Why can an insulated house overheat in summer? Thermal conductivity and the complete roof

A house can overheat in summer even when its roof is insulated. The cause is not simply that mineral wool stops working in warm weather. Insulation slows heat transfer, but indoor temperatures also depend on sunlight through windows, shading, ventilation, internal heat gains and the complete roof construction. Thermal conductivity λ can vary with temperature and moisture, so product data must be interpreted under the appropriate conditions. PIR, mineral wool, EPS, XPS and spray foam should be compared by their declared and design properties, thickness and intended use. This guide explains those distinctions without treating one insulation family as a universal solution.

What does thermal conductivity λ mean?

Thermal conductivity describes how readily a material conducts heat. It is expressed in W/(m·K). For the same thickness and conditions, a lower value means greater resistance to conductive heat flow. It is not the U-value of an entire wall or roof.

For a homogeneous layer, the simplified relationship is R = d/λ, with thickness d in metres. Whole-building elements also include other layers, surface resistances, joints and thermal bridges. Use the declared or design values appropriate to the calculation.

Declared and design values are different

A declared value relates to specified reference conditions. A design value accounts for the relevant application and conditions. Temperature and moisture corrections must come from recognised methods or product documentation, not an arbitrary percentage.

Temperature correction: use the right temperature

The temperature of a sunlit roof covering is not automatically the mean temperature throughout its insulation. Inner and outer conditions, layer thickness and the construction determine the temperature distribution.

ISO 10456 describes procedures for converting thermal values between conditions. Its scope includes design ambient temperatures from −30 to +60°C, but its stated temperature conversion coefficients apply to mean temperatures between 0 and 30°C. These are different ranges. Do not extrapolate a graph to a roof-surface temperature without checking the method’s validity.

For a particular product, use the relevant declared value, verified design conversion and assembly calculation.

Mineral wool and cellulose do not stop insulating in summer

Their thermal conductivity can change with conditions, but there is no basis for saying that every mineral-wool roof loses 30% of its insulation or that 15 cm universally becomes equivalent to 10 cm in summer. Product type, density, moisture and temperature must be considered.

Decimal placement matters. A value such as 0.033 W/(m·K) is not 0.33 W/(m·K): the latter is ten times larger. Use the exact value from the chosen product’s documentation. Do not treat an example for one wool product as the value for all stone wool, glass wool, wood fibre or cellulose.

A well-detailed mineral-wool roof can provide thermal insulation in both winter and summer. Summer comfort also requires control of solar gains and removal of accumulated indoor heat.

EPS and XPS: compare the actual product and application

EPS and XPS have different structures and product variants. Neither name supplies a single universal thermal conductivity or a fixed percentage loss at summer temperatures.

Check the declared λ, thickness, compressive properties, moisture exposure and permitted use of the actual board. The layer arrangement and water-management details are particularly important for roofs. A material’s low λ alone does not make it suitable for every roof construction.

PIR and phenolic boards: low λ is useful, but not a complete design

Rigid PIR and phenolic boards can provide substantial thermal resistance within limited thickness. Their properties still depend on the specific product, facings and conditions.

For example, the termPIR AL technical sheet gives λD = 0.022 W/(m·K) for its stated thickness range. That is a value for the identified product, not a declaration for every PIR board. It must not be written as 0.22.

Over-rafter boards can provide a continuous layer across the rafters, but joints, edges and fixings still require careful detailing. Boards do not automatically replace structural decking, underlay or waterproofing. Follow the specified roof system and compare complete installed costs rather than asserting that one method is always cheaper.

PUR spray foam may be open-cell or closed-cell

It is incorrect to describe all PUR spray foam as open-cell. Both open-cell and closed-cell systems exist and have different thermal, vapour and moisture properties. Factory-made PIR boards are another product form.

Compare the documented properties of the actual products and the required installation thickness. Spray application also needs appropriate substrate conditions and application control. Boards require suitable cutting, joints and fixing.

Do not assign one density, blowing agent or λ to an entire material family. Product documentation and the proposed construction determine the appropriate choice.

What should you check if an attic overheats?

• Sunlight entering through rooflights and other glazing, and the available external shading.
• Whether ventilation can remove heat when outdoor conditions are cooler.
• Internal heat gains from occupants and equipment.
• Continuity and thickness of insulation, gaps, air leakage and thermal bridges.
• The roof’s complete thermal and moisture design.

Insulation is part of the solution; it is not active cooling. Replacing wool with PIR alone cannot guarantee a cool room. Where overheating persists, assess the complete room and roof rather than choosing solely by a headline λ value.

Prepare the information needed for a quotation

Send SteelProfil the roof area, proposed layer arrangement, available insulation thickness and any project requirements. We can help identify suitable products and prepare a quotation. The final thickness, moisture detailing and fire requirements must follow the project and product documentation.