Insulation thickness is one of the easiest figures to compare when looking at different house quotations. One wall may contain 200 mm of insulation, another 250 or 300 mm – intuitively, it can seem that more must automatically be better.

In reality, the number of centimetres alone tells you surprisingly little. The same thermal performance can be achieved with different thicknesses depending on the insulation material used and the overall wall construction. Junctions, thermal bridges and airtightness are just as important.

In Happy Home CLT houses, we use PIR insulation in the external envelope. One of its key advantages is excellent thermal performance with a relatively slim insulation layer. Combined with the slender CLT load-bearing structure, this allows us to create a well-insulated but compact external wall, leaving more usable interior space within the same external dimensions of the house.

1. The requirement is about thermal performance, not wall thickness

Energy-efficiency requirements do not specify how many centimetres of insulation a wall must contain. What matters is how much heat passes through the complete construction – described by its thermal transmittance, or U-value.

The final wall thickness therefore depends on the materials used and their properties.

Two walls of different thicknesses can have very similar thermal performance. Conversely, two walls with the same overall thickness can perform very differently if their materials and construction differ.

So when considering insulation, the useful question is not only “how thick is it?”, but also “what performance does the complete building envelope achieve?”

We discuss how insulation works together with airtightness, windows, ventilation, heating and other systems in more detail in “Energy-efficient home: what really makes an A energy rating?”

2. Thermal conductivity: why the material affects thickness

Insulation materials are characterised by their thermal conductivity. Put simply, the lower the thermal conductivity of a material, the thinner the layer required to achieve the same thermal performance.

This is where the difference between materials such as PIR insulation and mineral wool becomes important.

PIR, or polyisocyanurate, is a rigid closed-cell insulation board with lower thermal conductivity than mineral wool. This means that the required thermal performance can be achieved with a thinner layer.

For Happy Home, this is one of the main reasons we use PIR specifically in the external envelope.

That does not mean PIR is automatically “better” than mineral wool in every construction. Mineral wool is non-combustible and, thanks to its fibrous structure, is also an effective sound-absorbing material. We therefore use it, for example, in internal partitions and floor constructions.

A good building does not come from choosing one material for everything – different materials are used where their particular properties provide the greatest benefit.

3. CLT walls: a slender load-bearing structure with continuous insulation

The load-bearing part of a Happy Home external wall is CLT, or cross-laminated timber.

In a conventional timber-frame wall, structural studs are often positioned at approximately 600 mm centres. Timber conducts heat better than the mineral wool between the studs, so each repeated stud interrupts the insulation layer and creates a recurring thermal bridge.

A CLT wall does not have this type of stud frame. Instead, the load-bearing layer is a continuous solid timber panel, with the PIR insulation installed as an uninterrupted layer on the outside.

The basic Happy Home external wall build-up, from inside to outside, consists of:

• the load-bearing CLT panel;
• PIR insulation;
• the wind-protection function provided by the PIR layer;
• battens creating the required ventilated cavity and fixing base for the façade;
• the façade finish.

CLT and PIR also work particularly well together for another reason: both allow a more slender construction than many conventional alternatives.

CLT provides high structural capacity with a relatively small cross-section, while PIR achieves the required thermal performance with a thinner insulation layer.

For the client, the benefit is very practical: a thinner external wall means more usable interior floor area within the same external dimensions of the house.

Across the full perimeter of a house, a difference of several centimetres in wall thickness can add up to several square metres of genuinely usable floor space.

We discuss the CLT structure and external wall build-up in more detail in “Benefits of a CLT house: why we use cross-laminated timber at Happy Home.”

4. Thermal bridges matter at least as much as insulation thickness

Thermal bridges are areas where heat passes through the structure more easily than through the surrounding construction. They can occur, for example, where the foundation meets the wall, around windows, at corners, at structural connections and around service penetrations.

This is why an energy-efficient house cannot be designed simply by making the insulation layer increasingly thick.

The insulation layer needs to remain as continuous as possible, and critical construction details must be properly designed. A poorly resolved junction cannot simply be compensated for by adding a few more centimetres of insulation to the rest of the wall.

The same applies to airtightness. If warm air escapes uncontrollably through junctions and penetrations, a thicker wall alone will not solve the problem.

Measured air leakage rates in Happy Home houses have been 0.13–0.35 m³/(h·m²) at 50 Pa, demonstrating how important precise design and execution are alongside the materials themselves.

5. Wall thickness directly affects usable interior space

This is one of the most practical but often overlooked aspects of comparing insulation systems.

The external dimensions of a house are usually limited in some way – by the permitted building area, plot boundaries and setbacks, the size of the site, or simply by the desired proportions of the architecture.

If the external wall becomes thicker, less interior space remains within the same external dimensions.

A difference of ten centimetres in a single wall may sound insignificant, but across the entire perimeter of a house it can amount to several square metres of usable space.

When comparing different construction systems, it therefore makes sense to look not only at the cost of the insulation itself, but also at how much usable floor area the solution preserves within the same external dimensions.

This is where the combination of CLT and PIR becomes particularly valuable: strong structural and thermal performance can be achieved without making the external wall unnecessarily bulky.

6. Roof and floor insulation follow a different logic

With external walls, every additional centimetre can affect the amount of usable interior space. The same relationship does not apply in exactly the same way to the roof and floor.

In a roof, it is often possible to use a thicker insulation layer without reducing usable floor area. Floor insulation, meanwhile, is linked to the foundation structure, finished levels and the build-up of the underfloor heating system.

For this reason, it does not make sense to look at wall, roof and floor insulation separately and assume that all three should follow the same approach or simply use the thickest possible insulation.

The energy performance of the house comes from the complete building envelope, and the designer can assess where improvements provide the most sensible result for the particular house.

We discuss the foundation and floor structure in more detail in “House foundations: what to consider for a CLT home?”

7. Windows, airtightness and ventilation influence the result just as much

An excellent external wall alone does not make an energy-efficient house.

The total heat loss of a building comes through the walls, roof, floor, windows and doors, while thermal bridges and airtightness also affect the final result.

Large glazed areas behave differently from a highly insulated external wall and therefore need to be included in the overall energy calculation. This does not mean that an energy-efficient house needs small windows – the glazing, structure and technical systems simply need to be designed together.

Heat-recovery ventilation is another important part of the whole. In winter, much of the heat contained in the outgoing warm air is recovered and transferred to the incoming fresh air, reducing the amount of heat the heating system needs to provide.

Walls, windows, airtightness, ventilation and heating work together in an energy-efficient house – it makes little sense to over-engineer one element simply to compensate for weaknesses elsewhere.

8. How much insulation should there be?

More is not automatically better.

Each additional layer of insulation provides a smaller improvement than the previous one, while the thickness and cost of the construction continue to increase. The aim is therefore not to make the external wall as thick as possible, but to find a technically and economically sensible solution for the house as a whole.

The moisture behaviour of the construction must also be considered. The insulation material, thickness and order of the layers need to form a functioning system – one layer cannot simply be changed independently of the rest of the wall.

The external envelope of each Happy Home house is designed according to the specific project and energy calculation. If the architecture, glazing or other elements change, the optimal wall construction may change as well.

The goal is not the maximum possible amount of insulation, but the right balance between thermal performance, wall thickness, cost and usable interior space.

What should you look for in a quotation?

When comparing house-building quotations, it is worth looking beyond a line that simply says “200 mm insulation” or “250 mm insulation”.

Much more useful questions are:

• What is the complete external wall build-up?
• Which insulation material is used?
• What is the calculated thermal transmittance of the construction?
• How are thermal bridges at the foundation, windows and other critical junctions resolved?
• How is airtightness achieved?
• Is the airtightness of the completed house actually measured?

The number of centimetres describes the quantity of material. A well-designed building envelope describes the actual quality of the house.

For more on comparing builders, quotations and technical solutions, see “How to choose the right builder for your home: what should you really compare?”

In summary

When it comes to insulation, the most important question is not how many centimetres of material are placed in the wall.

What matters is the thermal performance and overall thickness of the complete construction, and how well the load-bearing structure, insulation, junctions and airtightness work together.

In Happy Home CLT houses, we use PIR insulation in the external envelope because it allows us to achieve excellent thermal performance with a relatively slim construction. Combined with the slender CLT load-bearing structure, this means more usable interior space within the same external dimensions of the house.

Mineral wool is used where its properties – such as sound insulation or non-combustibility – make it the more suitable solution.

The aim is not to use as much of one material as possible, but to use the right material in the right place and make the entire construction work as one system.