Internal wall plasterboard insulation guide, how to fix cold internal walls, home brick lining

How to Insulate an Internal Wall Using Plasterboard with Insulation

30 April 2026

Cold internal walls are a common issue in older UK homes, especially where solid brick has little resistance to heat loss. If a room never quite warms up or you notice cold patches along external walls, installing plasterboard with insulation internally is one of the most effective fixes.

It improves thermal performance, cuts heat loss, and makes the space feel consistently warmer, all without changing the outside of the building.

Here’s what really matters:

Use the right board type, PIR is usually the best choice
Match the insulation thickness to your U-value target
Prepare the wall properly before fixing
Choose a fixing method that suits the wall condition

Assess the Wall Before Selecting a Board

Start with the wall itself. Most older properties have solid masonry walls, and these are ideal for internal insulation, as long as they are dry and structurally sound.

Damp is the one thing you cannot ignore. If moisture is present and you cover it, the problem gets trapped behind the boards and will cause damage over time. Deal with the cause first, then move forward once the wall is fully dry.

When measuring up, take your time and think through the practical details:

Floor to ceiling height and full wall width
Skirting boards, architraves, and sockets that need removing
Any cables or pipework that may need adjusting

It is a simple step, but it sets everything else up properly.

Choose the Right Plasterboard with Insulation for the Job

Insulated plasterboard combines rigid insulation with a plasterboard face, so you are insulating and finishing in one pass. It keeps the process efficient and avoids building up multiple layers.

Insulated plasterboard combines rigid insulation with a plasterboard face, so you are insulating and finishing in one pass. It keeps the process efficient and avoids building up multiple layers. Online Insulation Sales stocks Celotex and Gyproc-compatible systems across a full range of thicknesses. This allows you to match the board to your thermal target, without compromising on depth or performance.

PIR vs EPS: which should you use?

This decision comes down to performance versus space.

PIR backed boards deliver strong thermal performance at a thinner depth. That makes a real difference in retrofit work where every millimetre of room space counts. In most cases, this is the option people choose.

EPS boards cost less, but they need more thickness to achieve the same result. The extra depth can quickly eat into usable space, which is why EPS tends to be less common for internal walls.

Determine the Correct Insulated Plasterboard Thickness

Thickness is what drives performance, so this is not a step to guess your way through.

For a typical 215mm solid brick wall using PIR insulation, 50mm will get you to around 0.50 W/m²K. Increase that to 70 or 80mm and you are moving towards 0.30 W/m²K, which aligns with Part L retrofit targets.

Here is the important part: always base your decision on a proper U value calculation using the actual wall construction. This will tell you the minimum insulation depth required, without overspending or losing more space than necessary.

Also keep in mind that the total board thickness includes both the insulation and the 12.5mm plasterboard face. As the full depth is how far the wall will move into the room.

Prepare the Wall Surface

This is the stage that often gets rushed, and it usually shows later.

Boards need a clean and reasonably flat surface to bond properly. If the wall is uneven or dusty, the finish will suffer and adhesion can fail over time.

Work through the basics carefully. Remove anything loose, clean the surface, and check the wall for level. If areas are badly out of line, build them out with bonding compound first. On painted or sealed surfaces, apply PVA so the adhesive can grip properly.

Think about it this way, the boards will only ever be as good as what they are fixed to.

Fix the Boards – Dot and Dab or Mechanical Fix

There are two main ways to fix insulated plasterboard, and the condition of the wall usually decides it for you.

Dot and dab is the standard method. Adhesive is applied in a grid across the wall, then the board is pressed into place and adjusted until it sits plumb. It is quick and works well on reasonably even surfaces. Just make sure you leave a small gap at the floor and ceiling to allow airflow behind the board.

Mechanical fixing uses battens or a metal frame, with the boards screwed to the structure. It takes more time and adds depth, but it gives you control. If the wall is uneven or you need space for cables or pipes, this is often the better option.

Cutting Boards Cleanly

Cutting insulated plasterboard is straightforward, but it is worth doing neatly.

Score the face with a sharp knife, snap the board along the line, then cut through the backing paper. Clean edges make fitting easier and help keep joints tight.

Tape, Fill, and Prepare for Decoration

Once the boards are fixed, the job moves into finishing. This is where the final look is made or lost.

Start by taping all joints with scrim, then apply jointing compound and feather it out so the surface stays flat. Let each coat dry fully before adding the next. External corners should be protected with corner bead to keep edges sharp and durable.

For the final finish, a full skim coat gives the smoothest result. Tapered edge boards can sometimes be filled and decorated without skimming, which can save time on simpler jobs. Either way, make sure everything is fully dry before painting.

Ready to Order? Confirm Your Specification and Get the Right Boards

At this point, your decisions should be clear. You know the wall type, the insulation thickness you need, and how you plan to fix the boards.

Now it is about ordering accurately. Measure the wall area, add around 10 percent for waste, and check board sizes against your room height to reduce off cuts.

For most internal wall insulation projects, PIR insulated plasterboard is the standard starting point. If you are ordering from Online Insulation Sales, check your selected board against your U value target and Part L requirements before finalising the order.

FAQs

What thickness of insulated plasterboard do I need for an internal wall?

For a solid brick wall, 50mm PIR gives around 0.50 W/m²K. To reach 0.30 W/m²K, you will usually need 70 to 80mm, but this should always be confirmed with a U value calculation.

Can insulated plasterboard be fixed directly to a masonry wall?

Yes, dot and dab is the standard approach. Mechanical fixing is used where walls are uneven or where services need to run behind the boards.

What is the difference between PIR insulated plasterboard and EPS backed boards?

PIR delivers better thermal performance at a thinner depth. EPS costs less but requires more thickness to achieve the same level of insulation.

Do I need to address damp before fitting insulated plasterboard?

Yes. Any damp issues must be resolved first, otherwise moisture will be trapped behind the boards and cause damage over time.

What sizes does insulated plasterboard come in?

The most common size is 1200 x 2400mm with a 12.5mm plasterboard face. Total thickness varies depending on the insulation layer, typically from 37.5mm up to 102.5mm or more.

Comments on this guide to Internal wall plasterboard insulation guide article are welcome.