Building air tightness (also called envelope air tightness) can be defined as the resistance to inward or outward air leakage through unintentional leakage areas within the building envelope. This air leakage is driven by differential pressures across the building envelope due to the combined effects of stack, external wind and mechanical ventilation systems.
Why is air tightness important?
Up to the 1990’s, air tightness was warranted little attention for two reasons – lack of insulation and cheap heating costs. Especially increased heating costs created a demand for better insulation. New synthetical insulation materials, such as glass wool and PIR boards have since been created in order to provide affordable solutions for home owners to make their homes warmer and more comfortable. Unfortunately, draughts coming through the insulation or the transitions of building parts render even the best insulation materials ineffective. These draughts cool down the room temperature, which results in colder homes or increased heating costs, respectively, when the occupant tries to keep the temperature at a comfortable level.
Airtight sealing reduces the infiltration of cold air to a minimum and prevents the waste of energy.
How to create air tightness
There are different options available to create an airtight layer on the building envelope. External block walls are only airtight by having an internal plaster applied straight to the block work. Please keep that in mind in case you decide to use an internal insulation, such as insulated plaster board. In the case of timber frame structures, such as pitched roof areas or complete timber frame houses it is possible to apply an airtight membrane or OSB. Please note that not every type of OSB is automatically airtight; it is advisable to consult your OSB supplier for further details. Overlaps, joints and penetrations need to be sealed by using special airtight tapes or airtight materials like patches, sleeves or grommets. The joints around the external windows and doors need to be sealed as well. You can choose between various window sealing tapes for this purpose.
Where do I need air tightness?
Air tightness is advisable on every building envelope that bears the risk that cold air gets in contact with heated rooms. By contrast, air tightness is not a necessity between two floors with a similar room temperature.
How can I test the air tightness of my Building?
Preliminarily, air tightness can be tested and graded on every building. In order to arrive at an accurate testing result, it is highly recommended to consult an expert with special testing devices. By creating a pressure difference between the inside and the outside of your building, experts can make air leakages within your building structure visible through the use of equipment, such as a wind gauge, smoke or a thermal imaging camera to show you these air leakages clearly. It is mandatory to do an official air tightness test for new buildings or extensive renovations at the end of the construction phase. More detailed information about this procedure can be found in Part L of the Irish building regulations.
Is air tightness the only criterion for building sealing?
Air tightness is merely one part of a functional building sealing. Beside an airtight layer, every building envelope should have barriers against wind, driving rain, dampness rising of the ground and moisture caused by high internal humidity. Wind-tightness, water-tightness and vapor control are as important as air tightness to protect your building structure permanently.
Wind-Tightness (and protection against driving rain)
A roof underlay is a good example how to create wind-tightness and protection against driving rain. This underlay is installed straight under the roofing and protects the building structure against the ingress of water. Please note that a certain amount of water will find a way through your roofing, no matter whether it consists of slates or tiles. The roof underlay is the protection barrier for your roof structure, not the cladding. It is highly recommended to tape the membrane overlapping's and all transitions, e.g. ducts, wires or roof windows. Otherwise your roof will not achieve the full wind-tightness and protection against driving rain.
Prevents mostly the ingress of dampness through the foundation. Radon barriers act as a water-tight barrier as well. The most critical point is the water-tight sealing under the thresholds of any kind of external doors (entrance doors, back doors, French doors, sliding doors).
To prevent structural damages, moisture should be able to pass through the building structure to the outside in a controlled manner.
Therefore, the rule of thumb of sealing for most buildings means: ‘tight on the inside’ but ‘breathable on the outside’. Please consider peculiarities for some types of building structures, e.g. flat roofs; these are too tight on the outside to allow a controlled transfer of moisture through the external sealing layer. In order to prevent the moisture from being trapped within the structure it is necessary to create a ventilation layer or to use a humidity variable vapour control membrane to allow the re drying of moisture back to the room.
Our thanks to Prodomo Ireland for the above information on airtightness
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