Why Good IWI Is a Finishing Trade: Plaster, Skirtings and Sockets

IWI is judged by the finish, not the board

Nobody sees the insulation. Once the room is decorated, the only visible evidence of an internal wall insulation job is the finish: how flat the plaster is, whether the skirting sits tight to the wall, whether the window reveals are square, whether the sockets sit flush. The thermal work is hidden; the finishing work is on display for decades.

That has a practical consequence for how you should read quotes. Two installers can specify the identical board — PIR, wood fibre or insulated plasterboard, compared in our IWI boards guide — and quote wildly different prices, because one has priced a finished room and the other has priced boards on a wall. The gap is the finishing trades: plasterer, carpenter, electrician, decorator prep. On our quotes those trades are line items, not surprises.

Everything attached to the wall has to move

Insulating a wall internally makes it 60–100mm or more thicker on the room side. Everything that touches that wall is affected:

• Skirting boards and picture rails come off and go back on the new line.
• Sockets, switches and TV points move forward to the new surface.
• Radiators on external walls come off and are refitted on extended pipework.
• Window and door reveals get thinner insulation returns and new linings.
• Window boards (internal sills) are replaced wider to suit the deeper reveal.
• Curtain poles, blinds and shelving are refixed through the board into solid fixings.
• In period rooms, cornices and coving are either carefully stopped into or re-run.

None of this is optional if you want a room that works and looks right. It is precisely why a single room runs £1,800–£4,000 rather than the price of a few boards — and why our IWI service page talks as much about carpentry as insulation.

Plaster and skim: the surface you actually live with

Insulation boards are skimmed with a finish coat of plaster before decoration — that is the “plaster/skim” line that appears on 60 of our quotes. Done well, the skim hides every joint and fixing permanently. Done badly, or skipped in favour of taped-and-filled joints, the board grid telegraphs through the paint within a year or two, especially in raking light from a window.

An installer’s detail worth knowing: board joints get jointing tape bedded in before the skim, and the skim is applied in two passes, because a single heavy coat over rigid boards is prone to crazing as it dries. On breathable wood-fibre systems the finish is a lime or clay plaster instead of gypsum — the board and the plaster have to be specified together, which is one reason breathable build-ups cost more.

Skirtings, architraves and window boards

Skirting work appears on 51 of our quotes — more than nearly any other finishing item — because there is no honest way to insulate a wall without touching it. The insulation must run to the floor behind the skirting; stop it at skirting height and you have built a cold bridge along the entire base of the wall, exactly where cool air pools and condensation forms first.

In a Victorian or Edwardian room the original skirtings are often 200mm plus, moulded, and worth keeping. We remove them carefully, insulate to the slab or floorboards, and refit the originals; where they break, we match the profile. The same applies to architraves where a door sits in an external wall, and to window boards, which must be replaced wider — the reveal is now deeper than the original board. If your home has this kind of joinery, our Victorian terrace guide covers the wider picture.

Sockets, switches and radiators

Socket and electrical adjustments appear on 37 of our quotes. Every accessory on an insulated wall has to come forward: the back-box is re-fixed at the new depth, the cables are checked for adequate length and the faceplate ends up flush with the skimmed surface. Cables buried behind insulation also run warmer than cables in free air, which is an electrical consideration a boarding-only outfit will never mention.

Radiators on external walls come off before boarding and go back on extended tails afterwards. It is also the natural moment to ask whether the radiator should go back on that wall at all — once the wall is insulated, the old logic of putting heat under the coldest surface matters less, and an internal wall position can free up furniture space.

What making good looks like, room by room

• Living rooms: the highest finish standard in the house — cornice junctions, alcove returns beside chimney breasts, TV and socket positions re-planned on the new wall.
• Bedrooms: built-in wardrobes on external walls are the decision point — remove and refit, or insulate around. Behind-wardrobe walls are where mould shows first, so leaving them untreated defeats the point.
• Kitchens and bathrooms: the highest-moisture rooms. Units and tiling on external walls complicate boarding, and extract ventilation must be working before the room is sealed up tighter — see ventilation after insulating.
• Halls and stairs: long uninterrupted walls, so flatness matters; string lines and a straight edge, not eye.

We sequence occupied houses one room at a time — the practical rhythm is covered in insulating while you live there. A typical room is out of action for days, not weeks.

Why battened boards without detailing look cheap — and perform worse

The shortcut version of IWI is battens on the wall, boards on the battens, joints filled, done. It looks acceptable for about a season. Then the problems arrive in order: joints ghosting through the paint, skirtings standing proud of the new wall face, sockets sunk in little recesses, cold draughts at the floor edge. Cosmetically it reads as exactly what it is — boards stuck over a room.

The performance problem is worse than the cosmetic one. An unsealed batten cavity behind the boards lets warm room air circulate against the cold original wall, where its moisture condenses — hidden interstitial condensation on the old wall face. The insulation you paid for becomes a cavity that manufactures damp. Our guide to condensation, damp and mould after insulation explains the mechanism; the fix is continuous insulation, sealed edges and proper vapour control, not thicker boards.

The honest caveat: not every wall needs the full treatment. A garage-facing wall or a utility room can justify a simpler spec with a basic finish, and we will say so on the quote. But in living space, boarding without finishing is not a budget version of IWI — it is a different, worse product at a similar-looking price. If you want the finished-room version priced properly, get a free, fixed, itemised quote — plaster, skirtings, sockets and all.

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