If you live in a charming period property in the UK, chances are you've fallen in love with its character, high ceilings, and sturdy structure. But you've probably also shivered through a winter or two, battling draughts and sky-high heating bills. The culprit? Often, it's the solid brick walls.
Unlike modern homes with cavity walls, properties built before the 1930s (and many after) have solid external walls. This means there's no air gap to fill with insulation, making them notoriously difficult to heat efficiently. The solution: Internal Wall Insulation (IWI).
But here’s the challenge: how do you add effective insulation without eating significantly into your precious internal space? At Next Day Insulations, we get this question all the time. Let’s explore the best space-saving IWI options for your period home.
The Space-Saving Contenders for IWI
When insulating solid walls from the inside, the goal is to achieve maximum thermal performance with minimum thickness. Here are the top methods:
1. Insulated Plasterboard (Thermal Laminates)
The Concept: A rigid insulation board (usually PIR or Phenolic) is pre-bonded to a sheet of plasterboard.
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How it Works: These boards are typically fixed directly to the solid wall using the "dot-and-dab" method (adhesive dabs) or mechanically fixed to a thin timber battens.
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Thickness: Generally ranges from 27mm to 70mm (including plasterboard).
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Pros:
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Space-Efficient: One of the thinnest effective solutions, combining two steps into one.
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Fast Installation: Reduces labour time as you're insulating and drylining simultaneously.
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Finish-Ready: Provides an immediate surface for plastering or decorating.
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Cons:
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Penetrations: Requires careful sealing around electrical sockets and pipework to prevent thermal bridging.
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Moisture Risk: Requires good ventilation and a vapour control layer (VCL) if not already integrated.
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Best For: Living rooms, bedrooms, or any area where space is a critical factor and a quick finish is desired.
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NDI Pick: Knauf XPS Thermal Laminate for its excellent moisture resistance, or Celotex PL4000 for top-tier thermal performance.
2. Thin-Profile PIR Boards with Separate Plasterboard
The Concept: Installing a high-performance PIR (Polyisocyanurate) board first, followed by a separate layer of plasterboard.
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How it Works: Thin PIR boards (like Celotex or Kingspan) are fixed to a thin timber batten frame or directly to the wall, then plasterboard is fixed over the top. A vapour control layer (VCL) is often incorporated.
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Thickness: PIR boards can start from 25mm thick, plus a standard 12.5mm plasterboard. Total build-up around 40mm - 75mm+.
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Pros:
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Superior Thermal Performance: PIR offers the best thermal conductivity (around 0.022 W/mK), meaning you get more warmth from less thickness.
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Flexibility: Allows for services (electrics) to be run in the battens if used.
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Targeted U-Values: Easier to achieve specific, demanding U-values with thinner materials.
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Cons:
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Two-Stage Process: Takes longer than insulated plasterboard as it's separate insulation and drylining.
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Cost: Generally a higher material cost than EPS or mineral wool.
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Best For: Achieving the highest thermal performance in the thinnest possible profile, ideal for critical areas like listed buildings or where every millimetre counts.
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NDI Pick: Kingspan K8 Cavity Board (often adapted for IWI) or Celotex GA4000 in thin gauges.
3. High-Performance Multi-Foil Insulation
The Concept: Multi-foil insulation consists of multiple layers of reflective foil, wadding, and polyethylene foam, designed to reflect heat.
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How it Works: Multi-foils are typically stapled onto timber battens, creating air gaps on either side (which are crucial for their performance). Plasterboard is then fixed over the top.
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Thickness: The product itself is very thin (e.g., 25mm-50mm), but requires an air gap on each side, so the total build-up with battens and plasterboard might be closer to 60mm-100mm.
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Pros:
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Very Easy to Install: Lightweight, flexible, and simple for DIYers.
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Vapour Control: Often acts as a vapour control layer itself.
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Radiant Heat: Excellent at reflecting radiant heat.
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Cons:
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Dependent on Air Gaps: Performance significantly drops if air gaps aren't maintained.
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Lower R-Value per mm: While thin, you might need a thicker total build-up compared to PIR to achieve the same U-value in some applications.
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Best For: DIY projects, garden rooms, or when combining with other insulation types to boost overall performance, especially in roof spaces.
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NDI Pick: Superfoil SF19 for its versatility and ease of handling.
Important Considerations for IWI
Regardless of the method you choose, remember these crucial points:
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Vapour Control Layer (VCL): Essential to prevent moisture from condensing within the wall structure, which can lead to damp and mould. Many insulated plasterboards or multi-foils integrate this, but always check.
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Ventilation: Ensure adequate ventilation in the room. Sealing up walls without proper airflow can trap moisture.
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Thermal Bridging: Be meticulous about insulating around windows, doors, and floor/ceiling junctions to avoid "cold spots."
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Professional Advice: For listed buildings or complex projects, always consult with a conservation officer or an experienced professional.
Making the Right Choice
The "best" solution depends on your budget, the desired U-value, your DIY skills, and how much space you can realistically sacrifice.
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Tightest on Space & Top Performance: Go for Thin-Profile PIR Boards.
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Balancing Space & Speed: Insulated Plasterboard is your champion.
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DIY-Friendly & Radiant Heat Focus: Consider Multi-Foil Insulation.
At Next Day Insulations, we stock a comprehensive range of all these materials, ready for next-day delivery to get your renovation project moving. Don't let cold, solid walls deter you from enjoying a warm, energy-efficient home.
Ready to transform your old home? Explore our full range of Internal Wall Insulation products here or contact our expert team for tailored advice!