INSULATION / MY WARM HOME
Internal wall insulation.
Improve comfort from the inside — with moisture, junctions and living space considered from the start.
01
Understand
How IWI improves comfort and changes wall behaviour.
02
Detail
Moisture, airtightness and thermal bridges.
03
Materials
Compare systems before choosing a product.
04
Check
Assessment, installation, ventilation and safety.
THE SHORT VERSION
IWI is a complete wall system — not just insulated plasterboard.
Internal wall insulation (IWI) adds a thermal layer to the room side of an external wall. It can help reduce heat loss and improve comfort where insulating the outside is not practical.
Unlike EWI, the original masonry or other wall structure can become colder after internal insulation is installed. Moisture management, the wall’s ability to dry and details at edges and penetrations therefore need deliberate design.
An IWI scheme also affects the inside of the room: radiators, sockets, skirting, reveals, coving, joinery and floor area may all change. The full scope of reinstatement should be agreed before work begins.
01 / UNDERSTAND THE SYSTEM
Four principles behind a good internal insulation design.
IWI can be suitable for solid-wall homes and other constructions following assessment. No single board, adhesive or vapour-control approach suits every wall.
01
Insulate the room side
Improve the thermal resistance of the existing external wall and raise the temperature of the new inner surface.
02
Assess moisture first
Investigate external water ingress, existing damp, exposure and whether the original wall can dry safely after insulation.
03
Detail every junction
Reveals, intermediate floors, ceiling edges, skirting zones and internal-wall junctions need a continuous strategy.
04
Plan the whole room
Consider heating, services, room dimensions, ventilation, occupant disruption and reinstatement before selecting materials.
A thicker board is not automatically a safer or more appropriate result. Thermal benefit must be balanced against building-fabric and moisture performance.
BEFORE WORK STARTS
Health and safety comes first.
The work takes place inside occupied rooms. Plan access, isolation of services, dust control, safe removal of fittings and reinstatement before the installers arrive.
Existing services
Electrical cables, sockets, radiator pipework, heating equipment and concealed services should be surveyed and made safe before strip-out.
Hazardous materials
Older coatings, boards, ducts or other components may contain asbestos or other hazardous substances. Do not disturb suspect materials without competent assessment.
Fire and structure
Retain the necessary fire protection, compartmentation and safe detailing of wall and floor junctions. Do not cut or alter structural timbers or supports without appropriate advice.
Flues and ventilation
Combustion air, chimney breasts, flues and existing ventilation must not be obstructed or compromised by the work.
02 / MOISTURE MATTERS
The original wall becomes colder behind the insulation.
Heat flow from the room into the original wall reduces once IWI is installed. That is the point of insulation but the colder wall may dry differently and can be more vulnerable to moisture accumulation if the proposed system is unsuitable.
Start with the outside of the wall
Repair defective gutters, pointing, render, flashings and other sources of water ingress. Examine rain exposure and allow existing moisture problems to be understood and addressed before covering the internal surface.
Assess vapour and liquid moisture
Some designs use a specified air and vapour control layer, others use carefully matched vapour-permeable and capillary active materials. Neither approach is universally suitable. Follow the designed system and consider hygrothermal assessment where appropriate.
Stop warm air bypassing the insulation
Gaps behind boards, poorly sealed perimeters and unsealed service penetrations can carry warm, moist indoor air to cold surfaces. Airtight detailing is essential, whatever system is selected.
Do not confuse two forms of condensation
Surface condensation appears on an exposed cold surface. Interstitial condensation forms within a wall build-up or at a concealed interface. A newly warm room-side surface does not prove that the hidden assembly is moisture-safe.
02 / THERMAL BRIDGING
The weak points are usually around the edges.
IWI is interrupted by floors, ceilings, partitions and openings. The junction designs should show where the insulation and airtightness layers go including details that are not visible once the room is redecorated.
01
Window and door reveals
Use an appropriate thin or specialist insulating return and deal with frames, sills, condensation risk and opening clearances.
02
Intermediate floors and joists
Joist ends and the wall/floor junction need special moisture and thermal-bridge assessment; avoid trapping vulnerable timber in a colder, wetter zone.
03
Internal-wall junctions
A return of insulation on adjoining surfaces may be required to mitigate cold strips; detail the transition and its moisture behaviour.
04
Ceilings and ground floors
Plan how wall insulation meets loft, floor and perimeter insulation without leaving a cold edge or an unsealed air path.
Do not simply push insulation behind a socket, skirting or floorboard and assume continuity. The specification must explain how each junction is completed.
THE IMPORTANT DIFFERENCE
Airtight does not always mean vapour-tight.
Moisture safety is a property of the complete wall assembly and the external exposure, not a marketing claim made for one insulation product.
Air leakage
Warm, moisture laden air can travel quickly through cracks and gaps. A continuous airtight layer limits that unwanted movement.
Vapour diffusion
Water vapour can also move through materials. Whether a vapour-control layer is appropriate depends on the wall and the complete system.
Liquid moisture
Rain and other water ingress need independent consideration. A vapour-open system cannot compensate for an unrepaired leak.
Drying potential
Traditional solid walls may need materials and finishes that maintain suitable pathways for moisture transport and drying.
VENTILATION AND HEATING
Do not improve the wall and forget the air.
Insulation work can reduce uncontrolled leakage and change room temperatures. Before and after IWI, the ventilation strategy should be checked against the property, occupancy and moisture sources.
Background and transfer air
Check designed background ventilators and internal transfer routes where required. Do not cover or seal working air inlets with the new lining.
Extract ventilation
Confirm appropriate extract provision and performance in kitchens, bathrooms, WCs and utility rooms. Check the complete house strategy, not only the room receiving IWI.
Heating and controls
Radiators may need temporary removal, brackets and pipework extended, and room controls revisited. Heat-loss calculations can help establish the post-retrofit heating requirement.
03 / CONSTRUCTION METHODS
Common ways to install internal wall insulation.
Construction methods must be selected against wall condition, moisture performance, fire requirements, available room space, fittings and the intended finish.
01
Direct-bonded board system
Boards fixed to a suitably prepared wall using a manufacturer-approved method. Prevent unwanted air paths behind the insulation.
02
Framed lining system
A timber or metal frame carries the insulation and finish. The design must address thermal bridging through framing and service penetrations.
03
Capillary-active bonded system
Selected vapour-permeable materials and compatible adhesives/plasters may help manage moisture in suitable wall constructions.
04
Insulating plaster
A specialist lime or other formulated insulating plaster can follow irregular surfaces and may suit selected traditional interiors.
Adhesives, mechanical fixings, preparation, boards, air/vapour control, joint tapes, plaster and finishes are not interchangeable across unrelated systems.
03 / MATERIAL OPTIONS
Which internal insulation material?
Compare the complete specified system — insulation alone does not tell you enough about hygrothermal suitability, fire protection, thickness or installation detail.
01
Mineral wool
Fibrous insulation used in appropriately designed framed systems. Insulation continuity, frame thermal bridges and air/vapour-control detailing need attention.
02
PIR and phenolic boards
High thermal resistance at relatively modest thickness. Assess the full lining system, vapour strategy, fire protection and existing wall compatibility.
03
Wood fibre
A vapour-permeable option for some older or traditional walls when adhesives, plasters, rain exposure and moisture design are compatible.
04
Calcium silicate / mineral foam
Mineral, often capillary-active options used in selected moisture-conscious systems. Manufacturer-compatible preparation and finishes are essential.
05
Aerogel-based products
Specialist high-performance materials can help at reveals or other locations with little available depth; check tested build-up and cost.
06
Cork and other natural fibres
Can form part of vapour-permeable assemblies in suitable buildings. Confirm product properties, moisture exposure, fire and complete-system suitability.
07
Insulating lime plaster
A possible alternative where wall irregularity, internal features or heritage constraints make conventional boards difficult; thermal performance is design-specific.
A vapour-permeable board is not, on its own, proof that an IWI installation is safe. Installation detail, external exposure and moisture assessment still govern the design.
CHOOSE THE SYSTEM
Select for the building, not just the U-value.
The objective is comfort and lower heat loss without harming the original fabric or creating a concealed moisture defect.
Existing wall and weather
Solid brick, stone, cavity and non-standard structures have different moisture behaviour. Consider orientation, wind-driven rain and maintenance history.
Available depth
Allow for loss of floor area, window reveals, door clearances, kitchen units, fitted furniture and access routes.
Fire and acoustic needs
Assess the entire lining build-up, fixings, protection and compartmentation where relevant.
Thermal and moisture modelling
Specify a justified target and evaluate thermal bridges and the wall assembly. More insulation is not always the same as less risk.
WHAT CHANGES IN THE ROOM?
Agree reinstatement before anyone removes the skirting.
Unlike EWI, internal insulation normally involves disruption to lived-in rooms. The project quotation should clearly cover removal, protection, alterations, repositioning and the final finish.
- Radiators, brackets and heating pipes
- Sockets, light switches and wiring
- Skirting, picture rails, coving and architraves
- Window boards, sills and reveal finishes
- Curtain tracks, blinds and shutters
- Kitchen units, cupboards and fitted furniture
- Stair and corridor clearances
- Floorboards, ceilings and party-wall junctions
- Wall-mounted televisions and shelving
- Service and meter access
- Fire and acoustic junctions
- Decoration, dust management and occupant access
The specification should also identify suitable fixing zones for heavy items. Avoid puncturing a completed air/vapour-control layer with unplanned later fixings.
04 / DESIGN BEFORE PRODUCT
Assess the building first. Then specify the lining.
A well-maintained modern wall and an exposed traditional solid wall may need very different solutions. The survey should explain why the selected approach is compatible with the property.
- Wall construction and history: establish age, materials, cavity or solid-wall build-up and previous insulation or coatings.
- External condition: gutters, roof junctions, pointing, render, flashings and exposure to driven rain.
- Existing moisture: distinguish water ingress, plumbing leaks, condensation and any other defects before proposing insulation.
- Timber and structure: investigate embedded joist ends, structural members and damp-sensitive details.
- Current services: electrical, heating, gas and ventilation locations, safe isolation and relocation requirements.
- Heat and air: heat-loss calculations, thermal bridges, existing background/extract ventilation and controls.
- Occupancy and access: room-by-room sequencing, vulnerable occupants, temporary accommodation and practical disruption.
- Heritage: assess the impact on original finishes and whether listed-building or other permissions are required.
- Design package: whole-wall moisture strategy, installation drawings, junctions, specified system and required checks.
04 / INSTALLATION & QUALITY
Eight checks for a better IWI installation.
01
Repair first
Fix moisture sources and confirm the existing wall is suitable for insulation before covering it.
02
Prepare the substrate
Remove unsuitable finishes and prepare the surface as the approved system requires.
03
Follow the moisture strategy
Keep the specified air/vapour control or capillary-active build-up continuous, with compatible layers.
04
Detail the junctions
Complete reveals, joist zones, internal returns, ceiling and floor edges to the drawings.
05
Protect the services
Relocate sockets, pipes, radiators and fixings without compromising insulation, fire or airtightness.
06
Keep ventilation working
Check background openings, wet-room extract, internal transfer paths and combustion requirements.
07
Inspect before covering
Photograph and check concealed stages such as junctions, membranes, seals and service penetrations.
08
Hand over the evidence
Provide system information, product details, completion records, maintenance guidance and any required compliance documents.
COMBUSTION SAFETY
Open-flued appliances need particular care.
Changes to walls and air leakage must be coordinated with existing combustion appliances, not treated as a decorating task.
Combustion air
Fabric improvements and ventilation changes can affect the air available to an open-flued gas, oil or solid-fuel appliance.
Pressure and extract
Extract fans may depressurise a room and adversely affect an open-flued appliance, even if they are not in the same room.
Competent assessment
Arrange any required appliance assessment and spillage checks with suitably competent qualified people. Relevant gas work requires an appropriately registered Gas Safe engineer.
Keep flues and vents safe
Do not conceal, shorten, obstruct or reposition flues, air vents or terminals as part of IWI without specialist design and installation.
TRADITIONAL AND HISTORIC BUILDINGS
Preserve the building as well as the heat.
Traditional solid walls may depend on compatible materials and the ability to transport and release moisture. Their external condition and internal historic features are part of the assessment.
Original details may be lost
Cornices, skirting, picture rails, architraves, fireplaces and room proportions can be altered or hidden. Document what will be retained, carefully removed or reinstated.
Permeability needs a complete-system approach
Lime-based finishes, wood-fibre systems and other specialist options may be considered where appropriate. They still need proper moisture assessment, substrate preparation and continuous installation.
Check permission before work
Listed buildings and protected interiors can require consent. Building-control and conservation considerations should be addressed at the design stage.
1
whole wall system — insulation, moisture, junctions and the room around it.
MY WARM HOME VIEW
Good IWI is designed around the existing building.
The finished plasterboard might look simple. Behind it, the specification needs to protect the original wall, handle moisture and give the homeowner a comfortable space with working ventilation and properly reinstated services.
Understand the wall. Design the moisture and junction details. Then choose the product.
STANDARDS & GUIDANCE
Use the documents relevant to the property.
Guidance differs across UK nations and depends on the timing and scope of work. Earlier standards may apply to some projects. Confirm the applicable requirements before installation.
IWI best practice
Energy and ventilation
Approved Document L (2026) → and Approved Document F (2026) → for projects subject to those standards.
Moisture and combustion
Traditional buildings
KEEP EXPLORING
Plan the hidden details before fitting the lining.
Insulation, ventilation, heating and moisture management belong in one whole-home plan.