Humidities are the most common pathologies in buildings and, within these, those caused by condensations. Therefore, humidity control in construction is a factor which can’t be underestimated.
What is the steam flow?
In winter, indoor air in buildings has a steam pressure much higher than that in the outside air. This difference in steam pressure generates a flow of steam through the beneficial envelope, since it contributes to improving the healthiness of the interior environment and the durability of the envelope.
However, it is necessary to control its intensity, since excessive flow could cause superficial or interstitial condensations somewhere in the interior of the enclosure, contributing to its deterioration, the deterioration of adjacent materials and the proliferation of microorganisms and moulds.
We can distinguish the following types of humidity by condensation:
- Interior superficial condensation.
It occurs when the interior superficial temperature is lower than the dew temperature. The most obvious case is due to high steam production in bathrooms and kitchens, or due to the impermeability of the glass in the windows. Its main symptom is the drip. But it can also occur in places with a moderate steam production and with more porous surface finishes, such as bedrooms or living rooms. In this case, the cause is usually insufficient insulation of the enclosure and, above all, the existence of thermal bridges.
- Interstitial condensation.
It occurs when the physical phenomenon happens at some point inside the enclosure. This second case of condensation depends not only on the flow of water steam through the wall and its temperature gradient, but also on the constitution of the enclosure itself, the arrangement of the different layers that make it up and the permeability to water steam of each one of them, as well as its insulation capacity.
The interstitial and internal condensation may appear simultaneously, since part of the water steam continues to move from the inside to the outside despite the fact that condensation has already occurred on the interior surface of the enclosure.
The case of sprayed polyurethane foam
Sprayed polyurethane foam acts as a humidity regulating membrane –being impermeable to water, it is however permeable to water steam.
The degree of water steam permeability can be reduced by increasing the density of the foam. Thus, in any climatic condition, we can find the suitable level of resistance to the flow of water steam to avoid condensation and allow maximum steam flow.
The values that characterize the water steam resistance are: the dimensionless factor of resistance to water steam diffusion MU (W), or, the resistivity to the diffusion of water steam.
For sprayed polyurethane foam, with densities between 30 and 60 kg/m³, the resistance to the transmission of water steam ranges between 330 and 825 MN·s/g·m. That is, a resistance factor to the diffusion of water steam, W, between 60 and 150.
When should a steam barrier with sprayed polyurethane foam be used?
In those applications in which there is an element very resistant to the flow of water steam in the cold face of the enclosure, there will be a greater risk of condensation inside the insulation. This situation can occur in the polyurethane foam when projecting inside a sheet metal enclosure, or when placing on the foam an asphalt cloth in a cover, for example. In these cases, the interposition of a steam barrier between the hot face and the polyurethane foam will be necessary.
The most suitable steam barriers are in situ barriers, such as asphalt or bituminous emulsions, since they are chemically compatible with polyurethane, and maintain the advantages of adhesion and continuity of the latter, guaranteeing humidity control in buildings.