Is your energy bill skyrocketing? Moist insulation could be to blame

The job of insulation is to reduce the transfer of heat between the inside and outside of a building, thereby improving energy efficiency. This article delves into the mechanisms of moisture transport, the effect of moisture on thermal conductivity, and the comparative performance of various insulation materials. Research findings show that increased moisture in insulation degrades thermal performance and significant increases in thermal conductivity start to become noticeable at relative humidity levels of 60-80%, and the most dramatic increases occur at even higher humidity levels (85-100%) (Wang et al. 2023). Thus, increased moisture in the insulation layer leads to higher energy consumption and costs. Effective humidity management using systems like VILPE Sense can prevent such issues, maintaining the insulation’s efficiency and optimizing energy usage.

The relationship between moisture and thermal conductivity in building insulation materials is a critical aspect of building physics, influencing among other things energy efficiency. Thermal conductivity is a property of a material that indicates its ability to conduct heat. It is well-documented that moisture significantly impacts the thermal conductivity of building materials as dry air in the pores of insulation materials is replaced by water as moisture content increases. The physical reason is that the thermal conductivity of water is approximately 0.6 W/mK, vastly higher than that of air (0.026 W/mK) (Ducoulombier and Lafhaj, 2017). This means that as moisture content rises, the thermal conductivity of insulation materials increases meaning that the insulation becomes less effective at preventing heat transfer. For example, in winter, more heat will escape from the building, and in summer, more heat will penetrate the building.

Increase in thermal conductivity at higher moisture levels in insulation materials directly impacts energy consumption in buildings. For example, if the insulation’s thermal conductivity increases from 0.03 W/mK to 0.06 W/mK, the rate of heat transfer through the insulation layer will also double. This would directly increase energy bills and environmental impact. Consequently, increase in moisture content can markedly reduce the insulating efficiency of materials​ and it highlights the importance of low humidity levels in the insulation.

However, insulation materials differ in their sensitivity to moisture. Here, we report on research findings of the comparative performance of four insulation materials effect of moisture on thermal conductivity. The results highlight the importance of considering moisture sensitivity, early detection of moisture problems and sufficient ventilation of structures. Ventilation of structures is particularly important for moisture sensitive insulation material.

Moisture accumulation in the roof structures

In general, there are two main reasons for moisture accumulation in roof constructions: breaches or structural damages that occur through diffusion and capillary action (Ducoulombier and Lafhaj, 2017). Breaches occur when, for instance, someone steps on a nail and breaks the roofing material or due to a building error, such as faulty installed pass-throughs. Structural damages occur because of for instance air leaks or poor structure ventilation. During winter, indoor temperatures are typically higher than outdoor temperatures, while outdoor relative humidity is higher than indoor humidity. This creates opposing gradients that drive water vapor from the inside to the outside. Water vapor diffuses through material pores, where it can be adsorbed onto pore surfaces, forming a thin film. As humidity increases, the film thickens, reducing the adhesive forces and enabling more mobile water molecules to diffuse. When pores become nearly saturated, capillary transport predominates, reversing the moisture flow direction and leading to moisture accumulation within the structure​.

The role of relative humidity for understanding thermal conductivity changes

Understanding relative humidity (RH) is crucial for interpreting the effects on thermal conductivity. Relative humidity measures the amount of moisture in the air relative to the maximum it can hold at a given temperature, directly impacting moisture absorption by materials. More specifically, at 100% relative humidity the air is fully saturated with water vapor and cannot hold any more moisture (Wang et al. 2023). At this point, any additional moisture will condense into liquid water. However, it does not mean that the material is pure water, but it does indicate that the material’s pores are filled with water vapor, and liquid water may start to form within the material.

Relative humidity differs from absolute humidity that measures the total moisture content in the air. Absolute humidity does not account for temperature variations that affect how much moisture materials will absorb. Therefore, relative humidity is more relevant for understanding thermal conductivity changes.

Experimental findings on different insulation materials and thermal conductivity

Wang et al. (2023) compared the hygrothermal properties of glass wool, rock wool, expanded polystyrene (EPS), and wood fiberboard. The results highlighted significant variations in how these materials respond to moisture (See Table 1).

Glass and rock wool: Both materials showed an increase in thermal conductivity with rising moisture content. Due to its fibrous and porous structure, glass wool absorbs moisture readily, which can lead to a rapid increase in thermal conductivity even at lower humidity levels. Rock wool has slightly better moisture resistance than glass wool due to its denser structure, which slows down the rate of moisture absorption. As the materials become more saturated, the rate of increase in thermal conductivity slows down​.

Expanded polystyrene (EPS): EPS exhibited a smaller change in thermal conductivity with moisture compared to fibrous materials. Its closed-cell structure limits moisture penetration, resulting in a more gradual increase in thermal conductivity even at higher moisture contents​. EPS has good moisture resistance, making it a suitable choice for environments with high humidity. Its thermal performance remains relatively stable across a range of humidity levels.

Wood fiberboard: This bio-based material demonstrated higher sensitivity to moisture. Its natural fibers absorb moisture readily, leading to a significant increase in thermal conductivity. The material’s hygroscopic nature makes it particularly vulnerable to moisture-related performance degradation​. This makes it particularly sensitive to changes in relative humidity, impacting its thermal performance.

In conclusion, the results implicate the importance of early detection of moisture problems and sufficient ventilation of structures so that the structure can dry. Moreover, ventilation of structures seems to be particularly important for moisture sensitive insulation material.

How VILPE Sense helps maintain low thermal conductivity 

The VILPE Sense product family offers solutions for a wide range of moisture problems, which is crucial for maintaining low thermal conductivity in insulation materials. The product VILPE Sense humidity control system promotes adequate ventilation and drying of the insulation layer, thereby safeguarding against moisture damage from structural causes, including vapor barrier leaks. The system consists of sensors that collect data on moisture conditions and the roof fan which the system controls to operate at optimal performance.

Here is how it works:

  1. Continuous monitoring: VILPE Sense uses sensors to continuously measure the humidity levels in your building’s insulation. This helps detect critical high relative humidity levels that lead to higher thermal conductivity.
  2. Early detection: If the sensors detect high moisture levels, the system alerts you immediately. Early detection allows you to address moisture issues before they become severe, ensuring that the insulation remains dry and effective.
  3. Demand-based ventilation: VILPE Sense humidity control automatically adjusts ventilation to reduce humidity as needed. By smart ventilation, it helps keep the insulation dry, which maintains its low thermal conductivity and ensures your building stays energy efficient.

By keeping the insulation dry, VILPE Sense humidity control ensures that the material’s ability to resist heat and cold remains high, which helps keep your building warm in the winter and cool in the summer without using any extra energy.


  • Relative humidity. Relative humidity is a measure of the amount of moisture in the air compared to the maximum amount of moisture the air can hold at a given temperature. It is expressed as a percentage, with 100% relative humidity meaning the air is fully saturated with water vapor and cannot hold any more, leading to the possibility of condensation or precipitation.
  • Absolute humidity. Absolute humidity is the total mass of water vapor present in a given volume of air, usually expressed in grams of water vapor per cubic meter of air (g/m³). Unlike relative humidity, it does not depend on temperature; it simply measures the actual amount of water vapor in the air.
  • Thermal conductivity. Thermal conductivity is a property of a material that indicates its ability to conduct heat. It is defined as the amount of heat that passes through a unit area of the material, with a unit temperature gradient, over a unit time, typically expressed in watts per meter per degree Celsius (W/m·°C).

Insulation material:

  • Glass wool is an insulation material made from fibers of glass arranged into a texture similar to wool. It is produced by melting glass and spinning it into fine fibers, which are then bound together to form mats or rolls.
  • Rock wool, also known as mineral wool or stone wool, is an insulation material made from volcanic rock, typically basalt. It is produced by melting the rock and spinning it into fine fibers, which are then compressed into mats, rolls, or boards.
  • Expanded polystyrene (EPS) is a lightweight, rigid insulation material made from polystyrene beads that are expanded and fused together. Despite its lightweight nature, EPS offers significant structural strength and can be easily cut and shaped to fit various applications.
  • Wood fiberboard is an insulation material made from wood fibers that are bonded together using heat and pressure to form dense, rigid panels.