Vapor and wind insulation: purpose, application, which side to lay

Recommendation points

• Protective films and membranes: properties and differences
• Varieties and specifications
• What heaters require protection
• Basic concepts about the device of the insulation cake
• Subtleties of installation of protective films

Porous and vapor-permeable heaters often need protection from moisture saturation and weathering. Today we will share with the readers the general principles of wind and steam protection and tell you how to properly arrange protected insulation cakes.

Protective films and membranes: properties and differences

The main subject of our discussion today is the separation of air currents, slowly but almost always inevitably circulating within the structure of the building envelope. The membranes and separators used can either have an absolute degree of containment, like a polyethylene film, or restrict air flow, retain water vapor or only moisture droplets.

Heaters made of glass wool or polymer with an open type of cells greatly lose in thermal insulation properties if saturated with moisture. But if for synthetic materials this phenomenon is reversible, then cotton wool usually gets lost and does not restore its structure when it dries.

Wind and vapor barriers are mainly used in the process of assembling the “pie” of enclosing structures, since it is rather rare to separate air flows inside buildings. The exceptions are rooms with high natural humidity: steam rooms, showers, pool rooms.

Basically, the task is not reduced to the complete locking of moisture inside the premises, but to its accumulation and removal at a controlled point. This can be condensation behind an external hydro-barrier, which allows water vapor to pass through, but not water droplets, followed by draining or drying with street air..

Ventilated facade cake: 1 – ventilation facade system; 2 – mineral insulation; 3 – vapor barrier

More complex cakes will trap most of the moisture indoors. In this case, the boundary between humid and dry air is maintained at a temperature where water cannot condense. The operation of the entire complex of protection of heaters and enclosing structures and the operation of the internal ventilation system are inseparable from each other, they should be developed together, establishing a mutually beneficial mode of operation.

Varieties and specifications

For continuous hydrobarriers (films), the main parameter is the amount of diffuse penetration: water vapor particles can leak between polymer chains, especially if there is a pressure difference.

Membranes that allow air and steam to pass through, but retain water in a liquid state, differ in their normal throughput. They are used to forestall the weathering of insulation particles by intense air currents, while maintaining sufficient ventilation. These are a kind of check valves, but they only work if the leaving moisture is condensed out or removed in another way..

Vapor barriers are used to limit the penetration of moisture into the insulation or the supporting structure. Their effectiveness is determined by the ability to transmit steam, expressed in grams per area and per unit time. Usually, it is the excess of water vapor that is removed through the vapor barrier, thus maintaining a comfortable level of humidity inside the premises..

In addition to the main parameters for protective membranes, strength (breaking load) and resistance to various kinds of influences, from chemical to temperature and fire, are important. The most versatile material for most types of barriers is polypropylene and polyethylene..

The material itself can have a woven structure (membranes), characteristic of wind protection and high-capacity vapor barriers, or a continuous cellular structure (film), typical of more expensive vapor barriers with precise values ​​of the moisture transmission volume. Another difference between films and membranes is the need to provide the former with space for ventilation, while the latter are not designed to retain condensate.

What heaters require protection

The only exception when protective barriers for insulation are not used is the example of using synthetic materials such as PU foam, EPS, or foam glass. These materials are able to take on the dew point and tolerate freezing well even when saturated with moisture, which would be detrimental for supporting structures..

However, it is worth remembering that if the synthetic insulation has zero permeability and there is a risk of the dew point going deep into the supporting structure, the flow of moist air from the inside will still have to be limited. This is one of those cases where a high-quality ventilation device is considered a must..

In general, all types of stone wool and fiber insulation similar to it require protection. In this case, the permissible amount of steam passed through should be directly proportional to the density of the insulation.

Basic concepts about the device of the insulation cake

When composing an insulation cake with high-quality protection, you should adhere to three basic rules:

1. Ensure sufficient water vapor permeability through the building envelope. Recall that the throughput is calculated as an excess of moisture, which the ventilation system cannot cope with. So that the throughput is not limited by other sections of the wall, materials are arranged according to the degree of increase in their permeability from the inside out..
2. Limit the penetration of water vapor to such a value at which it will naturally evaporate without harming the insulation and the supporting structure. As a starting point for calculations, the difference in relative humidity inside and outside the building and the division of the total mass of excess moisture by the surface area of ​​the walls, ceiling and floor is taken.
3. Protect the insulation located on the outside of the wall from direct contact with water. Damage can be caused by both rainwater, which migrates from the surface into the depths of a wet facade, and condensation droplets formed on the wrong side of metal or plastic sheathing / roofing.

Subtleties of installation of protective films

Let’s clarify the difference between films and membranes:

1. Films designed to act as a moisture barrier to trap condensed and migratory moisture should be provided with a ventilation gap on both sides.
2. Vapor barriers can adjoin insulation and other structures closely, but not on the side with which condensation is potentially possible. That is why they are mounted mainly on the inside of the wall..

Since the design of the cake is carried out under the initial condition that there is a pressure difference between a warm room and a cold street, the tightness of the barriers cannot be neglected. In particular, vapor barriers should not be fastened with staples, they are glued to the crate or supporting structure, fixing them with an overhead shingle. The thickness of the fixing strips is selected based on the required amount of space for ventilation.

The joints between the canvases must be glued with special adhesive tape that is resistant to getting wet. If it is planned to drain off the liquid along the hydrobarrier or vapor barrier, the canvases are placed horizontally, with each upper row superimposed on the previous one from the “wet” side of the order of 100–150 mm with obligatory gluing. This technique is widely used in the preparation of a cake for the insulation of sloped roofs and mansard roofs..

When the presence of a crate on the outside of the vapor barrier is undesirable, it is allowed to sew it to the base with staples or nails. But at the same time, each of the attachment points must be covered with a flap of aluminum adhesive tape..

Pay special attention to corner fittings. The vapor barriers are glued to each other between the wall and ceiling. At the abutment to the floor, the vapor barrier is glued to the enclosing structure after preliminary priming, but without turning it onto a horizontal plane. When framing window openings, the insulation is wrapped on the slopes and tightly pressed against the foam seam.

The specified installation requirements apply not only to vapor barriers, but also to all types of protective membranes. Despite the different principle of operation, they all require the preservation of integrity in order to perform their functions. Please note that for some types of waterproofing and vapor barrier that trap condensed moisture from draining, it may matter which side the membrane is turned to the condensate formation zone.