrete blocks get a primer. The boards are cut accurately to size and the adhesive goes on the board, not the wall. On narrow reveal strips the adhesive is usually spread over the whole board with a notched trowel. Wider boards get a continuous bead around the edge and a few dabs in the middle. Boards are butted tightly together, and small gaps are filled with low-expansion PU foam made for polystyrene.

Once the adhesive has set, high spots are sanded down and the dust brushed off. Then comes the reinforcement layer: basecoat, glass-fibre mesh pressed into it and a second levelling coat. Corner beads with mesh go on the edges. Before the main mesh layer, diagonal mesh patches are placed at the corners of the window opening on the facade, which helps prevent cracks running out from the corners. Finally a primer and a finishing render are applied: outside, one that belongs to the same insulation system as the walls; inside, an ordinary interior plaster or skim.

Joining the insulation to the window frame

Between the insulation and the frame there is a joint, and this joint decides whether the detail stays tight for years. A window connection bead with a sealing strip and mesh is stuck to the frame. It copes with the frame moving as temperatures change. A bead of acrylic or silicone on its own will crack in this joint sooner or later. When the window was fitted, the gap around it should have been sealed with an airtight tape on the inside and a breathable tape on the outside. If these tapes are missing, insulating the reveal will not make up for them.

Joining the insulation to the facade

If the walls have an external wall insulation system (often called ETICS), the reveal insulation must run on from the wall insulation without any gap. Any gap creates a thermal bridge, a spot where heat escapes faster, exactly where you least want one. It is far simpler to get this right when windows, reveals and facade are planned as one job.

A ground-floor reveal with a flat, sound background, a window already in place and intact tapes on the frame is straightforward. These things add time and effort:

• Work at height that needs scaffolding or a lift
• Loose or damp old render that has to be hacked off
• Deeply set windows with a large reveal area
• Uneven, crooked or rounded reveals that need many small cuts
• Missing tapes around the frame or a damaged sill
• Very little visible frame, which means thin boards and precise cutting

In a block of flats the facade is usually shared property, so any external work has to be agreed in advance with the building management or the owners' association. If the reveals are part of wider external wall insulation or other building work, check with your local building authority what permission or notification your local building regulations require. Listed buildings and conservation areas often have extra rules for changes to the outside.

• Fixing boards to a dusty, damp or flaking background, which often shows as boards coming loose after the first winter
• Using an adhesive not suitable for XPS, or solvent-based foam that eats into the board
• Leaving out the mesh or just laying it on top instead of embedding it in the basecoat, so the surface cracks
• Filling the joint at the frame with sealant only, without a connection bead
• Working in frost, rain or strong direct sun, so the adhesive cannot cure properly
• Cutting short the drying time between layers
• Choosing a board so thin that the reveal only looks finished while the thermal bridge remains

Doing it yourself only makes sense for windows you can reach from the ground or from a low, stable platform. Leave work at height and the connection to a whole new facade to a contractor. Work within the temperature range the manufacturer gives, usually roughly +5 to +25 °C, and avoid rain and direct sun.

1. Check that the window frame has its sealing tapes and that the background is sound and dry.
2. Remove loose render, dust and grease, and prime absorbent surfaces.
3. Measure each surface separately and cut the boards, ideally with a hot-wire cutter.
4. Roughen any smooth board surface and apply an adhesive made for XPS.
5. Press the board into place, check it with a spirit level and adjust before the adhesive starts to set.
6. Fill gaps between boards with polystyrene-safe PU foam and trim it flush once cured.
7. Sand the surface once the adhesive has hardened and brush off all dust.
8. Fix the connection bead to the frame and the mesh corner beads to the edges.
9. Apply the basecoat, embed the mesh with overlaps and add the levelling coat once dry.
10. After the recommended drying time, prime and apply the finishing render.

• Boards do not sound hollow when tapped and nothing is coming away
• No mesh shows through and the surface has no cracks
• Edges are straight, protected by corner beads and free of chips
• The joint at the frame is neat, with a connection bead and no gaps
• The window opens freely and the insulation does not cover hardware or drainage slots
• The reveal insulation runs on smoothly from the wall insulation
• The sill is sealed tightly and water runs off it away from the wall