SWIP IWI is a BBA-certified internal wall insulation system that uses thermally engineered composite studs to reduce thermal bridging. Stay Warm Insulation installs the system as a manufacturer-approved installer. This guide explains how SWIP IWI works, its typical thermal performance, what it may cost and when it may be suitable.
What is SWIP insulation?
SWIP IWI is a complete internal wall insulation (IWI) system manufactured by SWIP. It is primarily designed to improve existing solid masonry walls and uninsulated cavity walls where cavity wall insulation or external wall insulation may not be appropriate.
Rather than using a single insulation board, the system combines specified components that must be installed together in accordance with the current SWIP specification. These include:
- SWIP EcoStuds: thermally engineered composite studs manufactured from rigid insulation bonded to an OSB facing. Their insulated core eliminates the repeating thermal bridging associated with conventional timber or metal studs.
- SWIP EcoBatt insulation slabs: water-repellent glass mineral wool slabs fitted between the EcoStuds. The EcoBatt has a Euroclass A1 reaction-to-fire classification.
- Vapour control layer: installed on the warm side of the insulation and sealed at laps, edges, junctions and penetrations to help control water vapour and prevent uncontrolled air movement through the construction.
- Plasterboard lining: mechanically fixed to the EcoStuds, sealed around the perimeter and finished ready for decoration.
- Insulated reveal boards: used around window and door openings where required to maintain insulation continuity and reduce cold spots.
The SWIP IWI system is covered by BBA Agrément Certificate 18/5506 when the specified system components are installed in accordance with the certificate and SWIP's current installation requirements.
A SWIP 25-year product guarantee may also be available, subject to the installation being completed by an approved installer and meeting the applicable registration requirements, terms and conditions. This should not be confused with the installer's workmanship guarantee or any separate insurance-backed guarantee. We install it with our own PAS 2030:2023 certified, TrustMark-registered team.
How the system works
SWIP EcoStuds are normally installed vertically at approximately 600 mm centres, together with the required perimeter studs, rails and additional supports.
SWIP EcoBatt slabs are cut and friction-fitted between the studs so that the insulation zone is completely filled without gaps. A suitable vapour control layer is then fitted over the insulated construction and carefully sealed at all laps, edges, junctions and service penetrations.
Plasterboard is mechanically fixed to the OSB face of the EcoStuds. A small gap is left around the perimeter of the plasterboard so that it can be sealed using the specified sealant. This helps prevent uncontrolled air movement behind or through the finished IWI system.
Where the existing wall is uneven, suitable moisture-resistant packers may be required behind the EcoStuds. The important requirement is that the completed construction is properly supported, insulated and sealed against air leakage.
Window and door reveals, intermediate floors, ceilings, party-wall junctions and other adjoining elements must also be considered. These areas can create thermal bridges if the insulation is stopped without suitable junction detailing.
Thermal performance
The thermal performance achieved by an IWI system depends on several factors, including the construction and condition of the existing wall, the selected EcoStud and EcoBatt thickness, stud and rail spacing, the plasterboard or insulated lining used, window, floor, ceiling and party-wall junctions, installation quality and airtightness, and the continuity of insulation around the complete heat-loss area.
The following figures are typical design values shown in SWIP technical literature for example wall constructions:
| SWIP system | Indicative U-value |
|---|---|
| 65 mm SWIP EcoStud and EcoBatt | Approximately 0.43 W/m²K |
| 95 mm SWIP EcoStud and EcoBatt | Approximately 0.30 W/m²K |
| Two layers of 65 mm EcoStud and EcoBatt | Approximately 0.24 W/m²K |
These values are indicative and should not be treated as guaranteed results for every property. A project-specific U-value calculation should be completed using the actual existing wall and proposed system build-up.
For comparison, an uninsulated 225 mm (9-inch) solid brick wall may have a U-value of approximately 2.0 to 2.1 W/m²K. Improving the wall to approximately 0.30 W/m²K would substantially reduce heat loss through that element.
SWIP technical literature also states that its thermally engineered studs can make the system almost 13% more thermally efficient than a comparable timber-stud construction of the same thickness. The main benefit is reduced thermal bridging through the stud lines, allowing strong thermal performance without unnecessarily increasing the overall system depth.
Moisture, condensation and airtightness
Moisture management is one of the most important parts of any internal wall insulation project. Installing IWI changes the temperature and moisture behaviour of the existing wall. If the system is incorrectly designed or poorly sealed, there may be an increased risk of moisture accumulating within the construction.
The SWIP system addresses this by using a vapour control layer on the warm side of the insulation. The membrane must be continuous and properly sealed around membrane laps, floors and ceilings, window and door openings, service penetrations, electrical boxes, timber joists and other structural elements, and junctions with adjoining walls and materials.
Depending on the property and proposed construction, the specification may use a conventional vapour control layer or a vapour-variable membrane such as Intello. Intello changes its resistance to water vapour according to the surrounding conditions, helping protect the construction during colder periods while providing greater inward drying potential when conditions allow. However, membrane selection must not be based on exposure alone.
The appropriate membrane and wall build-up should be determined by considering the existing wall construction and materials, internal and external wall finishes, wall orientation and exposure to wind-driven rain, existing moisture or salt contamination, indoor humidity and ventilation, the condition of gutters, roofs, pointing and external finishes, and the drying potential of the completed construction.
The system should be designed and installed in accordance with the current SWIP specification and the principles of BS 5250. Where required, a project-specific moisture-risk or hygrothermal assessment should be completed.
No IWI system should be used to conceal an unresolved damp problem. Leaking gutters, defective pointing, damaged render, plumbing leaks, rising damp and other sources of moisture must be properly investigated and rectified before insulation work begins. Ventilation must also be assessed. Making a home more airtight can reduce uncontrolled draughts, but adequate background and extract ventilation must still be provided.
For more information, see our guide to condensation, damp and mould after insulation.
Is SWIP suitable for period and solid-wall homes?
SWIP IWI can be suitable for many properties with solid masonry walls, including Victorian, Edwardian and other traditionally constructed homes. It may also be used on certain uninsulated cavity walls where the existing construction and condition have been properly assessed.
Because the insulation is installed internally, the external appearance of the property can usually be retained. This can make IWI an option for properties in conservation areas or where the existing brick or stone frontage needs to remain visible.
However, listed buildings and properties with protected features may still require listed building consent, planning advice or input from a conservation specialist. Traditional stone and lime-built walls may also require a more detailed moisture assessment before the system and membrane are selected.
Work may sometimes be completed in phases, but each phase must be properly designed. Particular attention is required where insulated areas meet untreated walls, floors, ceilings, reveals and separating walls. To identify the type of wall in your property, see cavity wall vs solid wall. If you're weighing internal against external insulation, our EWI vs IWI comparison sets the two side by side.
For converted lofts, SWIP also provides a separate room-in-roof insulation specification covering sloping ceilings, stud walls, residual loft areas and associated junctions. See our guide to SWIP Room in Roof insulation.
Fire safety and building height
Building height and fire requirements must be assessed for the whole building. The 11 m reference does not mean that the system can automatically be installed on the lower floors or lower 11 m of a taller building. For buildings exceeding the permitted height, or buildings connected to taller buildings, suitability must be established through the project fire strategy, applicable Building Regulations and written confirmation from SWIP.
Combustion appliances and flues
There is no universal 200 mm clearance that applies to every stove, boiler, flue or chimney arrangement. Required separation distances depend on the appliance, flue construction, temperature designation and the manufacturer's installation instructions. Work must also comply with Approved Document J and be assessed by an appropriately competent person.
Combustible insulation must not be exposed to temperatures beyond its permitted service conditions. Any necessary non-combustible protection, clearance zone or alternative detail must be designed for the specific appliance and flue arrangement.
Radiators and wall-mounted fittings
Radiators, cupboards, televisions and other heavy fittings must not be supported by plasterboard alone. Suitable supports should be incorporated into the system or fixings designed to transfer the load into the EcoStuds, supporting masonry or other structural background. The required support arrangement should be selected according to the weight, fixing position and anticipated load.
Lightweight pictures and similar fittings may use appropriate plasterboard fixings, subject to the fixing manufacturer's instructions.
Does SWIP help reduce noise?
The glass mineral wool insulation can absorb sound energy, while the plasterboard lining adds mass to the existing wall. SWIP technical literature states that the airborne sound insulation of a suitable solid masonry wall may improve by up to approximately 5 dB when the system is correctly installed and made airtight.
This is an additional potential benefit rather than a guaranteed acoustic performance. Results will depend on the existing wall, windows, floors, ceilings, ventilation openings, junction details and workmanship.
What SWIP insulation costs
The cost of internal wall insulation depends on more than the square metre area of insulation. Preparation, finishing and junction details can account for a significant proportion of the total price. As an initial guide, Stay Warm Insulation estimates the following installed costs:
| Scope | Indicative cost |
|---|---|
| Per square metre | £100–£140 / m² |
| A single room | £1,800–£4,000 |
| A full 3-bed house | £8,000–£15,000 |
These figures are indicative only and are not a quotation. The final price may be affected by the number and area of external walls, the required insulation thickness, wall condition and preparation work, window and door reveals, radiator and service alterations, electrical work, skirting boards and joinery, ventilation upgrades, plastering and decorative finishes, access and waste removal, and whether VAT is included.
Stay Warm Insulation provides a written quotation following a free survey, with no obligation. The quotation should clearly identify the proposed system, thickness, included preparation work, finishing requirements and any exclusions.
Ready to find out whether SWIP is suitable for your home? Get a free quote and we'll arrange a free survey at a time that suits you.
SWIP insulation FAQs
What is SWIP insulation?
SWIP IWI is a BBA-certified internal wall insulation system used to improve suitable solid masonry and uninsulated cavity walls from inside the property. The system combines thermally engineered EcoStuds, water-repellent glass mineral wool EcoBatts, a specified vapour control layer, system tapes and sealants, insulated reveal boards where required and a plasterboard finish. Stay Warm Insulation installs the system as a manufacturer-approved installer.
How much does SWIP insulation cost?
Stay Warm Insulation estimates an indicative installed cost of approximately £100 to £140 per m². A single room may cost approximately £1,800 to £4,000, while a complete three-bedroom house may cost approximately £8,000 to £15,000. Actual costs depend on the property, system thickness, wall condition, junction details, service alterations and required finish. A survey and written quotation are needed before an accurate price can be provided.
Is SWIP suitable for period homes?
SWIP may be suitable for Victorian, Edwardian and other traditionally constructed homes with solid masonry walls. Because the system is installed internally, it can retain the building’s external appearance. The existing wall must first be assessed for damp, exposure, condition and moisture behaviour. Listed buildings and protected properties may also require consent or specialist conservation advice.
How is SWIP different from standard internal wall insulation?
The main difference is the thermally engineered EcoStud. Conventional timber and metal studs create repeating thermal bridges through the insulation layer. SWIP EcoStuds use an insulated core bonded to an OSB facing, significantly reducing heat loss through the stud lines. SWIP technical literature states that the system can be almost 13% more thermally efficient than a comparable timber-stud construction of the same thickness. Actual performance must still be confirmed using a project-specific U-value calculation.
Does SWIP prevent condensation?
No insulation system can guarantee that condensation will never occur in every building. When SWIP IWI is correctly assessed, designed and installed with the appropriate vapour control layer, airtightness detailing and ventilation, it can help limit the risk of surface and interstitial condensation. Existing damp defects must be repaired before installation, and a project-specific moisture assessment may be required.
Does SWIP help reduce noise?
It may provide an additional acoustic benefit. The mineral wool absorbs sound energy and the plasterboard lining adds mass to the wall. SWIP literature refers to potential improvements of up to approximately 5 dB in suitable constructions, but the actual result will depend on the complete building and installation.
Does SWIP come with a guarantee?
A 25-year SWIP product guarantee may be available where the system has been installed by an approved installer and all registration requirements and guarantee conditions have been met. The product guarantee, installer workmanship guarantee and any insurance-backed guarantee are separate forms of protection. Customers should be given written details explaining exactly what each guarantee covers.