timber framing

Naturally, timber frame structures have a low thermal mass which significantly reduces the amount of heat absorbed. However, the choice of timber frame insulation is important for any kind of building structure, so you can meet the required standards of fire safety, sound insulation, thermal performance, and durability.

In this article, we will look into what is the best timber frame insulation between two of the most common types: PIR insulation and mineral wool.

//How to Choose the Best Insulation for Your Timber Frame Project?

Timber frame insulation enhances timber’s natural thermal performance, and by choosing the right insulation it performs better than traditional masonry walls of the same thickness. How do you know whether to choose between PIR or mineral wool though? Here are their benefits, considerations and how they affect wall thicknesses.

What is PIR Insulation?

Polyisocyanurate (PIR) insulation is a type of rigid board insulation made from a synthetic material. Some of its main characteristics include:

  • Low thermal conductivity
  • Lightweight
  • Easy to cut and install
  • Resistant to moisture and mould

It is often used between the studs to provide pre-insulated open panel or as part of a closed panel system, like our NHBC Accepts approved Future-Frame system. To reduce the risk of thermal bridging – the transfer of heat through the structural elements of the building – it can also be installed as an additional layer to the inner face of the studs.

Benefits of PIR Insulation for Timber Frame Construction

High Thermal Performance

PIR insulation can help you achieve the required U-values with a thinner layer of insulation which can, in turn, save you space and materials. This allows housebuilders to create a more spacious and comfortable living environment.

Flexible & Versatile

This type of insulation can be cut to fit your dimensions of your timber frame construction and fixed with nails, screw, or adhesives meaning it is easy to install. Its flexibility is also highlighted with the ability to be used in combination with other types of insulation to create a hybrid system.

Prevents Air Leakage

PIR can prevent air leakage and increase air-tightness due to its closed-cell structure and dense packing when installing. Air leakage, like draughts and cold spots, can reduce the energy efficiency of your building so this is a benefit of using PIR insulation for your timber frame structure.

Considerations for PIR Insulation


PIR insulation is more expensive than mineral wool, so your project budget may affect your choice of insulation. However, by having thinner walls, you will increase the saleable floor area and save money.

Sound Transmission Issues

Although it is less effective at reducing sound transmission, it isn’t used in party wall situations, so it is still a good choice for external wall environments.

Fire Spread

There is a full suite of tested details that demonstrate that a single layer of 15mm plasterboard will provide the required level of fire protection for insulation and overall structure. For further advice and information on the tested build-ups, please contact us.

What is Mineral Wool Insulation?

Mineral wool is a type of fibrous insulation made from natural or synthetic materials such as rock, slag or glass. Some of its main characteristics include:

It is often used as a site fitted element between the studs of an open panel timber frame or factory fitted as part of a closed panel solution like our NHBC Accepts approved Future-Frame system.

Benefits of Mineral Wool for Timber Frame Construction


Compared to PIR insulation, mineral wool is a lower cost choice so this may be a better option for your project budget.


As mentioned above, mineral wool insulation is incredibly effective at reducing sound transmissions, especially in party walls and floors.


Similar to PIR insulation, mineral wool is also flexible in the way that it can be more easily fitted into small recesses when frames are already built on site.

Considerations of Mineral Wool Insulation

Lower Thermal Performance

Due to its lower thermal performance, timber frame construction requires a thicker layer of wool insulation to achieve the required U-values. With thicker insulation, you will have thicker walls and ultimately, a smaller amount of saleable floor.

Difficult to Install

Because it needs to be cut and fitted between the studs of timber frame with no gaps or voids, mineral wool insulation is more difficult to install. Also, if it sags or settles over time, this can reduce its effectiveness and create thermal bridges. However, with careful detailing and robust QA processes, like our Future-Frame system, this can be avoided.

//What is the Best Timber Frame Insulation for Your Project?

The answer to this question depends on your budget, design, site conditions, and performance requirements. Pinewood Structures are here to help though; with over 40 years of experience within the timber frame building industry, we are well versed in all types of insulation and can help guide you to find the right one for your project. We are also familiar with UK building regulations and requirements, so you can always trust our solutions.

Looking to maximise the thermal performance of your timber frame building and space? PIR insulation is best for this as it provides a high level of insulation with a thin layer, but you will need to consider the cost implications.

If your budget is your priority rather than wall thickness, then mineral wool is best as it is cost-effective and can provide good insulation with a thicker layer. You need to make sure it’s installed carefully and correctly though.

Advanced reflective membranes and vapour control levels (VCL) are other options for augmenting the thermal performance of walls which we will cover in more depth in the future. This is part of the Future-Frame system that we have approved by NHBC Accepts.

Get in touch with us today if you have any further questions or would like some advice with your next development.