Roughly a third of the houses in Britain have solid walls. No cavity, nowhere obvious to put insulation, and a heating bill that reflects it. If yours is one of them, insulating those walls is probably the single biggest improvement you can make to how the place feels and what it costs to run.
It is also the job that goes wrong most often. And when internal wall insulation fails, it fails quietly, behind the boards where nobody can see it, sometimes for years.
This is a guide to getting it right. What internal wall insulation actually is, the three main systems you will be choosing between, and how to work out which one suits your building.
How much it actually saves
Which? put the saving from insulating solid walls at between £150 and £550 a year on heating. Energy Saving Trust figures work out at around £310 a year for a typical semi and £450 for a detached house.
Treat those as the ceiling rather than the expectation. They assume the whole house treated by a professional installer, with a mains gas boiler and average occupancy. A DIYer doing one bedroom will not get near them.
The money is only part of it. A cold wall surface is where condensation forms, so warming that surface up removes the conditions mould needs. Done properly, internal wall insulation often fixes a damp problem rather than causing one. Your boiler also has less work to do, cycling less often and running at lower output for the same result, which tends to extend its working life.
Do you actually have solid walls?
Look at the brick bond. A solid wall shows header courses, which are the short ends of bricks appearing at regular intervals in the pattern. A cavity wall is stretcher bond, so you only see the long faces.
As a rule of thumb, houses built before the 1920s are usually solid walled, and by the mid 1930s cavity construction had become the norm. Between those dates it varies by region and by builder, so check the brickwork rather than trusting the date alone. If the wall is rendered and you cannot see the bond, measure the thickness at a window or door reveal instead.
The mistake almost everyone makes
People think insulation solves problems. Quite often it hides them.
Leaking gutters, cracked render, rising damp, blocked drainage, poor ventilation, existing mould. Board over any of those and you have not fixed anything. You have buried it, and the problem carries on out of sight in a wall that is now colder and slower to dry than it was before.
So the first job is not choosing a product. It is assessing the building. Are the gutters and downpipes clear and running properly? Is the render sound, with no blown or cracked sections? Is the pointing in good order? Is there a working damp proof course, and will the new insulation sit above it rather than bridging it? Is there any sign of damp, salting or mould on the internal face?
If any of those is a no, that is the job to do first.
Why moisture is the whole story
Warm air inside a house carries moisture, and warm moist air always moves towards colder, drier air. When it reaches a cold surface, it condenses.
Before you insulate, that cold surface is the inside face of the wall, where you can see it and wipe it. Add insulation and you move the cold surface behind the boards, where you cannot. Condensation forming inside the build-up is called interstitial condensation, and it is the single biggest risk with this kind of work.
This is why detailing matters more than product choice. A modest system installed carefully will outperform an expensive one installed badly, because every system on the market fails if warm moist air is allowed to reach a cold surface behind it.
The three systems
Every internal wall insulation system answers the moisture question in one of two ways. Either it stops moisture getting into the wall at all, or it lets moisture move through and dry out again. Both approaches work. They work in different buildings.
1. Vapour closed, rigid board
Typically a phenolic or PIR foam core with a foil facing, bonded to plasterboard. The foil acts as a vapour control layer, so the system works by keeping moisture out of the wall entirely.
This gives you the best thermal performance per millimetre, which means the thinnest build-up for a given result. That matters in small rooms where every millimetre you add is floor space you lose.
The trade-off is that it is the least forgiving of poor workmanship. Every joint, socket, reveal and perimeter has to be sealed, because one gap undermines the whole system.
2. Vapour open, fire rated
Usually high density mineral wool between timber studs, with a vapour control layer, plasterboard and sealant. Mineral wool is non-combustible, which is where the fire rating comes from.
Rather than sealing the wall, this works with it. Moisture can move and the wall can dry, which suits older solid walls that were built to breathe. It is thicker than option one for the same performance, but considerably more forgiving.
3. Vapour open, natural
Hemp or wood fibre. Breathable, moisture buffering, and with far lower embodied carbon than the foam based systems.
This is the best match for traditional and heritage buildings, and the obvious choice if the environmental impact of the material matters to you. Natural does not mean foolproof, though. Fitted badly onto a damp wall it fails exactly like anything else.
Matching the system to the building
There is no best system. There is only the system that suits your wall, your room and your budget.
Older solid brick, exposed to driving rain, generally wants a vapour open build-up so the wall can dry. A rendered or clad wall in a sheltered spot, with a proper ventilation strategy, makes vapour closed viable and gives you the thinnest build-up. Traditional, listed or conservation area properties point towards natural materials. Flats and anything with height or boundary restrictions need the fire rated option, and the limits checked carefully.
If there is any sign of damp at all, the answer is none of them until that is dealt with.
How much space you will lose
People underestimate this. What comes off the room is not just the insulation thickness. It is the insulation, plus the plasterboard, plus whatever holds it on the wall.
A rigid board with 50mm of insulation bonded to 12.5mm plasterboard is 62.5mm before it goes near the wall. Bed it on adhesive and you are at roughly 77mm off the room. Put 80mm on timber battens and you are close to 120mm, which is the best part of five inches. Do that on two external walls of a small bedroom and you have lost close to a square metre of floor.
More is not automatically better either. On a typical solid brick wall, going from 50mm to 80mm improves the U-value, but not in proportion to the extra 30mm of room you give up. Work out what the thickness is worth in that particular room before defaulting to the thickest board.
Remember the things that move with it. Window and door reveals need insulating too, in a thinner board. Skirting, architrave and window boards all come off and go back. Radiators, sockets and switches all move forward. And check critical dimensions in bathrooms and kitchens before you start.
Ventilation is not optional
Insulating and sealing a room changes how the building handles moisture. An old draughty house ventilates itself, badly and expensively, through gaps. Take those gaps away and the moisture people generate by breathing, cooking, washing and drying clothes still has to go somewhere.
A tighter room with the same moisture load and nowhere for it to go is how you end up with mould you never had before. Extract fans, trickle vents, airbricks and subfloor ventilation all need looking at as part of the job, not as an afterthought.
Where it goes wrong
Almost every failure traces back to the same five things. Nobody assessed the building first. The vapour control layer was punctured and not resealed. The insulation was not continuous, leaving cold bridges at reveals, corners and junctions. Ventilation was never upgraded. Or the wrong system went onto the wrong wall.
What that looks like, eventually, is mould growing behind the boards where you cannot see it, joist ends rotting in a wall that is now cold and damp, bathrooms steaming up worse than before, a musty smell nobody can trace, and black spots creeping out around the window reveals.
By the time it shows on the surface, the damage behind it is done, and putting it right usually means taking the whole lot out.
None of which is an argument against doing it. It is an argument for assessing the building first, because that single step prevents most of what goes wrong.
Watch the fitting video
In the follow-up episode Roger fits two of these systems start to finish, covering the studwork, the vapour control layer, taping, reveals and the detailing that decides whether it works: Internal Wall Insulation: The Bit Everyone Gets Wrong
Download the free guide
We have put together a free 13 page guide with the experts at Ezy Fit, covering all of this in more detail. It includes two printable checklists, one for assessing whether your building is suitable and one for preparing a room before the first board goes up. Walk round the house with a pen and be honest about what you find.
📕 Download the guide:
go.skill-builder.uk/iwi-guide
Thanks to Ezy Fit Insulation Systems for supporting this video.
go.skill-builder.uk/ezy-fit
