Internal Wall Insulation Guide

By applying a layer of high-performance insulation to the inside of your external walls, you can significantly reduce heat loss and improve your Energy Performance Certificate (EPC) rating. While it involves some disruption and a slight reduction in room size, the increase in thermal comfort and reduction in energy bills is often substantial.

What Does the Work Involve?

• Initial Assessment: A specialist checks for existing damp, as IWI should never be installed over active rising or penetrating damp.
• Preparation: Removal of skirting boards, radiators, door frames, and electrical sockets from the target walls.
• Fixing Insulation: Rigid PIR boards are either "dot and dabbed" or fixed to timber battens, depending on the wall's condition.
• Vapour Control: Installation of a vapour control layer (VCL) to prevent warm, moist indoor air from reaching the cold masonry and causing rot.
• Plastering: Fitting moisture-resistant plasterboard followed by a multi-finish skim for a smooth decorative surface.
• Reinstatement: Refitting all fixtures, extending electrical back-boxes, and adjusting plumbing to accommodate the new wall thickness.

Typical Costs

The cost of internal wall insulation varies based on the materials used and the complexity of the room. Moving radiators and electrical points often accounts for a significant portion of the labour cost.

Pricing is largely driven by the finish required. If you choose breathable lime plaster or natural insulation like wood fibre to protect a period property, expect costs to sit at the higher end of the scale.

How Long Does It Take?

• Single Room: 3–5 days from strip-back to final plaster skim.
• Full House (Empty): 2–3 weeks depending on the number of external walls.
• Drying Time: 3–7 days for plaster to dry before any painting can begin.

DIY or Professional?

While a confident DIYer can fix insulation boards, IWI is a high-risk project that is usually best left to professionals. Incorrect installation can trap moisture between the insulation and the wall, leading to structural rot or hidden mould growth.

Note: Professional installers will ensure the vapour barrier is continuous, which is the most critical step in preventing long-term damage to your home.

Furthermore, any electrical or gas work (like moving radiators or sockets) must be carried out by qualified Gas Safe engineers or Part P certified electricians to comply with UK law.

Choosing the Right Tradesperson

Look for installers with specific experience in "Solid Wall Insulation" rather than general builders. Ensure they understand the difference between breathable and non-breathable systems.

• Check Credentials: Look for TrustMark registration or membership in the National Insulation Association (NIA).
• Ask about PAS 2035: If you are seeking government grants (like the Great British Insulation Scheme), the installer must be PAS 2035 compliant.
• Questions to Ask:
  • How will you maintain ventilation in the room after the walls are sealed?
  • Will you be using a vapour control layer or breathable materials?
  • How do you treat "thermal bridges" like floor joists and internal wall joins?

UK Regulations

• Building Regulations Part L: If you are renovating more than 50% of a wall's surface, you are legally required to upgrade its thermal insulation to meet current U-value targets.
• Ventilation: Improving airtightness often requires the installation of trickle vents or mechanical extract fans to prevent condensation.
• Building Control: You should notify your local Building Control office, who will issue a completion certificate—essential for when you sell the property.

Common Problems

• Interstitial Condensation: Moisture getting trapped behind the insulation. Avoid this by using a high-quality vapour barrier.
• Thermal Bridging: Forgetting to insulate "returns" (the small sections of internal wall where they meet the external wall), which leads to cold spots and mould.
• Loss of Space: High-performance PIR boards are usually 60mm–100mm thick. Ensure you are happy with the reduced floor area before starting.
• Fixing Issues: Standard wall plugs won't work on insulated walls; you will need special "heavy-duty" fixings to hang TVs or heavy mirrors in the future.

Frequently Asked Questions

Will internal wall insulation cause damp?

If installed correctly with a vapour barrier, it prevents damp. However, if the wall was already damp or the barrier is breached, it can trap moisture. Always fix external leaks or rising damp before insulating.

How much space will I lose?

Expect to lose between 65mm and 120mm per external wall. While this sounds small, it can impact the placement of furniture and the swing of doors near corners.

Is IWI better than external wall insulation (EWI)?

EWI is generally more effective and doesn't reduce internal space, but it is significantly more expensive and changes the look of your home. IWI is better for conservation areas or where external access is restricted.

Do I need to move my radiators?

Yes, in almost all cases. The insulation sits behind the radiator, so the pipework must be extended to bring the radiator forward onto the new wall surface.

Can I hang heavy items on the new wall?

Yes, but you cannot use standard nails or screws. You must use specialist deep-reach fixings that anchor into the original masonry behind the insulation.