How far should you insulate a building? Finding the right balance between energy performance and investment

Who it’s for: Architects, design offices and building owners.

Thermal insulation is essential to a building’s energy performance. But should its thickness always be increased to get better results? Between reduced needs, the cost of the works, summer comfort and long-term profitability, the real challenge is to find the level of insulation suited to the building, its climate and its use.

In brief

• Each extra centimetre of insulation reduces losses through the wall, but less and less: this is the principle of diminishing returns.
• The most cost-effective thickness is not necessarily the one that saves the most energy: the two optima can be far apart.
• In a Mediterranean climate such as Tunisia’s, insulation is sized together with the roof, glazing, solar shading and building controls, not in isolation.

Better thermal insulation is often presented as self-evident: the higher a wall’s thermal resistance, the less heat it lets through. The reasoning is physically correct, but it is not enough to determine the best solution for a project.

Every extra centimetre of insulation has a cost. Its value depends on the savings it still makes possible, how long those savings will last, the price of energy, the characteristics of the construction and the other options available to improve its performance.

The question is therefore not whether to insulate more, but:

up to what level does additional insulation remain worthwhile, in energy, economic and environmental terms?

It is this search for the right balance that deserves to be at the heart of high-performance building design.

Why more insulation does not bring proportional savings

To understand the phenomenon, two notions must be separated: the thermal resistance of a wall and the energy savings it delivers.

Thermal resistance, expressed in m²·K/W, describes a material layer’s ability to resist heat transfer. For a homogeneous insulation material, it is calculated simply:

R = e / λ

• e is the thickness of the insulation, in metres;
• λ (lambda) is its thermal conductivity, in W/(m·K).

The greater the thickness, the higher the thermal resistance of the insulation.

For a wall whose other layers remain unchanged, the thermal transmittance follows:

U = 1 / total R

The lower the U-value, the less heat the wall transmits, for the same temperature difference and under the same conditions.

But the reduction in heat transfer becomes progressively smaller as resistance increases. A theoretical example is enough to show it: if a wall’s total resistance goes from 1 to 2 m²·K/W, its U-value goes from 1 to 0.5 W/(m²·K), and transmission is halved. If it then goes from 2 to 3 m²·K/W, U goes from 0.5 to about 0.33 W/(m²·K). The improvement is still there, but it is smaller in absolute terms.

This is the principle of diminishing returns: each additional improvement brings a smaller marginal benefit on heat transfer through the wall.

What the figures show

Take a wall whose existing layers and surface exchanges amount to a resistance of 0.5 m²·K/W, and a common insulation material with a conductivity of 0.035 W/(m·K). Steady-state calculation, without thermal bridges:

The first four centimetres divide the wall’s transmission by more than three. The next four reduce it by a further 0.25 W/(m²·K), then by 0.10, then by 0.06. Each layer remains useful, but its effect shrinks, while its cost rises roughly in proportion to thickness.

Be careful, though: this does not mean the energy savings become negligible. Their value also depends on the climate, the surface area concerned, conditions of use and the performance of the heating and cooling systems.

The question of optimal thickness therefore cannot be settled by the thermal resistance formula alone.

The real goal: reducing the building’s needs, not just losses through the walls

A building does not use energy only because of its walls. Its thermal behaviour depends on the whole envelope:

• The external walls;
• The roof and floors in contact with the outside or with unheated spaces;
• Windows and doors;
• Thermal bridges;
• Airtightness and air renewal;
• Solar gains and internal gains from occupants and equipment.

Heating and cooling needs result from the interaction of these elements with the climate and the use of the building.

Extra wall insulation may therefore be of limited value if the roof is the weakest point of the envelope, if the glazing is unsuitable or if solar gains cause significant overheating. Conversely, improving a particularly leaky wall can deliver significant benefits.

The main sources of heat loss and gain should therefore be identified before deciding where to invest.

This approach avoids a common mistake: spending a large budget on improving an element that already performs well, when another intervention would have delivered a greater benefit for the same investment.

The climate changes the answer completely

An insulation thickness suited to a building in a cold climate is not necessarily optimal for a building in a Mediterranean climate.

In cold regions, heating needs can dominate much of the year. Reducing losses through the envelope is then a major lever.

In a Mediterranean climate, the picture is more complex: winter heating needs coexist with significant summer cooling needs. Sunshine, night-time temperatures, humidity and occupancy habits strongly influence the result.

A study published in Building and Environment illustrates this very well. For five buildings (a flat, a detached house, a clinic, a secondary school and a supermarket) in three locations representative of Portugal’s climates, it compared insulation thicknesses from 0 to 12 cm. Its conclusions:

• The optimum depends on the point of view: depending on the climate zone and the building type, the most cost-effective thickness ranges from 0 to 10 cm, whereas the best thickness in energy terms ranges from 0 to more than 12 cm;
• Climate matters: regions with harsher climates require more insulation;
• Use matters too: homes, and buildings occupied permanently, require more insulation than buildings used intermittently.

In other words, the thickness that saves the most energy and the one that costs the least over time are not necessarily the same.

The particular case of Tunisia

In Tunisia, it would be simplistic to apply a single insulation thickness to every project.

A building in an inland region, exposed to colder winters and large temperature swings, does not have the same needs as a coastal building facing hot summers, higher humidity and different ventilation conditions.

The roof deserves particular attention, as it is heavily exposed to solar radiation.

A study published in the journal Energy looked precisely at the roofs of homes in Tunisia, combining thermal insulation with a reflective coating (cool roof), using dynamic thermal simulations. Its findings are valuable:

• In summer, reflection wins: the gains from a more reflective roof outweigh the winter penalties;
• Moderate insulation, a highly reflective roof: for the Tunisian climate, the authors recommend moderate roof insulation combined with a surface that is as reflective as possible;
• A fast payback on uninsulated roofs: for these roofs, the reflective coating is cost-effective, whether aged or restored, with net savings of up to 44.53 TND/m² over 20 years and a payback period of 3.4 years;
• A precise optimal combination: maximising net savings and including environmental benefits, the study recommends a concrete roof with a restored reflective coating, combined with 5.4 cm of rock wool;
• Results sensitive to assumptions: economic parameters and temperature setpoints noticeably change the optimum.

Read these figures carefully: the 5.4 cm correspond to the configuration studied, a concrete residential roof with a restored reflective coating, under the conditions of the study. It is not a recommended thickness for every Tunisian roof: each project must be assessed with its own structure, local climate and use.

This approach does not mean insulating less in hot regions. It means sizing insulation consistently with the other design strategies.

External wall insulation: a real advantage, but not a universal answer

External wall insulation (EWI) has several advantages when it is properly designed.

It limits heat transfer through the walls and can reduce certain thermal bridges, particularly at junctions between floors and façades. It also keeps much of the walls’ mass on the inside of the insulated envelope.

This mass can help stabilise indoor temperatures and make use of the building’s thermal inertia. But the value of that inertia depends on real conditions: solar gains, ventilation, night-time temperatures and the ability to remove stored heat.

In a Mediterranean building, good external insulation combined with suitable solar shading can be particularly relevant. Conversely, a well-insulated envelope is not enough to prevent overheating if large glazed areas are exposed to the sun without effective protection.

External insulation is therefore not, on its own, a guarantee of low consumption. It must be part of a coherent envelope design.

The choice between external, internal or in-wall insulation also depends on the existing structure, architectural constraints, humidity, condensation risks, the cost of the works and regulatory requirements.

The right question is not which method is always superior, but which one best meets the constraints of the project.

How do you determine the optimal insulation thickness?

Optimal thickness is not a universal physical constant. It is a result that depends on the objectives chosen and the calculation assumptions.

For the same building, you can aim to minimise energy needs, reduce total cost over several decades or limit the project’s environmental impact. The solutions obtained may differ. A rigorous method is to compare several scenarios on the same basis.

Step 1: establish a baseline

You need to know the composition of the existing walls, the surface areas, the characteristics of the glazing, thermal bridges, climatic conditions and how the building is used.

In renovation, actual consumption data is particularly useful, provided it is interpreted in light of the weather, occupancy and temperature setpoints.

Step 2: compare several thicknesses

You could, for example, study variants of 4, 8, 12 and 16 cm of insulation, as in the table above. These values are comparison scenarios, not recommendations for a real project.

For each variant, assess the thermal resistance achieved, the installed cost, the technical constraints and the expected effect on heating and cooling needs.

Step 3: simulate the building as a whole

A dynamic thermal simulation makes it possible to compare annual energy needs, indoor temperatures and overheating risks. It is particularly useful when solar gains, inertia, solar shading and air conditioning play a significant role.

A study based only on the heat losses of a wall cannot predict the annual consumption of the whole building with enough accuracy.

Step 4: calculate the whole-life cost

For each scenario, add the initial cost of the insulation and future energy spending, taking relevant maintenance or replacement costs into account.

Future cash flows must be assessed over a defined period, with consistent assumptions about energy price trends and the discount rate. You then keep the scenario that minimises whole-life cost over the period, while meeting regulatory requirements, comfort and the project’s other objectives.

Step 5: check how robust the result is

An optimal thickness calculated on a single economic scenario may become less attractive if energy prices, the cost of works or conditions of use change. The Tunisian study shows this too: economic parameters and temperature setpoints noticeably change the result.

It is therefore worth testing several assumptions, to check that the choice holds up under different conditions. This is what avoids presenting a theoretical value as a universal truth.

Is the optimal threshold the same for energy, cost and the environment?

No. This is one of the most important points in the reasoning.

The energy optimum

It is the level of insulation that most reduces energy needs, within the objectives and limits set.

When only heat transfer through a wall is considered, increasing thermal resistance keeps reducing it. There is therefore not necessarily a physical maximum corresponding to a single thickness.

The economic optimum

It is the level at which the building’s discounted whole-life cost is lowest over a defined period.

It weighs the cost of the insulation against the value of the savings it brings. Beyond a certain level, the extra cost of additional insulation can exceed the discounted value of the additional savings. This optimum depends on the building, the climate, prices, financing and the length of the analysis.

The environmental optimum

It seeks to reduce impact over the whole life cycle: emissions from energy use, but also those from manufacturing, transporting, installing and disposing of materials.

Thicker insulation can reduce operational emissions while increasing those from materials. The balance depends on the product, the period of use and the energy actually saved.

These three optima can be close, but they are not necessarily identical: the Portuguese study showed this for energy and cost. The right choice is therefore a balanced solution that meets regulatory requirements and the project’s objectives, rather than maximising a single indicator.

What if the best investment is not always another layer of insulation?

Once the main weaknesses of the envelope have been corrected, other improvements sometimes offer a better ratio of cost to benefit. Depending on the building, it may be worth studying:

• External solar shading for glazing;
• Improving the roof and choosing its covering;
• Reducing uncontrolled air infiltration;
• Suitable ventilation, particularly where night cooling is possible;
• Control of heating and air conditioning;
• Coordination between blinds, lighting and thermal equipment.

These solutions do not replace suitable insulation: they complement it. Their relevance must be checked case by case. Night ventilation, for example, is only worthwhile if outdoor conditions can genuinely cool the building, and if security, air quality and humidity are properly taken into account.

This is where the envelope and building controls meet. Motorised blinds driven by sunshine and occupancy, night ventilation triggered automatically when the outdoor air is cooler, zone-controlled air conditioning and energy monitoring before and after the works: these are the functions that make it possible to check, on real data, what the insulation actually delivered, and what remains to be gained. The same whole-life cost reasoning applies to automation itself, as we explained in our article on the profitability of a smart building.

The design of a high-performance building must therefore be holistic: choices about the envelope, equipment and controls are interdependent.

Frequently asked questions

Why can a well-insulated building still use a lot of energy?

Because insulation only reduces losses through the walls. Consumption also depends on glazing, solar gains, air infiltration, ventilation, equipment efficiency and how the equipment is controlled. A well-insulated but poorly controlled building, or one exposed to the sun without shading, can use a lot of energy, particularly for air conditioning.

What insulation thickness should be planned in Tunisia?

There is no single thickness: it depends on the region, the wall or roof, the type of building and its use. For residential roofs, a Tunisian study recommends moderate insulation combined with a reflective coating, rather than a large thickness alone.

Is external wall insulation always the best solution?

No. It limits certain thermal bridges and preserves the walls’ inertia, but its value depends on the structure, architectural constraints, humidity and cost. Nor does it prevent overheating caused by glazing exposed to the sun without solar shading.

Conclusion: do not insulate to the maximum, insulate intelligently

Thermal insulation is one of the fundamental levers of energy performance. Increasing a wall’s thermal resistance reduces heat transfer, but the additional benefits shrink progressively and must be weighed against the cost involved.

The optimal level depends on the climate, the building, its uses, its energy systems and the objectives pursued.

In some projects, additional insulation will be a particularly worthwhile investment. In others, it will be wiser to complement already satisfactory insulation with better solar shading, a more coherent envelope or higher-performance controls.

The best insulation is therefore not necessarily the thickest: it is the one that contributes most effectively to the project’s energy, economic and environmental objectives, while respecting comfort and technical requirements.

At ExaBuilding, this approach is part of a holistic view of the building, in which architecture, the thermal envelope, equipment and controls are designed together, from the audit to design and integration.

Because a building’s performance does not depend on a single isolated solution, but on the coherence of all the decisions that shape it.

Planning a construction or renovation project? Talk to an expert

Sources

Scientific studies

• A. M. Raimundo, N. B. Saraiva, A. V. M. Oliveira, “Thermal insulation cost optimality of opaque constructive solutions of buildings under Portuguese temperate climate”, Building and Environment, vol. 182, 2020, article 107107: https://doi.org/10.1016/j.buildenv.2020.107107
• K. Saafi, N. Daouas, “A life-cycle cost analysis for an optimum combination of cool coating and thermal insulation of residential building roofs in Tunisia”, Energy, vol. 152, 2018, pp. 925-938: https://doi.org/10.1016/j.energy.2018.04.010