Contrary to what happens with liquid water, the water vapor spreads more easily inside the structures, when a gradient of pressure and temperature is established between the sides of the material.
The water vapour modifies the behaviour of the insulator, increasing the thermal conductivity value and thus increasing the energy losses and the risk of condensations. The equivalent thermal conductivity of the water is 0,56 W/mK, a very high value if we compare it to the values declared by extruded polystyrene (XPS) 0,032 - 0,036 W/mK, or those declared by Poliiso ® 0,022-0,028 W/mK. This is the reason why a small amount of water, even less than 5% of the total volume of the insulator, results in a considerable increase (worsening) of the conductivity.
We summarize the most important basic concepts to understand the control of the passage of water vapour through thermal materials and insulators:

 

• "Water vapour permeability (δp)": Measures the behaviour of a material at the passage of moisture, i.e. the amount of water vapour that crosses, per unit of time, a unit of surface of the product, for a sample of unit thickness, when there is a difference in unit vapor pressure.         

 

The different and complex units of measurement used in each country have given rise to a dimensionless factor (without unit) which is the factor of water vapour resistance or µ factor.

 

 

                        δ air ( vapour permeability of water in air )

         µ =  ----------------------------------------------------------------------------

                   δ product (vapour permeability of water in the product)

 

 

By their nature, all materials, except glass and metals, are permeable to water vapor. However, a good insulator must avoid the penetration of water vapour, in order to avoid a significant increase in thermal conductivity during the lifetime of a product. If we consider an open cell thermal insulator and low-factor µ, in this case the water vapor will penetrate quickly into the material and moistens it. An insulating material with closed cellular structure, instead, and high factor µ, is characterized by a high resistance of the penetration of the water vapor.

 

The table below compares the behaviour of different insulating materials:

 

PROPERTIES	EXTRUDED POLISTYRENE(XPS)	EXPANDED POLISTYRENE(EPS)	POLYURETHANE FOAM (PIR/PUR) with permeable coatings	POLYURETHANE FOAM (PIR/PUR) with water proof coatings	MINERAL WOOL (MW)	WOOD WOOL
Thermal conductivity	0.032 - 0.038	0.033 - 0.050	0.025 - 0.028	0.022	0.034 - 0.044	0.038 - 0.050
Water vapour resistance factor	80 - 150	20 - 60	30 - 50	∞	1 - 2	4 - 8