How to Insulate a Cold Wall from the Inside

Tired of cold, draughty walls or worried about rising heating bills? For many homeowners, particularly those living in one of the UK’s 8.5 million solid-wall properties, adding insulation to internal walls can make a huge difference. Done properly, it improves comfort, reduces heat loss, and helps keep energy costs under control.

In this guide, we’ll look at the most effective methods and materials for insulating solid walls from the inside. We’ll also explain the latest standards, highlight the risks (especially around moisture), and share best practices to ensure your project delivers lasting results.

Why Wall Insulation is More Important Than Ever

Insulation is your home’s first line of defence against heat loss. For solid walls, which lose heat twice as fast as modern cavity walls, internal insulation is often the most practical solution. It not only makes your home more energy-efficient but also prevents the cold interior surfaces that can lead to condensation and mould, creating a healthier living environment.

With government schemes available and clear standards to follow, there has never been a better time to tackle those cold walls.

Understanding Your Walls and Insulation Options

• Solid Wall Insulation: Most older homes (pre-1920s) have solid walls, consisting of a single layer of brick or stone. Internal solid wall insulation involves adding an insulation layer to the inside face of these walls. This is our main focus, as it requires careful management of moisture.
• Cavity Wall Insulation: Homes built between the 1920s and 1990s often have cavity walls (two layers of brick with a gap). Cavity wall insulation involves filling this gap and is typically a job for specialist installers.
• Stud Wall Insulation: Internal partition walls or timber-frame walls can be insulated by fitting materials like mineral wool fibre between the wooden or metal studs.
• External Wall Insulation: An alternative for solid walls is external wall insulation, which involves fixing insulation to the outside of your home. While highly effective, it changes the building’s appearance and is often more expensive.

The Critical Role of Moisture Management

When you add insulation to the inside of a solid wall, you make the original wall colder. This increases the risk of “interstitial condensation”—moisture from inside your home passing through the insulation and condensing on the cold surface of the original wall. If not managed, this can lead to damp, mould, and structural damage.

Updated UK regulations, including BS 5250:2024 (Management of Moisture in Buildings), stress the importance of a “whole-building” approach. The key is to ensure your insulation strategy is breathable, allowing moisture to escape safely.

Key principles for solid walls include:

• Use Vapour-Permeable Materials: Choose insulation and finishes that allow water vapour to pass through them. Materials like wood fibre, cork, and sheep’s wool are excellent choices, as are traditional lime plasters.
• Avoid Air Gaps: Any gap between the insulation and the wall can become a trap for damp air. For this reason, the “dot and dab” method of applying plasterboard is strongly discouraged for insulation projects. Instead, a full bed of adhesive should be used to ensure a tight, continuous fit.
• Rethink Vapour Barriers: A vapour barrier or Vapour Control Layer (VCL) is not always the right answer. While they can block moisture, an improperly placed or non-breathable VCL can trap moisture and cause significant harm. A professional condensation risk analysis is needed to determine if a VCL is beneficial for your specific wall.

Choosing the Right Insulation Materials

Your choice of material will impact performance, cost, and moisture management.

• Rigid Insulation Boards: Boards made from materials like PIR or fibreglass offer high thermal performance in a slim profile. Phenolic boards in particular are valued for delivering very low thermal conductivity, meaning you can achieve excellent insulation with less thickness – ideal when internal space is limited. They must be fitted perfectly to avoid air gaps.
• Natural Insulation Materials: For a breathable system, especially in older homes, natural materials are ideal. Sheep’s wool, wood fibre, and cork are “hygroscopic,” meaning they can absorb and release moisture, helping to regulate humidity.
• Insulated Plasterboard: A convenient option, insulated plasterboard combines insulation and plasterboard into a single product, speeding up installation. Where moisture is a concern, such as kitchens, bathrooms, or external-facing solid walls, variants like XPS-backed boards (extruded polystyrene backed plasterboard) are often preferred, as their closed-cell structure provides far better resistance to damp compared with standard EPS.
• Multifoil Insulation: Another option is multifoil systems, such as YBS Insulation and Actis Insulation, which combine insulation, airtightness, and vapour control in a single product. These can be particularly effective in retrofits where space is limited and moisture management is a priority.
• Advanced Materials: For situations where space is extremely limited, advanced materials like aerogel insulation offer exceptional thermal performance with minimal thickness. One study showed aerogel insulation reduced a room’s heat loss coefficient by over 60%.

Step-by-Step Guide to Insulating an Internal Wall

Installing internal wall insulation is a major renovation project. Under the PAS 2035:2025 framework for retrofitting homes, projects are assessed for risk, and a professional Retrofit Coordinator may be required to oversee the design and installation.

1. Assess Your Walls and Conduct a Risk Analysis

First, confirm you have solid walls. Before you buy any materials, you must assess the risk of condensation. You can use industry-recognised software like Knauf Insulation’s online tool or the Celotex Condensation Risk Calculator to model your wall’s performance. This analysis will inform your choice of materials and whether a VCL is needed.

2. Prepare the Wall Surface

Clear the area, removing furniture, radiators, and skirting boards. Electrical sockets and switches will need to be moved forward. The wall itself must be in good condition – sound, dry, and free of any existing damp or mould issues.

3. Apply the Insulation

Following the manufacturer’s instructions, apply the insulation directly to the wall. To prevent air gaps, use a continuous layer of a suitable adhesive rather than the “dot and dab” method. Ensure boards are cut precisely and butt up tightly against each other.

For extra security, especially with rigid boards, consider using insulation support fixings. These specialised fixings are designed to hold insulation panels tightly against solid walls, preventing movement and reducing the risk of air gaps that can cause damp issues later on.

4. Detail Around Openings and Junctions

Thermal bridges (or cold bridges) are spots where heat can bypass the thermal insulation. Pay close attention to window and door reveals, as well as the junctions with floors and ceilings. These areas must be insulated to ensure a continuous thermal layer.

5. Finish the Wall

Apply your final wall finish. If you’ve used a breathable insulation system, it’s vital to use a breathable finish like lime plaster or clay-based paint to maintain the wall’s ability to manage moisture. If you’ve used a rigid board system, you can finish with standard plasterboard and skim.

6. Reinstall Fittings and Enjoy the Benefits

Re-fit skirting boards, sockets, and radiators. Remember that the wall will be several centimetres thicker, which may require adjustments. Once complete, you’ll notice a significant improvement in comfort and energy efficiency.

Costs, Savings, and Grants

Installing internal wall insulation is an investment. The typical cost for a three-bedroom semi-detached home is around £7,500. However, this can lead to annual energy bill savings of approximately £380 and reduce your home’s carbon emissions by 880 kg per year.

You may be able to get help with the cost through the Great British Insulation Scheme (GBIS). Running until April 2026, this government scheme offers grants to eligible households. Eligibility often depends on your property’s Energy Performance Certificate (EPC) rating (D or lower), your Council Tax band, and sometimes household income.

Conclusion

Insulating a cold wall from the inside is one of the most effective upgrades you can make to an older home. It boosts comfort, cuts energy bills, and creates a healthier living space.

However, it is not a simple DIY job. A successful project depends on careful planning, a thorough understanding of moisture risk, and adherence to modern building standards. By following the principles in this guide, you can transform a chilly interior into a warm and inviting space for years to come.