FAQ: Quick help provided by ISOVER

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Adequate supply of fresh air

Oxygen is our most important nutrient. Experts recommend a fresh air supply of 30m3 per person per hour, based on a maximum CO2 concentration of 0.15% or 1,500 ppm (parts per million). For some activities (e.g. schools, offices) experts recommend even 800 ppm. Such a room climate requires a high rate of proactive fresh air exchange.

Airtight bonding

Airtight seams with VARIO KM or VARIO KM Duplex UV are achieved by bonding with VARIO MultiTape.

Airtight construction

Airtight construction prevents air from moving out of the building. The airtight membrane, which also acts as a water vapour retardant, is installed on the warm side of the construction. Typical solutions for airtight construction incorporate:

Avoiding penetrations

It is recommended that mains supplies and cabling etc. are run outside the airtight layer. If this is not possible, then penetrations need to be sealed with Mangete Passelec or VARIO Stos.

Blower Door Test procedure

With windows and doors closed, a fan creates a pressure difference of 50 Pascal (Pa) between inside and outside the building, and then determines the flow rate needed to maintain this pressure difference. The test indicates the degree of leakiness, since the volume of air needed to maintain the pressure difference is equal to that leaking out of the building.

Bonding VARIO membrane seams

VARIO KM and VARIO KM Duplex UV membrane seams are overlapped by 10-15 cm and bonded with VARIO KB1, VARIO MultiTape or VARIO Silverfast.

Careful workmanship

The VARIO effect is subject to careful and effective bonding of overlapping seams in the membrane and sealing of junctions with components and around all penetrations, such as chimneys, pipes and services, with appropriate VARIO System products.


The planning and building authorities in some European countries have made a certificate of airtightness a requirement as part of a drive to increase quality and workmanship in buildings.

Comparison of thermal needs of different house standards

Old restored house
Average house (post-1980)
Low-energy house
ISOVER Multi-Comfort House (passive house)
Uncontrolled leakage


approx. 300 kWh/m²a
approx. 100 kWh/m²a
approx. 50 kWh/m²a
≤ 15 kWh/m²a
≥ 20 kWh/m²a

Correct assembly on site

The sealed timber frame construction is assembled on site with the other building components. Joints between the elements and around doors and windows etc. are properly sealed. The separate services layer has additional insulation installed, making sure that this accounts for no more than one third of the total insulation inside the element.

Damages caused by wet air

Leaks are a potential cause of damage in a building. When damp, warm air moves through gaps and cracks from the interior into colder areas of the un-protected building, the vapour it contains can condense into water. This provides an ideal habitat for moulds and other fungi.

The affected material can suffer damage and insulation performance can be reduced by up to a sixth, leading to further dampness and setting in motion a chain reaction that leads to serious damage.


VARIO KM is laid over double-layered thermal insulation, installed between and across the rafters, with 3 cm of slack.

Fixing VARIO KM Duplex UV

VARIO KM Duplex UV is a specially laminated non-woven membrane which is extremely tear-resistant. For this reason it is laid over insulation without slack. The dotted guideline markings make installation much easier.

Fixing VARIO membranes to sub-constructions

With metal frame sub-constructions, VARIO KM and VARIO KM Duplex UV are fixed with VARIO DS or VARIO ProTape. In the case of timber sub-constructions, VARIO KM or VARIO KM Duplex UV are stapled directly to the timber framework.

How can I find out whether my house is already airtight?

Many weak spots in the building fabric, such as cracks, gaps or holes, are easy to spot. Obvious draughts can also be easily pinpointed by the layperson, either using a wet finger or by the strategic positioning of tealight candles.

However, to identify and locate all problem areas and to measure airtightness precisely, requires a more sophisticated approach, using the Blower Door Test >>

How wet is air

The warmer the air, the more water vapour it can contain. At 30°C, for instance, water vapour content is approximately 30 g/m3, whilst at 0°C it is just 5 g/m3, and at -10° C no more than 2 g/m3. In a 10 m² room at 30°C with a 2.5 m ceiling height, approximately 750 g of water vapour is present in the air.

Pleasant air temperature

A person’s individual perception of comfort depends on room temperature, the activity they are undertaking and their clothing. An air temperature of approx. 22°C is generally perceived as “comfortable” in sedentary living areas, whereas with physical exercise, air temperatures of between 16-19°C can be perceived as “pleasant”.

Sealing timber frame construction

The entire timber frame construction is sealed and protected with VARIO KM or VARIO KM Duplex UV. Overlapping seams are taped with VARIO KB1 and all openings and penetrations are sealed with VARIO DS.


With high humidity and temperature during the summer months, the diffusion resistance of the VARIO membrane is reduced, i.e. the molecules move apart, and allow trapped moisture to pass through the membrane into the room. Moisture that had penetrated the structure can now escape.

The difference between constructional and service leaks

Constructional leaks are leaks at junctions between the outer walls and the roof, or through perimeter joints in windows and doors. Service leaks are due to mains installations, chimneys and internal openings, such as interior doors.

The requirements of airtightness materials

Generally, materials used to produce flat surfaces are airtight (foils, plaster, etc.).

All materials, particularly ceiling membranes and adhesives, must be compatible and selected to work together.

UV and moisture resistance are important, as is tear resistance. In cold regions, where the airtight layer is applied to the warm side of the building, the materials must also be water vapour retardant.

The „breathing building“ myth

Construction physics and practical experience have shown, that air exchange between inside and outside through walls, floors and ceiling cannot provide enough fresh air. Moisture equilibrium generally occurs only within the first 8-13 mm of the interior building lining, particularly plaster, plasterboard and wood cladding.


VARIO DB is a single-sided, self-adhesive sealant tape specially designed for producing airtight joints in uneven building components. If the substrate is uneven, VARIO DB, combined with clamping strips, produces long-lasting airtight seals.


VARIO DS is a durable elastic, self-adhesive sealant, in cartridge or sausage bag format, that is ideal for use with VARIO KM and VARIO KM Duplex UV climate membranes. It is used to produce long-lasting airtight seals with gable walls, ceilings or chimneys.


A special water vapour retardant, non-woven laminated climate membrane for sealing and moisture protection in lightweight and solid construction

More about VARIO KM Duplex UV >>

VARIO MultiTape SL

VARIO MultiTape SL is flexible and has a split backing strip, making it easier to work in areas where access is difficult. It is used to produce airtight seals around penetrations and junctions, e.g. around windows and mid-purlins.


VARIO ProTape is a self-adhesive sealant on a roll, and is twice as fast to work as cartridge sealants. It is used to produce durable airtight junctions between VARIO KM and VARIO KM Duplex UV climate membranes and all building components.

VARIO – unique variable diffusion resistance

Only VARIO intelligently adapts to the prevailing climatic conditions, preventing moisture from penetrating into the building structure and allowing trapped moisture to diffuse into the room. Conventional water vapour retardant membranes have a diffusion resistance that remains static, regardless of climatic conditions, and function only as a water vapour stop. Moisture that has become trapped in the building structure cannot escape, which can lead to damage.

Ventilate properly

The best solution to replace moisture-laden interior air by oxygen-rich air from outside is controlled room air ventilation. If this is not available, rooms must be ventilated by fully opening windows and doors for a few minutes at intervals during the day.

What does kWh/m2a mean?

kWh/m2a means kilowatt hours per sq. metre per annum

What does n50 mean?

n50 expresses airtightness in numbers, and indicates how often the air volume of the building concerned is exchanged per hour at a pressure difference of 50 Pa.

Windtight construction

Wind-tight construction prevents outside air from seeping into the building. The wind-tight membrane is installed on the external face of the structure and protects it from cooling and dampness. Typical solution for a wind-tight construction:


At the relatively low humidity levels present during colder months of the year, the so-called diffusion resistance of the VARIO membrane is increased, i.e. the molecules close up and stop the diffusion of water vapour, thereby working as an effective water vapour retardant. This prevents moisture in the room from penetrating into the structure.