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There are many disadvantages of air flowing in through joints and gaps in the building envelope. A large percentage of building damage is caused by leaks in the building envelope. Sound insulation is reduced, drafts cause discomfort for occupants and there are high heat losses. That is why airtightness standards have been set for many years and why we still have them in current regulations.
It is still widely beleived that poor airtightness ensures good ventilation in dwellings. However air movement is greatly dependent on outside wind speeds and the stack effect within the building (where warm air tends to rise). There are substantial drafts in poorly airtight old buildings even at moderate wind speeds but during periods of mild and calm weather air flows are inadequate. The air flow rate is too variable to maintain a consistant hygienic level of air exchange and high ventilation heat losses are inevitable.
This applies just as much to new buildings as to old ones. In energy efficient buildings an increased level of airtightness is particularly important. The air change rate necessary for good quality air can be ensured with mechanical ventilation.
At the very least, ventilation through joints causes discomfort and heat losses because the heat cannot be recovered from air lost through building joints (also known as uncontrolled ventilation).
In particular, airtightness has the following advantages:
An adequate level of airtightness is the basis for:
Two similar terms should be clarified: the windtightness of a building component protects it from external air flowing in through the thermal insulation, which would otherwise impair the insulating effect and lead to increased energy consumption. This should not be confused with airtightness that is being discussed here, which is used for the movement of air through the building envelope, from the inside towards the outside or vice versa.
Windtightness is completely different from airtightness, which is an important complement of the concept, particularly for energy-saving buildings.
The airtightness of a building can be measured by means of an air pressure test (airtightness test or “blower door test”) which determines the overall remaining leakage of a building. A fan is installed into a door or window to create a negative pressure in the whole house. The fan has a measuring device which gives the air flow rate for the pressure being tested. Flow rates are recorded for pressure differences at several points between 10 and 70 Pascals (Pa) negative pressure. The value at 50 Pa is calculated from the range of readings. The whole process is then repeated using a range of positive pressures [EN 13829] .
|Figure 1: Basic measurement setup for testing airtightness; [Peper/Feist/Sariri 1999] .|
The negative and positive air pressure test results at 50 Pa are then averaged to give a single result, the leakage rate n50 at 50 Pascals. The units are air changes per hour, written 1/h or h-1. This is the flow rate at 50 Pa (average of negative and positive tests) in m³/h divided by the building air volume VL in m³. A similar measure, q50 value with units m³/(m²h) uses the air pressure result per unit area of the building envelope A [m²].
Pressure test results of old buildings that have not been modernised are often in the range between 3 and 6 h-1; however, much higher values are also acheived. Energy efficient buildings should reach values less than 1 h-1; the target value for Passive Houses is <0.6 h-1. This high requirement is frequently fulfilled (see [Peper 2000] ). Such excellent airtightness levels can also be achieved for modernised buildings if airtightness is considered from the very start. Some good examples of this are the well-documented refurbishment projects in Frankfurt a.M., Ludwigshafen and Nuremberg. In these complete refurbishments, values between 0.4 and 0.7 h-1 were measured ([Kaufmann/Peper/Pfluger/Feist 2009] [Peper/Feist 2008] [Darup et all 2005] ).
With professional planning and implementation of the airtightness measures using the appropriate materials, permanently high airtightness values of the building can be expected ([Peper/Kah/Feist 2005] ).
Depending on the refurbishment project (partial or complete refurbishment), the airtightness of the building is either improved or completely newly planned. In the case of complete refurbishments at least, it makes sense to ascertain the airtightness before and after the modernisation measures. During the measurement before the start of modernisation work (so-called initial or preliminary measurement), any problematic areas where airtightness may be inadequate can be checked or detected. These findings should then be incorporated into the planning for airtightness.
Example: there are visible cracks in the surface of the floor on the ground floor above an unheated basement and at the connections to the internal and external walls in a building that needs to be refurbished. It is not known what effect this will have on the airtightness of the building, therefore airtightness in this area is tested during the initial measurement in order to ascertain the course of action for the modernisation procedure.
At the same time, the initial value of the project is documented which will then be used to ascertain the improvement after implementation of the modernisation measures. Even during the initial measurement for the planning, the building components that will represent the airtight layer must be specified. If specific areas prove to be airtight enough (e.g. intact interior plaster), they can be incorporated into the new concept.
If a particularly low level of airtightness is expected for a building (many leaks) or if the building is particularly large, the pressure test can be carried out using several blower fans. In accordance with the [EN 13829] standard, the measurement is also valid if a pressure difference of at least 25 Pa between the inside and outside is achieved for large buildings (more than 4000 m³).
In order to reduce the volume to be tested in large buildings it is possible to divide the building into several zones and test them one after another. It must be made sure that dividing the building is technically feasible. In most buidlings it will not be possible to achieve a perfect airtight separation of the zones. In this case the leakage between the zones will affect the measured value. To prevent this leackage the zones that are not tested at that time can be pressurized by another fan to the same preasure as the tested zone (guard-zone-measurement). Because of the huge extra effort dividing a building into several zones and testing them seperately this method is only recommended in special situations.
Measurements should take place in the state of use. This means that during the preparation of the building for the test, only those openings should be sealed which would normally be closed tight. Supply air openings for a non-room-sealed fireplace (e.g. apartment heating or coal stove) may not be closed or sealed for the test. These openings are essential during operation and always remain open and therefore influence the airtightness of the building and its thermal assessment.
During the measurement it is always advisable to carry out a series of negative AND excess pressures so that the building’s performance can be better represented and to increase the measurement accuracy. A major part of the time is required for the preparation of the building and for detecting leaks; in contrast, the actual test normally takes only about 30 to 40 minutes, therefore saving time here wouldn’t be appropriate.
The volumetric flow of the leakage V50 [m³/h] that is ascertained during the measurement is usually based on the actual heated volume of the building or part of the building that is being examined. This is used to calculate the parameter n50 [1/h]:
This does not depend on the size of the building and can be used for comparisons between buildings or for comparisons before and after the modernisation. The actual heated volume of the building is the result of the heated living space multiplied by the clear room height.
The volume in a wall that results from the installation of a window or a door is not taken into account for the calculation of the building volume. For suspended ceilings only the clear measurement up to the suspended ceiling is considered. This always applies regardless of how airtightly the suspension has been carried out. Among other things, this stipulation according to [FliB 2002] in addition to the [EN 13829] standard facilitates the test in old buildings for which no detail plans are available.
Beams, visible rafters etc. are not deducted. The actual size of the volumes under sloping ceilings etc. are taken into account. If there are staircases inside the airtight layer these are also added with their base area and clear height without considering the stairs themselves (i.e. as a simplification the volume of the steps is not subtracted from the building volume).
If the floor construction or suspended ceiling is not yet present or complete at the time of measurement after the modernisation, the volume of the completed state is still added.
The volume must be determined by the inspector himself and must be documented comprehensibly, or if a calculation by a third party is used, it must be checked. The room-by-room listing must be attached to the test protocol in both cases, and the source for the dimensions should also be given.
For large buildings (greater than ca. 4000 m³) it is always easier to implement low n50-values due to the increasingly favourable SA/V ratio (surface area to volume). However, based on the envelope surface, the building becomes increasingly non-airtight – with a constant n50-value (see Figure 2 with Table 1). It is therefore also necessary to determine what is known as the q50-value [m³/h/m²]. This is obtained from the volumetric flow of the leakage V50 in proportion to the whole envelope area of the building:
The q50-value is more suitable for calculating the air transporting devices (fans) required for the test in large buildings.
The calculation for the envelope surface is given in [EN 13829] . The envelope surface is the total area of all floors, walls and ceilings that enclose the volume (in a terraced house this includes the partition walls of adjacent houses). The areas below the ground level are also included. The “interior dimensions through everything” are used for the calculation. The abutting face of integrated interior walls, ceilings, and floors are not deducted, thus simplifying the calculation.
|Figure 2: Interrelation of the q50-value for 6 sample buildings of different sizes
(simple cuboid shapes of various dimensions) with a constant n50-value.
|Building 3||4 080||1 568||0.38||0.6||2 448||1.56|
|Building 4||9 000||2 820||0.31||0.6||5 400||1.91|
|Building 5||25 200||5 500||0.22||0.6||15 120||2.75|
|Building 6||62 500||10 000||0.16||0.6||37 500||3.75|
Table 1: Data for the 6 buildings shown in Figure 2.
There are two central planning fundamentals for implementing an airtight building envelope (based on [Feist 1995] ):
|1. The “pencil rule”: it must be possible to trace the airtight layer of the envelope in the plan (for each building section) using a pencil without lifting the pencil – except for any planned ventilation openings.|
|2. There must be only one single uninterrupted airtight layer. Leaks CANNOT be remedied by another airtight layer before or after the first one (e.g. double lip seals at windows, vestibule door behind the front door). A comparison to illustrate this point: water won’t stop leaking from a bucket with a leak if the bucket is placed inside another bucket with a leak.|
Besides the basic principles, the following guidelines are helpful for successful planning of the airtightness – whether for a new construction or for a refurbishment of an old building (based on [Feist 1995] ):
When planning an airtight building, three construction elements should be specified and taken into consideration:
During the planning stage, the airtight layer for each external building component must be clearly specified. It does not matter which layer of a component (load-bearing structure, interior cladding, etc.) is used as an airtight layer, but care should be taken that it can be connected as easily and securely with the airtight layers of the adjacent building components as possible.
The determination of the precisely defined airtight layer depends on the materials used, i.e. the structure of the walls, roof or floor. Common construction materials have varying degrees of permeability. Four material groups can be used to implement an airtight layer:
Airtightness concepts of all kinds usually consist of a combination of these materials. In principle it should be possible to use these materials without any joints or to seal the joints permanently without great effort.
Interior plaster is normally used as the airtight layer in solid constructions. A continuous layer of plaster is necessary because an unplastered brick wall generally isn’t airtight. The uninterrupted interior plaster should be applied from the unfinished floor (before applying the screed!) right up to the unfinished ceiling and joined together tightly. It is important to ensure that “invisible” areas, like those behind stairs and prewall installations in bathrooms, are correctly plastered. This is necessary for an airtight application of the plaster and not for decorative reasons. For such areas, it has proved to be practicable if a smooth layer of cement is applied on the unfinished surface as a “preliminary layer”. Force-fitted interconnected concrete elements are the only supporting structures that are airtight by themselves.
For lightweight or mixed constructions, chipboard, plywood, OSB boards and tempered wood fibreboards are used for the airtight layer. These are normally mounted on cross battens and the joints must be bonded or connected airtightly. Prefabricated foils, cardboard strips and adhesive tapes are available for this purpose.
A reliable solution for lightweight constructions is the “double use” of the vapour barrier [Feist 1997] . This is located on the inside of the load-bearing structure and is normally separated from the wall cladding on the inside by the lathing. This results in a gap which can be used for the building services installations. Another possibility is to position the vapour barrier immediately behind the interior cladding. Only diffusion-resistant materials can be used for vapour barriers so that airtightness can be ensured. The use of foils or reinforced building boards is common. If continuous polyethylene sheets are used (e.g. for wooden constructions such as rafter roofs), these should be applied on the inward side of the thermal insulation.
If basic airtight constructions have been chosen for the different building components, the airtight connections between the building components must also be carefully planned, because this is where significant leaks are often found later on. Even a compact single-family house with a simple layout can be used as an example: depending on the type of construction, there are 150 to 300 m of leak-prone component connections [Zeller etc. 1995] . It is obvious that this will greatly influence the overall airtightness of the building, therefore particular attention must be paid to these areas during the planning and implementation phases. For connections in particular, recourse should be taken to a few, easily implemented and reliably airtight details.
In the publication [Peper 2008] , it was explained that good planning for airtightness begins with the identification of the airtight layers of the building components. The following example is given there to illustrate this point.
Common errors: the attempt is made to connect the frame “tightly“ with the bare brickwork using construction foam, filling, sealing tape or adhesive tape. This will not succeed; it is not a question of the materials used, but rather an error in the specification of the airtightness layers. In an external masonry wall, the brickwork is not the airtightness layer. The whole brickwork area is interconnected through a network of gaps and hollow spaces containing air – in other words, the brickwork is an air conducting layer. Therefore the airtight layer of a component abutting a brickwork wall cannot be connected to the bare brickwork; instead, it must be connected to the airtight layer – which is usually the interior plaster.
Since the frame and plaster can move in relation to each other as a result of the different thermal expansion coefficients and mechanical strain (weight of window or door wing when opened), the airtight connection must be able to accommodate relative movements of up to 2 mm without tearing. Therefore direct plastering in of the frame is not possible. Better solutions would be:
I. Adhesive tape applied securely to the plaster which is stuck to the frame and can be plastered over later (fleece-laminated adhesive tape).
II. A plastic plastering strip, one side of which has a a flexible airtight sealing insert with enough give (≥ 2 mm) which is glued to the window frame, the other side of which is rigid and is plastered over inside the interior plaster.
III. A plaster end-strip which is applied at a distance of ≥ 8 mm from the window frame, creating a defined groove between the plaster strip edge and the frame. Tape (e.g. consisting of paper or fabric) is inserted into this groove to prevent the joint filler from sticking to the brickwork of the reveal. Then the space between the plaster end-strip and frame is filled with the flexible joint filler (silicone or acrylic filler) so that it adheres to the plaster end strip and the frame (two-flank bonding).
|Figure 3: The possibilities (see above (I) to (III)) for a permanently airtight
connection of the window frame in plastered solid masonry
(adapted from [Peper/Feist/Sariri 1999] ).
Based on the basic principle given in the example described in [Peper 2008] , it becomes clear that switching the location of the airtight layer between the inside and the outside of the supporting structure should be avoided as far as possible. If this rule is adhered to, connections between similarly constructed components (e.g. solid external wall to solid ceiling or solid interior wall, or lightweight external wall to lightweight roof) will be less complicated in terms of planning. Connections between lightweight and solid construction methods require particular attention.
It is obvious that after all the efforts for a continuous airtight layer, any penetrations must be avoided or minimised. In this context, it is often forgotten that this plainly constitutes a planning task. It should not be left to the construction workers to “find” some kind of way through the airtight layer in an aimless manner, but rather the planner should indicate clearly and unambiguously where and how pipes and cables should be conducted, for example. In any case, it has proved to be easier and more cost-efficient if only a few points are assigned for this purpose (e.g. an opening through the floor slab which can later be sealed) and good connection details are worked out instead.
Therefore, in new constructions as well as in modernisations, in practice particular attention must be paid to the following points for implementing an airtight building envelope: