Why does the density of insulation change in unused attic space?

Identical R-value ≠ identical comfort: why insulation density really makes a difference in unused attic spaces

In thermal insulation, thermal resistance (R) is often the first criterion analyzed. It determines regulatory compliance and allows for quick comparison of materials. However, even with equivalent thermal performance, the comfort experienced by occupants can vary significantly.

The reason is simple: R only describes static behavior in winter. It does not fully reflect the dynamic thermal behavior of a material in summer.

In unused attic space, this nuance becomes decisive.

Thermal resistance (R): an essential indicator... but only part of the picture

The thermal resistance of a material is calculated using the following formula:

R = thickness / thermal conductivity (λ)

It measures the ability of an insulator to slow down heat transfer through a wall. The higher the R-value, the more the wall limits heat loss in winter.

Historically, French thermal regulations have relied heavily on this indicator. Even today, RE2020 imposes high energy performance levels.

However, RE2020 also introduces a new requirement: summer comfort, measured using the DH (Degree-Hours of Discomfort) indicator.

In other words: limiting winter losses is no longer enough. Summer overheating must also be controlled.

See our infographic: Extreme temperatures and building comfort

Thermal phase shift: the key to summer comfort

The key parameter for summer comfort is thermal phase shift.

The phase shift corresponds to the time it takes for a heat wave to pass through a material. The longer this time, the more the peak outdoor heat is delayed inside the building.

In concrete terms:

• If the outside temperature peaks at 2 p.m.,
• An insulator with low phase shift will transmit this heat at the end of the afternoon.
• An insulator with a high phase shift will transmit it late in the evening or even at night, when natural ventilation can remove it.

This mechanism is essential in unused attic spaces, which are particularly exposed to solar gain.

Why does density influence phase shift?

Thermal resistance depends mainly on:

• The thickness
• Thermal conductivity (λ)

On the other hand, the phase shift depends mainly on:

• The density of the material
• Specific heat capacity
• The thickness

Density directly influences a material's ability to temporarily store thermal energy. The denser a material is, the more heat it can absorb before releasing it.

Two insulating materials may therefore have the same R-value but behave very differently in summer due to their different densities.

In other words:

Identical R ≠ identical phase shift ≠ identical comfort

What institutional studies show

The work published by ADEME in 2024 highlights that insulation materials with higher density maintain more stable performance in real-world conditions, particularly during periods of high heat.

For its part, the CSTB points out in its technical publications that the dynamic thermal behavior of walls depends heavily on the thermal capacity of the materials.

These data confirm a physical reality: a denser material improves the damping of thermal variations.

Focus on unused attic space: a strategic area

According to ADEME, the roof accounts for up to 30% of heat loss in a poorly insulated home in winter.

In summer, it is also the main surface exposed to solar radiation.

In single-family homes, low structural inertia further increases sensitivity to overheating.

The choice of insulation for unused attic space therefore becomes strategic, not only to achieve the regulatory R-value, but also to ensure lasting thermal comfort.

See the Igloo memo on summer comfort

At the same R-value, what does a denser insulation provide?

With equivalent declared thermal performance:

• It further delays heat penetration,
• It improves summer comfort,
• It helps stabilize the indoor temperature in winter.

This stability reduces temperature fluctuations and improves overall comfort for occupants.

Incorporating density into technical analysis

Density does not replace thermal resistance. It complements it.

A relevant analysis must include:

• Thermal conductivity (λ)
• The targeted R
• Density
• Specific heat capacity
• The climate context
• The building's layout

FDES data sheets and technical opinions validated by the CSTB are reliable sources for objectively comparing materials.

Conclusion: moving beyond a single reading of the R

Thermal resistance remains a fundamental indicator. However, it is not sufficient to describe the overall comfort of a building.

In unused attic spaces, density and thermal capacity directly influence phase shift and therefore summer comfort.

In a context of increasing heat waves and stricter regulatory requirements, it is essential to incorporate these parameters into technical reasoning.

Identical R ≠ identical comfort.