Board thickness is one of the most important specification decisions you’ll make before ordering insulated plasterboard. It’s also one of the most misunderstood. Choose the right board, and you’ll achieve the thermal performance your project requires. Choose the wrong one, and you could end up missing your U-value target, losing more room space than expected, or creating installation issues on site.
Before ordering, it’s worth understanding four key factors:
- What the thickness figure actually includes
- How U-value targets affect board selection
- Why wall type influences the thickness you need
- How fixing methods can limit your options
This guide explains how these factors work together so you can choose the right insulated plasterboard with confidence.
The Thickness Figure on the Board Is Not All Insulation
One of the most common misconceptions is that the stated board thickness refers entirely to the insulation layer. In reality, insulated plasterboard consists of two components: a PIR insulation core bonded to a plasterboard facing. The thickness shown in product specifications is the combined depth of both materials. For example, a 62.5mm board typically contains 50mm of PIR insulation and 12.5mm of plasterboard.
This distinction matters when comparing products and calculating thermal performance. Contractors and renovators can browse the full range at Online Insulation Sales when choosing insulated plasterboard for wall insulation, where the PIR depth for each board is listed alongside the overall board thickness. Two boards with similar overall dimensions can deliver different thermal performance if the PIR insulation depth varies, so it’s important to check the breakdown before ordering.
Your U-Value Target Should Drive the Thickness Decision
A U-value measures how quickly heat passes through a building element, such as a wall. Lower U-values indicate better thermal performance, which is why most renovation and retrofit projects begin with a target U-value set by Building Regulations or the project specification.
Rather than choosing a board thickness first and hoping it performs well enough, it’s usually more effective to work backwards from the required U-value. Manufacturers such as Celotex and Gyproc publish technical data showing the thermal performance their insulated plasterboard products can achieve across different wall constructions and insulation depths. Using this information before ordering removes guesswork and helps ensure the selected board will meet the project’s requirements.
The Wall Type Will Narrow Down Which Thicknesses Are Practical
The type of wall being insulated plays a major role in determining which board thicknesses are suitable. Solid brick walls, blockwork, timber frame systems and other constructions all have different thermal characteristics, meaning the same board thickness will not deliver identical results in every situation.
Older solid-wall properties often require thicker PIR insulation to achieve a meaningful improvement in thermal performance because the existing wall offers relatively little resistance to heat loss. Installation requirements also vary.
Dot-and-dab applications place greater demands on adhesives as board thickness and weight increase, while mechanically fixed timber frame systems typically allow for a wider range of board thicknesses. Confirming the wall construction before specifying a product helps ensure the board is both practical to install and capable of achieving the desired thermal outcome.
Thicker Boards Reduce the Room (Factor This In Before Ordering)
Improved thermal performance generally requires more insulation, but every additional millimetre of insulated plasterboard reduces the usable space within the room. In larger rooms, this may have little impact, but in smaller bedrooms, hallways and compact living spaces, the cumulative effect can become noticeable.
Before committing to a particular thickness, it’s worth calculating how much floor area will be lost across all walls being insulated. In some situations, a slightly thinner board may provide a better balance between thermal performance and preserving usable space. The right specification is rarely about achieving the lowest possible U-value alone. It’s about finding the most practical solution for the room and the project as a whole.
The Fixing Method and Board Thickness Need to Be Decided Together
The installation method should always be considered alongside board thickness. Dot-and-dab adhesive fixing and mechanical fixing are the two most common methods used with insulated plasterboard, and each has its own practical limitations.
As boards become thicker, they also become heavier. Heavier boards place greater demands on adhesive systems, particularly when the substrate is uneven or in poor condition. In these situations, mechanical fixing may offer a more reliable long-term solution.
Many manufacturers also specify maximum board weights for adhesive fixing systems, making it important to check product guidance before placing an order. Confirming the fixing method and board thickness together helps avoid installation issues and ensures the chosen system is suitable for the application.
Confirm the Specification and Order With Confidence
Choosing the right insulated plasterboard thickness comes down to four connected decisions: the PIR insulation depth within the board, the target U-value, the wall construction and available space, and the fixing method that will be used during installation. Each factor influences the others, which is why reviewing the complete specification before ordering is so important.
Taking the time to verify these details can prevent costly delays, incorrect material purchases and avoidable site issues later in the project. Review the manufacturer’s technical data, confirm the required thermal performance and ensure the selected fixing method is suitable for the board thickness you’ve chosen.
FAQs
What thickness of insulated plasterboard do I need for internal walls?
The right thickness depends on the wall construction and the U-value target for the project. Most internal wall applications use boards with a PIR insulation depth ranging from 25mm to 75mm.
What U-value should insulated plasterboard achieve on an internal wall?
The required U-value varies depending on the project and any applicable Building Regulation requirements. Manufacturers publish performance data showing the U-values their boards can achieve across different wall types and thicknesses.
Does thicker insulated plasterboard always perform better thermally?
A greater PIR insulation depth generally delivers a lower U-value and improved thermal performance. However, thicker boards also reduce room dimensions and may affect fixing requirements, so thermal performance is only one part of the decision.
Can insulated plasterboard be used on all wall types?
PIR-backed insulated plasterboard is commonly used on solid brick, blockwork and timber frame constructions. The wall type will influence both the practical board thickness and the most suitable fixing method.
Where can I buy insulated plasterboard in a range of thicknesses?
Online Insulation Sales supplies insulated plasterboard across a range of PIR depths and overall board thicknesses, covering the specifications most commonly required for internal wall insulation projects.
