Airtightness Window Detail

Airtightness: A focus on detail – Windows and Doors

It is now widely accepted that if we are going to be serious about designing and building truly low energy buildings, we have to prevent the uncontrolled leakage or infiltration of heated air or cool external air out of or into the living space. Previously I outlined the basic principles and advantages of airtightness. These were primarily:

  • Avoidance of moisture related building damage
  • Avoiding drafts and discomfort
  • Avoiding high heat losses due to air leakage
  • Introducing a controlled ventilation system
  • Improved acoustic performance
  • Improved indoor air quality

This article will focus on a particular detail that often leads to air leakage and the inevitable, whodunit dispute on site, the window or door junction to the building structure. The following are the primary steps to achieving higher levels of airtightness in construction particularly at window and door junctions:

1. Forethought, planning of works schedule and clear simple design principles

2. Quality control on site and good workmanship with the use of appropriate materials to achieve an airtight seal. This includes identifying at the beginning of the project, who is responsible for sealing the window to the buildings airtightness layer.

3. A final measurement which confirms we have achieved the required level of Airtightness which we were aiming for in the first place.

Communication and coordination of works on site and a high level of consciousness and awareness regarding the importance of airtightness from the design stage to the build process are essential components of attaining a positive result.

How to ensure that airtightness is maintained around the windows?

Attaining best practice airtightness on site encompasses the various trades which may interact with the external envelope of the building. This may be the electrician who fits the plug sockets, the plumber who installs the piping or the ventilation supplier who must install extract or intake vents. The window installer is also one of the key personnel on site which can make or break the airtightness strategy. Windows are one of the most common failure points when an airtightness test is been carried out on a building.

While a window may, in theory, have an excellent level of airtightness, it is essential to ensure the INSTALLED window attains an airtight seal to the existing structure. If this is not attained the integrity of the thermal performance of the window can be compromised. A Passive window in particular can be the most expensive component within a low energy building. There is little point in investing so much in a building component if it is not installed correctly. It therefore pays to ensure it is installed correctly. At the end of the day this is what airtight building is all about, good building practices and getting things done correctly.

One of the main reasons a window often proves to be one of the most common leakage points in a building is not just down to the complexity of the junction, but it is due to the transfer of responsibility on site in dealing with this detail. It is important from the outset to identify who is responsible for sealing the window to the buildings airtightness layer when it’s installed. Otherwise, the window installer and the internal membrane installer or plasterer will more than likely rely on each other to seal the junction which leads to a dilution of responsibility, and in many cases, to no one sealing it in the end and the inevitable leak which ensues.

While it is impossible to provide guidance in relation to airtight window installations on every form of construction within the limits of this article, the following sections will provide general guidelines to two of the most common forms of construction on the market today, Timber Frame and Masonry construction. The key to attaining an airtight seal on any construction is to try to ensure that the air barrier layer on the wall or roof connect to the window or door frame seamlessly. The air barrier layer on a timber frame building tends to be either a vapour control layer membrane or an OSB sheathing board. In masonry, this tends to be the plaster on the block. Plasterboard tends to provide an unreliable level of airtightness and certainly shouldn’t be relied upon for those seeking to attain best practice air tight results over the lifetime of the building.

Timber frame window junction:

In Timber frame construction the airtightness layer tends to also act as a vapour control layer on the internal side of the construction. More details regarding the role of a vapour control layer are available on my previous article Airtightness Part 3

I will now describe 2 different approaches to sealing a window airtightly in timber frame construction.

Applying airtight seal before the windows are installed:

Figure1: Applying airtightness tape to window prior to installation offers greater flexibly and efficiency

Applying airtightness tape before the window is installed is the preferable method of attaining airtightness on site. Pro clima Contega SL window sealing tape is presented in Figure 1.

Here we can see the window installer has pre-applied the tape on the edge of the window frame. This has the following advantages:

  • It is very quick to apply
  • The edges of the tape are easily concealed once the internal lining is applied
  • It is easy to seal around tight corners

Once the window is installed the tape can be bonded directly to the internal OSB or vapour control with the integrated adhesive tape on the opposing side as shown in figure one. It is also important in all applications of such a tape, to ensure the window is sealed to the airtightness layer below the sill. One disadvantage of this approach is that if the tape is not bonded correctly to the window initially, then the installer has to try and seal the tape to the frame later. If the internal lining is not thick enough, this may lead to tape been exposed on the window frame when the building is complete.

Applying airtight seal after the windows are installed:

Figure2: Applying airtightness tape to window frame after the window is installed

Applying airtightness tape after the window is installed is also possible in cases where it is not possible to install the tape prior to the installation of the window. Figure 2 illustrates this approach. This method tends to be more time consuming as the installer has to ensure that excessive tape is not applied to the frame leading to it been exposed when the building is complete. This is particularly critical on expensive Passive House windows! Tight corners can also be cumbersome and time consuming.

Masonry window junction:

Masonry construction is the most common method of building in Ireland. The typical cavity block wall connection to the window frame can often lead to air leakage on site. This is due to the varying quality of the finish on the inside of the blocks been used which may of been damaged during installation, the skill of the builder to ensure the tolerance between the block window reveal and the window is not too high and the width of the cavity required to conform to the building regulations or to build low energy buildings means cavity’s have increased in size from 50mm to 300mm in some cases!

Bridging the gap between the window frame and the internal plaster on the block is the key to attaining a reliable seal. When one accounts for the range of factors outlined which may occur on masonry walls it is safe to say there is no single solution for every masonry wall. The following section will describe two ways of effectively sealing masonry walls to window frames airtightly.

Applying airtight seal before the windows are installed:

Figure 3: Applying airtightness tape to window before installation

Applying airtightness tape before the window is installed is the preferable method of attaining airtightness on site. Again, Pro clima Contega SL window sealing tape is presented in Figure 3.

Here we can see the window installer has pre-applied the tape on the edge of the window frame. This has the following advantages:

  • It is very quick to apply
  • The edges of the tape are easily concealed once the internal lining is applied
  • There is easy to seal around tight corners

Once the window is installed the tape can be bonded directly to the masonry with a continuous bead of high performance acrylic glue. The tape should be bonded to the wall completely with the glue. Then the plaster can be applied directly to the tape, ensuring a reliable, durable airtight seal is attained. It is also important in all applications of such a tape, to ensure the window is sealed to the airtightness layer below the sill.

Applying airtightness tape to window after installation:

Figure 4: Applying airtightness tape to window after installation

Applying airtightness tape after the window is installed is also possible in cases where it is not possible to install the tape prior to the installation of the window. Figure 4 illustrates this approach. This method again tends to be more time consuming as the installer has to ensure excessive tape is not applied to the frame leading to it been exposed when the building is complete. Again the tape is then fully bonded to the surrounding block-work with suitable airtightness glue, such as pro clima Orcon F. Then the fleece can be fully plastered at a later stage.

How do I know if my windows leak?

Once a window is installed and sealed it is important to confirm there is no leakage occurring either through the window itself, or the connection between the window and the adjoining structure. It is best to ascertain this prior to installing the internal finish. This is because it is easier to locate the leak at this stage and if any adjustments or remedial work is required it can be carried out more cost effectively without damaging the finished product.

Otherwise, the phenomena of dancing curtains in front of closed windows may occur when the building is complete!

A leak can be identified by depressurising or pressuring the building with a blower door or a WINCON fan.

This induces a negative or positive pressure on the building envelope and allows leaks to be detected with ones hand, a smoke pencil or fog emitting device such as a wizard stick shown below, or by using a thermal imaging camera, if the conditions are suitable.

Figure 5: Fog emitting device, Wizard Stick, often used to highlight air leakage around windows

As with all buildings, when high levels of airtightness are attained this lends itself to a high standard of construction and as a result unintended gaps and cracks will be minimal. It is therefore essential to ensure that a clearly defined ventilation strategy is designed into the building from the outset by passive or mechanical means. The key is to build tight and ventilate right!

Conclusion

It is clear that in order to attain optimal airtightness in buildings, the junction between windows and the building structure must be sealed continuously. It is no longer acceptable to install windows and rely on temporary seals such as poorly fitted expanding foam or poor quality sealing glues and tapes. These provide at best, a short term seal. The airtight seal attained when the building is constructed shouldn’t just last for the day of the test, but for the lifetime of the build.

Ensuring windows are installed and sealed airtightly have the following benefits:

a. Improved thermal efficiency

b. Increased comfort levels (No moving curtains on windy nights!)

c. Improved build quality and less builder call backs due to poor installation

d. Reduced risk of condensation between the window and the building structure and hence less risk of mould

e. Improved durability

f. Superior acoustic performance, particularly in urban areas

It is clear that airtight windows should be a priority from the outset, but there is little point in specifying or procuring exceptionally high performing windows if they are not installed correctly.

Niall Crosson

Technical Engineer BTech, MEngSc, MIEI

Pictures and graphics provided courtesy of Ecological Building Systems Ltd.

August 2011