Which side to lay the vapor barrier to the insulation

Recommendation points

• Temperature and humidity conditions
• Vapor barrier and its functions
• Correct positioning and installation
• Exceptions

In a thermal protection device, one of the most important issues, if not the most important, is the correct protection of the insulation. Today we will briefly and easily tell you on what principles moisture accumulation in the wall occurs, and on which side of the insulation a vapor barrier should be placed to prevent moisture condensation.

Temperature and humidity conditions

The design of thermal protection has two main goals: to reduce heat loss through the enclosing structures and at the same time to prevent the formation of a dew point in the layer of insulation or carrier material. This balance is quite delicate, but the main tendency is to determine the dynamics of the drop in the condensation temperature and ensure that it does not coincide under any circumstances with the actual temperature of a single layer of the wall. In order not to burden your time with studying formulas and searching for reference data, you can use online calculators for these purposes..

Modern heaters such as XPS and PUR are calculated according to the simplest scheme. They have a zero ability to accumulate moisture, there is simply nothing to condense in them, respectively, the entire route of the dew point can be placed entirely in the insulation belt. It is more difficult with mineral heaters: they are very much afraid of getting wet, therefore moisture condensation must be excluded by all possible means, which becomes more and more difficult to do with increasing thickness of thermal protection. In such cases, air convection comes to the rescue: if a sufficiently intensive air exchange remains in the condensation zone, the moisture will evaporate immediately without damaging the insulation.

Vapor barrier and its functions

The practice of drying mineral wool with street air is found both in roof insulation and in ventilated facades. The main problem of this method is that the intense flow and evaporation of moisture contribute to its even faster penetration from the inside of the building. To ensure that the moisture intake does not exceed its ability to evaporate, the saturation of the insulation with steam is limited using special materials..

Roof insulation: 1 – filing; 2 – vapor barrier; 3 – rafters; 4 – insulation; 5 – windproof membrane; 6 – counter lattice; 7 – crate; 8 – roof covering

There are also several nuances in this business. For example, on the reverse side of the insulation loop, there is an increased saturation of air with moisture, which must be taken into account in a new recalculation of the dew point route. On the other hand, you should correctly select the throughput of the barrier: no more than it can evaporate from the insulation to the street, but not too small for the wall to continue to “breathe”.

Ventilated facade of a house made of expanded clay concrete: 1 – wall made of expanded clay blocks; 2 – ventilated facade subsystem; 3 – moisture from the room; 4 – distance strips for the membrane; 5 – vapor-permeable membrane; 6 – insulation

The vapor barrier should not be placed in an area where moisture condensation is possible, otherwise the latter will most likely settle on the inside of the barrier. This behavior is sometimes used to their advantage: if the vapor barrier has air from the side of the room, the critical structures will not get wet, while the vapor permeability of the membrane is naturally regulated.

Insulation cake of the frame wall of the house: 1 – outer cladding (for example, siding); 2 – crate; 3 – wind protection; 4 – insulation; 5 – wall frame; 6 – vapor barrier; 7 – crate; 8 – interior decoration

Correct positioning and installation

The vapor barrier should not be confused with windproof films. In contrast to them, the vapor barrier is always laid on the side of the insulation, from where the warm and humid air flows. Therefore, the golden rule is this: vapor-limiting materials should always be placed only under a layer of interior decoration..

The obvious problem here is that not all types of building structures allow such placement. What can you do with a brick wall, which is insulated with mineral wool from the outside? It is too expensive and unreasonable to specially equip hollow cladding. In such cases, you can install a vapor barrier next to the insulation, but, again, only from its warm side. In this case, usually the vapor barrier is located in the calculated condensation zone, respectively, it needs to be provided with air or more accurately determine the appropriate throughput.

Brick wall insulation: 1 – facade cladding (ventilated facade); 2 – wind protection; 3 – insulation; 4 – brick wall; 5 – vapor barrier; 6 – interior decoration; 7 – removing moisture from the room

The main difficulty in such extraordinary situations is the correct fastening of the vapor barrier. Under normal conditions, it is fixed with brackets to the frame insulation system, while the joints between the canvases are connected hermetically. The latter can be achieved by folding into a joint and double-sided gluing, or by an overlap with a lubricant with special mastic..

Exceptions

There is none of them. A different layout or confusion in the functions of the material almost always leads to deterioration of thermal protection. Therefore, the vapor barrier is placed strictly on the side of the heated room. It can be argued that in summer mode, this protection will work in the opposite direction, but the manifestations of the “summer” dew point are extremely rare and occur only in the coastal zone or tropical climate.