Internal Wall Insulation

IWI

What Is Internal Wall Insulation?

Internal Wall Insulation (IWI) is the process of insulating the inner surfaces of external walls from the inside of a property. It is commonly used on solid wall properties (often built before 1920) that have no cavity, or in situations where cavity wall insulation isn’t suitable. IWI greatly improves thermal performance, helping to reduce heating bills and increase comfort.

Under PAS 2035:2023, IWI must be installed by certified professionals and designed with whole-house considerations in mind, as part of the PAS 2035 retrofit framework.

INSTALL

What’s Involved in Internal Wall Insulation?

The process typically includes:

Installing insulated plasterboard systems (e.g. Kingspan K118, Celotex PL4000) directly onto the interior wall.
Or building a timber frame with insulated studs, mineral wool inserted between the studs, then finished with plasterboard and skim (e.g. SWIP systems).
Adding vapour control layers to prevent condensation and protect the building fabric.
Repositioning or adapting fixtures such as radiators, sockets, skirting boards, and window reveals.
Making good, where possible, all decorative finishes after installation.

The insulation must achieve a U-value of 0.30 W/m²K or lower to meet funding and Building Regulation standards.

COMPLIANCE

PAS 2035 Compliance

Internal Wall Insulation under PAS 2035 involves:

Installation by PAS 2035 certified and TrustMark registered contractors.
Oversight by a Retrofit Coordinator, who ensures the measure is appropriate for the building, based on a full PAS 2035 assessment.
Use of approved insulation systems that meet fire and thermal performance standards.
Design strategies to avoid issues like thermal bridging, moisture build-up, and reduced indoor air quality.
Compliance with Building Regulations (especially Part L and Part B for fire safety).

PROS

Benefits of Internal Wall Insulation

Suitable for Solid Wall Properties

One of the best solutions for homes without a cavity— common in older brick or stone-built houses.

Energy Efficiency & Cost Savings

Can reduce heat loss by up to 45%, significantly cutting heating bills.

Enhanced Comfort

Warmer rooms, more stable indoor temperatures, and fewer cold spots.

Funding Eligible

Qualifies for support under ECO4, GBIS, and LAD schemes— often fully funded for eligible households.

No External Alteration

Ideal for homes in conservation areas, listed buildings, or those where external insulation is not permitted.

CONS

Considerations & Potential Drawbacks

Reduced Room Space

Typically reduces internal room dimensions by 50–100mm depending on the system.

Installation Disruption

Rooms need to be cleared and may require temporary relocation during work.

Costly Without Funding

Labour-intensive and more expensive than loft or cavity wall insulation if not grant-funded.

Moisture Risk

If not correctly designed, IWI can lead to condensation issues— especially in older properties with moisture-sensitive walls. PAS 2035 assessments address this risk.

Fixtures Need Alteration

Radiators, sockets, and skirting boards will often need repositioning or extension.

ALIGN

Is Your Home Suitable?

IWI is recommended if:

Your home has solid walls or uninsulated cavity walls that are not suitable for CWI.
External insulation is not practical or permitted (e.g. due to planning constraints).
Your property is hard-to-heat, and you’re eligible for funded retrofit upgrades.
A PAS 2035 assessment confirms the measure is technically appropriate.

SUMMARY

Conclusion

Internal Wall Insulation is a powerful solution for making older, solid-walled homes warmer, more efficient, and cheaper to heat. When installed to PAS 2035 standards and assessed under the whole-house approach, IWI becomes a safe, compliant, and long-lasting measure that integrates into your broader retrofit strategy.

With government funding widely available, this high-impact energy improvement is more accessible than ever— especially for vulnerable or low-income households.