- How membranes work
- What is the insulation afraid of?
- Special properties of films and membranes
- Types of building membranes
- Frequently asked questions about the installation of building membranes
- On which side of the insulation to attach the membrane?
- Which side to lay the membrane?
- Do I need a ventilation gap near the membrane?
- What should be the overlap of canvases?
- Do I need to glue the joints? If so, what?
- How to fix the membrane?
- How long can the membrane be left open?
- Instead of an epilogue
The house is insulated with mineral wool, serious funds and a lot of time have been spent, but for some reason there is no expected effect. It’s cold in the rooms, the walls and the roof are damp … This is a fairly common situation for irresponsible builders and too economical customers. But it was only necessary to take one more step – to close the heat insulator with membranes …
Every year, modern housing is becoming more complex and technologically advanced. It is not surprising, because recently the requirements for insulating materials, characteristics of almost all elements of buildings and structures have increased significantly. The issues of thermal insulation of residential buildings, in particular, in many countries have become the subject of government regulation. As a result, multi-layer structures with the use of fiber insulation have become widespread. These are frame external walls, ventilated facades, insulated pitched roofs and ceilings.
However, a mineral wool insulator itself needs reliable protection. The fact is that wind pressure, atmospheric moisture, vapors from the premises significantly reduce the thermal characteristics of mineral wool and the building as a whole. The use of construction films and membranes allows to preserve the design efficiency of multilayer structures, to avoid the formation of condensation on the building elements. At one time, membranes became a real breakthrough in building heat engineering, now it is impossible to imagine a residential building built without the use of this material. Membranes have proven themselves in practice, they continue to improve.
How membranes work
What is the insulation afraid of?
It is believed that rock wool does not absorb water, but it contains many pores and air channels, so that moisture can move inside the material and stay inside it. The mass of stone wool insulation can increase up to 5% of its own weight. Moisture displaces air from the fibers – the thermal insulation characteristics fall (by 20-30% even with one percent humidification, many technologists say), cold bridges are formed. With significant temperature fluctuations, the water repeatedly freezes and melts, expanding, destroying the internal structure of the insulation. If the enclosing and drainage structures are working properly, water can diffuse into the cotton wool from the premises, as a product of human activity, or outside – with humid air.
Air can penetrate from outside into insulated facades and roofs, and then into rooms under the influence of wind and temperature pressure. The wind not only pushes against the walls, but also creates swirls. Somewhere cold and humid air is blown into the structure, somewhere it is sucked out of the insulation, taking heat with it. This is how unplanned infiltration of structures occurs with a deterioration of their thermal resistance..
Ventilated roofs and facades have air spaces that act as convection channels. The air, passing through the ventilation gaps, even at a low speed of movement, “draws” heat from the unprotected cotton wool, which immediately reduces the thermal insulation of the building as a whole to 30–40% of the design values. Moreover, convective air currents are capable of “weathering” binders, as well as fibers of most types of cotton wool, also destroying the structure of the insulation.
Special properties of films and membranes
The main task of building membranes is to protect building structures from wind and atmospheric moisture. But at the same time, the films used on the outer walls and roofs must allow water vapor to pass through them from the premises to the outside. From the point of view of physics, any membrane is a semi-permeable film, a shell that separates two media, regulating the unidirectional transport of substances from one zone to another.
The main feature of most building membranes is the presence in their structure of diffusion layers with microperforation and micropores, which are able to conduct water vapor in one direction. Most often, vapor permeable membranes have one thin functional layer and one or more protective layers that provide physical and chemical stability..
Some membranes (often called construction films) do not allow steam or water to pass at all. They consist of several non-perforated polyethylene layers, usually on a mesh backing. This is the so-called “vapor barrier”.
When choosing construction films and membranes, special attention should be paid to two main consumer properties:
- vapor permeability
- moisture resistance
Construction membranes are made from synthetic fibers (polypropylene, polyethylene) in the form of textile woven or non-woven fabrics. Depending on the tasks, building membranes can have a single-layer or multi-layer structure, including with a reinforcing mesh made of polyethylene fibers or an additional aluminum coating. With a small thickness, the membranes have very high strength and low elongation. They are resistant to ultraviolet radiation for a certain time, are not affected by fungi and microorganisms..
Some manufacturers offer membranes that not only regulate humidity conditions, but also have their own resistance to heat transfer, which makes it possible to compensate for heat losses in the area of air spaces. These are multilayer needle-stitched materials with a thickness of 10-15 mm, made on the basis of polypropylene.
The fire resistance of building films is also a rather relevant issue, which is solved in two ways. There are membranes, the polymeric materials of which in the mass contain fire retardants, the second option is the impregnation of finished cloths or the application of protective compounds on their surface.
Another important nuance is the service life of the membrane. Obviously, the membrane should work as long as the enclosing structure as a whole. You should not use materials whose manufacturers are silent about the service life, or limit it to 10-15 years.
The performance of the membranes is significantly reduced due to the aging of the material under the influence of high temperatures. Common declared indicators “up to + 80 °” are not always enough, especially in an insulated metal roof, where temperatures can reach much higher values.
So, a building membrane is a film that lets in or out vapors, but always stops water and wind. This is the basis of film technology.
Types of building membranes
Depending on their purpose and, accordingly, some structural features, building membranes are divided into:
- vapor barrier
- vapor permeable
A vapor barrier is installed from the inside of the insulation, it must isolate the cotton wool from moisture vapor that occurs in the premises of the building. An example of an application is a heat-insulated roof or an overlap of the “attic” floor, where the cotton wool from below must be covered with a film. Also, a vapor barrier is necessarily used when insulating walls from the inside. The vapor barrier membrane has no pores and perforations, the lower its vapor permeability, the better. These materials are either reinforced or unreinforced polyethylene film, sometimes with a layer of aluminum foil. Note that the use of vapor barrier significantly increases the level of humidity in the building, so special attention will have to be paid to the ventilation of the premises..
Films with an anti-condensate coating can be considered a separate type of vapor barrier membranes. They are used under roofing materials that are afraid of corrosion – corrugated board, galvanized iron, some types of metal tiles without an inner coating. Such a membrane does not allow vapors to pass to vulnerable metal elements. The anti-condensation film is laid with a rough textile (absorbent) layer downwards, where moisture accumulates and is gradually removed without flowing back into the insulation and without contacting with the metal. There must be a gap of 20-60 mm between this membrane and cotton wool..
Vapor permeable (vapor release) membranes are used on the outside of the insulation. They serve as protection against wind pressure on the enclosing structures and are an auxiliary waterproofing layer in pitched roofs, as well as facades with non-hermetically connected cladding elements. Due to the fact that such films act as a buffer between the insulation and the environment, it is necessary that they freely pass moisture from the cotton wool into the ventilated space. A certain vapor permeability of these materials is ensured by the presence of microperforations and micropores. Naturally, the more active the diffusion of steam outward, the better, the drier and more effective the insulation will be. In accordance with the degree of vapor permeability, membranes are divided into:
- pseudodiffusion (up to 300 g / m22 per day)
- diffusion (300-1000 g / m2)
- superdiffusion (from 1000 g / m2)
Pseudo-diffusion membranes have good waterproofing characteristics, therefore they are often used as external under-roofing, and with the organization of a mandatory ventilation gap under them. The use of such films as an external vapor barrier for the facade is a mistake due to the minimum permissible throughput. The fact is that in dry weather, the micropores can become clogged with dust from the ventilation gap. As a result, moisture is not completely removed from the insulation, and condensation is possible.
Diffusion and superdiffusion membranes are free from this disadvantage. Here the vapor permeability characteristics are presented, as they say, “with a margin”. In addition, vapors are discharged through larger perforated micro-holes that are not susceptible to clogging. These materials do not require an additional ventilation gap at the bottom, so there is no need to mount all kinds of counter battens and additional battens.
A special type of vapor release materials are volumetric diffusion membranes. Due to its volumetric structure (the height of three-dimensional mats made of polypropylene filaments is 8 mm), this membrane is a specific separating layer that itself forms a ventilation gap and facilitates the removal of condensate from the metal roof. In fact, it performs the same function as a vapor barrier film with an anti-condensation coating, only it releases moisture from the insulation. The fact is that on sheets of metal roofing with a small angle of inclination (3-15 °), the condensate that has fallen from the bottom does not flow down and does not drip down, but is in direct contact with the zinc coating, destroying it. The volumetric membrane is fastened with nails to a solid base.
The main manufacturers of diffusion membranes for roofs and facades produce products that are relatively similar in their technical and operational characteristics. The differences concern only functionality, cost and quality of their films. This is due to the peculiarities of technological processes, the type of raw materials and additives, the type of insulating films, the number of layers and methods of their bonding..
Frequently asked questions about the installation of building membranes
On which side of the insulation to attach the membrane?
On the insulated facade, mineral wool is covered with steam films only from the outside.
In insulated roof structures, diffusion, anti-condensation or volumetric membranes are mounted on top of mineral wool, similar to installation in ventilated facades.
Roof elements without insulation are protected with vapor barrier membranes from the bottom of the rafters.
If the walls are insulated from the inside, a continuous vapor barrier is needed – a non-perforated film is installed over the cotton wool from the side of the room.
Insulation of the upper floor with a cold attic located above is closed by a vapor barrier from below.
Which side to lay the membrane?
Vapor barrier films are usually two-sided (no matter which side the material is facing), but there are exceptions. Anti-condensation membranes are attached inside the room with a textile absorbent layer. Metallized films are also one-sided – the foil should be facing the rooms.
Installation of vapor (diffusion) membranes by one side or the other must be carried out according to the manufacturer’s instructions. The same company can produce both double-sided and unidirectional films. The reference point is usually the different coloration of different sides of the membrane, one of which most often has a pronounced marking. In most cases, the “colorful” side of the membrane should be facing outward.
Do I need a ventilation gap near the membrane?
An air gap (about 50 mm) must necessarily be arranged below the vapor barrier films to weather any condensation. It is not allowed that the inner lining touches the vapor barrier.
Diffusion membranes are mounted directly on top of insulation or a solid OSB coating, moisture-resistant plywood. But on top of such membranes, it is simply necessary to make a ventilation gap to remove moisture. The ventilation gap in the roof is made with the help of counter-lattice bars; in the structure of the ventilated facade, the required spacer is provided by racks or perpendicularly located horizontal profiles.
Anti-condensation foil on both sides should have an air gap of about 40-60 mm.
What should be the overlap of canvases?
Construction films and membranes are often marked with a line along the edge of the web, which indicates the size of the overlap – from 100 to 200 mm. For the roof, the membrane performs a waterproofing function, therefore this size can vary depending on the slope of the slopes (from 30 ° – 100 mm; 20-30 ° – 150 mm; up to 20 ° – 200 mm).
The diffusion membrane in the ridge area overlaps by 200 mm. In valleys, the material is overlapped by 300 mm, plus, with small slopes, the second layer is laid along the entire length in the form of an additional strip extending 300-500 mm on both slopes.
Note that the membranes should cover not only the total area, but also the ends of the insulation. Roofing membranes are led out to a drain gutter or to a metal drip.
Do I need to glue the joints? If so, what?
The fabrics of building membranes must be glued together. The joint must be sealed. For these purposes, special self-adhesive tapes are used, which are made on the basis of various non-woven materials: polyethylene, polypropylene, expanded polyethylene, butyl, butyl rubber. They can be bilateral or unilateral. These tapes are used to repair tears and damage to the canvases..
The choice of a specific type of connecting tape should be made in accordance with the recommendations of the manufacturers..
The use of packaging tape (especially narrow widths) for joining construction films and membranes is a common cause of joint depressurization..
How to fix the membrane?
You can use wide-headed nails and construction staples as temporary fasteners. However, a really reliable fixation can only be ensured with the help of counter rails.
The situation is somewhat more complicated when equipping hinged facades. After installing the brackets, mineral wool slabs are laid, each of which is fastened with one or two disc dowels. Next, a diffusion membrane is rolled out over the insulation, cut through at the points of passage of the brackets and through a layer of cotton wool is fixed to the wall with the same dowels. The number of fasteners must be at least four pieces per square meter. If there is a choice, you need to drill at the junction of the canvases.
On roof slopes, the membranes are glued to structures along the entire perimeter using double-sided tapes. With the same materials, regulating construction films are fixed to various elements of the building: windows, doors, pipes, ventilation ducts, antenna racks … On rough surfaces, tapes do not help – they use polyurethane, acrylic, rubber adhesives, “fixers”.
How long can the membrane be left open?
The UV resistance of building membranes is limited. Usually it is up to 4–5 months, then the material loses its thermal stability, aging of the material occurs with the loss of most of its useful characteristics. Obviously, it is necessary to minimize the illumination of the membranes and install the cladding as soon as possible. No matter how hard we try to seal all joints and holes, these roll materials work only in tandem with the finishing outer layers, so heavy rain can cause the heat insulator and structural elements to get wet. That is why it is better to install insulation, films and membranes in stages, and not immediately for the whole house..
Instead of an epilogue
The use of building films and membranes is a prerequisite for the correct functioning of multilayer structures. Only with their help it is possible to ensure the proper temperature and humidity conditions inside the building. When working with membranes, there is usually no particular difficulty, you just need to choose the right material for a particular case and mount it correctly.
Practice has shown that it really makes sense to protect the insulation, especially when you consider that the cost of films and membranes during the construction of a cottage does not exceed the mark of 0.5% of the total estimate. But a lot is at stake – the microclimate of the premises, the durability of building elements, the level of energy costs.