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What are the methods of insulating a single skin wall to meet building regs?
Updated 07/26
By: Imogen Beech
Reviewed: Content Team
Single skin (or "solid") walls are just one brick thick, so cavity wall insulation is off the table straight away. But if you're renovating one, building regs may require you to insulate it anyway, and even where they don't, it's usually worth doing for the energy savings alone.
Here's what the regulations actually require, the methods available, and how to choose between them.
When building regs require you to insulate a single skin wall
The internal and external methods available
Which option suits your budget and disruption tolerance
What can go wrong if it's done badly
Roughly what it costs
How to find a qualified installer
Find a local tradesperson
When do building regs require you to insulate a single skin wall?
Under Approved Document L (Conservation of Fuel and Power), renovating a "thermal element" such as a wall, roof, or floor counts as building work, and it triggers a requirement to upgrade the insulation once the work passes a certain size.
The trigger is:
More than 50% of the surface area of that individual wall is being renovated, or
More than 25% of the total building envelope is being renovated at once
If either threshold is crossed, the whole wall (not just the renovated section) needs to be brought up to a minimum U-value of 0.30 W/m²K or better. The lower the U-value, the better the insulation performance.
This typically applies when you:
Add cladding or render to the outside of the wall (unless it's purely decorative)
Dry-line the inside of the wall
Strip a wall back to brick and rebuild it
The 50% rule applies per wall, not to your whole house. Insulating one wall while leaving others untouched can still be a sensible, regs-compliant way to phase the work if budget's tight.
You'll also need to notify building control before starting, unless the work is carried out by an installer registered with a competent person scheme, in which case they can self-certify it for you, similar to how Part P works for electrical work.
Methods of insulating a single skin wall
There are two main routes: insulate from the inside, or insulate from the outside. Both can meet the U-value requirement, but they suit different situations.
1. Internal wall insulation
Internal wall insulation goes on the indoor face of the wall. It's typically done one of three ways:
Insulated plasterboard, fixed directly to the wall. Insulation boards are attached straight onto the wall using adhesive. It's the least disruptive option but adds the least depth, so check it can hit the required U-value on its own.
Wall battening. A timber frame is built against the solid wall, insulation is fitted between the battens, and plasterboard goes over the top.
A new stud wall. A full stud wall is built a short distance in from the solid wall and filled with insulation such as mineral wool.
Single skin walls are far more prone to condensation and damp than cavity walls, especially in older buildings. A vapour control layer and a small air gap behind the insulation are usually essential, not optional. Don't let anyone skip this step to save time.
2. External wall insulation
External wall insulation goes on the outdoor face instead. Insulation boards, typically PIR, are mechanically fixed and adhered to the outside of the wall, then finished with a protective layer of render or cladding.
Because it changes the outside of the property, this route often needs planning permission, particularly on the street-facing side of a house or in a conservation area.
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Internal vs external: which should you choose?
Internal wall insulation | External wall insulation | |
Cost | Usually cheaper | Usually more expensive |
Appearance | Preserves the exterior look | Changes the exterior finish |
Room size | Reduces floor area slightly (allow roughly 75-100mm depending on method) | No impact on room size |
Disruption | Skirting, door frames and fittings need removing and refitting | Can often be done with minimal internal disruption |
Weatherproofing | No change | Improves weatherproofing and protects existing brickwork |
Condensation risk | Needs careful detailing (vapour control layer, air gap) | Can reduce condensation on internal walls |
Planning permission | Rarely needed | May be required |
Single skin walls aren't the only part of the house worth checking. See how this compares to cavity wall insulation or loft insulation if you're planning a wider energy efficiency upgrade.
How much does it cost?
There isn't a dedicated cost guide specifically for single skin wall insulation, since pricing depends heavily on which method and finish you choose. As a starting point:
Our external wall insulation cost guide breaks down pricing for the external route, including render and cladding finishes.
Our retrofit insulation cost guide gives a broader sense of typical insulation project costs, including internal options.
Get quotes based on the specific method you're considering, since board thickness, finish, and the amount of prep work (like fixing damp before insulating) can all move the price significantly.
On Checkatrade, you'll only find trades who meet our high standards and pass up to 12 checks.
Find a professional to insulate your single skin walls
A qualified installer will not only fit the insulation to a professional standard, they'll also make sure it meets the U-value building regs require, and can arrange notification through a competent person scheme where applicable.
Adding insulation to a solid brick wall underneath a bedroom bay window
If you're considering adding insulation to a solid brick wall underneath a bedroom bay window, this is worth doing properly rather than as an afterthought. Most pre-1920s properties have solid brick walls with no cavity, and the wall section under a bay window is often the coldest, dampest part of the room, since bay windows expose more external wall surface to the elements on three sides instead of one.
Before you start
Frequently asked questions about single skin wall insulation
Only if the work crosses the 50%-of-wall or 25%-of-building-envelope threshold, or involves other regulated work like re-cladding. Smaller patch repairs may not trigger it, but check with your installer or local building control if you're unsure.
You can, but the U-value must still meet building regs, and if the trigger applies, the work needs notifying to building control unless a registered competent person carries it out. Get this confirmed before starting rather than after.
Neither is automatically better, but older solid-wall buildings were often designed to be breathable, so getting the vapour control detailing right matters more than which side you insulate from. A professional survey before work starts is worth the cost.
It depends on the method, but budget for roughly 75-100mm depending on whether you're using insulated plasterboard, battening, or a full stud wall.
Often yes, especially on street-facing elevations or in conservation areas, since it changes the appearance of the property. Always check with your local planning authority before booking work.
Yes, they're more prone to this than cavity walls because there's no gap to break the cold bridge. This is exactly why vapour control layers and air gaps matter so much with internal insulation.
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