Since our last article, the windows and doors in the external envelope have started to be fitted, the first steps to making the building air tight in preparation for the stringent air tightness tests.
All the windows and doors utilised in a Passivhaus certified building have to meet certain performance criteria. The wall openings have been discussed in previous diary entries but here we will detail the exact capabilities of the window and doors units. The design and selection of the window and door units needs to take into account the risk of thermal bridges.
What exactly is a thermal bridge?
A thermal bridge or ‘cold bridge’ as it is often called, is created when materials which are poor insulators come into contact. Thermal bridges are either a ‘repeating’ thermal bridge, such as wall ties or continuous ‘non-repeating’ thermal bridge such as window sills or window lintels. Thermal bridges can also occur geometrically at an opening or wall corner junction.
When determining the thermal bridge heat loss, a one dimensional calculation is not adequate so two dimensional or three dimensional calculations are usually required for an accurate result.
We have to acknowledge that all buildings have junctions; there is no way around it. So how can a project be successfully constructed without thermal bridging? The answer is carefully considered design complimented by thermal bridge modelling, which can then be successfully translated on site.
We can reduce the affects of thermal bridging by ensuring that the outside ‘cold’ elements are suitably insulated from the ‘warm’ side of the building. Understanding and accurately modelling thermal bridging has become crucial the accurate prediction of building heat loss. The heat lost through thermal bridges in a Passivhaus building are significantly reduced. Attention to detail where thermal bridges are involved is the secret to successful passive house design.
The windows and doors of a building do not usually have the same thermal performance as the envelope. However, the right design and appropriate positioning will maintain the passive house performance and reduce the thermal bridge effects. All the external doors comprise of fully glazed panels they are essentially oversized window units, there are no traditional doors used on the external envelope. These high performance glazed door units facilitate additional solar gains from the south facing elevation.
All the windows and doors are supplied by Munster Joinery / Baskil Window Systems. The windows and doors units that are being used have to be certified by the Passivhaus Institut in Germany, assuring they meet the requirements necessary for Passivhaus certification and insuring their performance capabilities.
The windows and doors, similar to all the other building components, have been modelled using PHPP software to analyse their performance. All windows and doors must perform as part of the overall building envelope. In our build the windows and doors achieve a U-value of 0.75Wm2K, the SIP panels a U-value of 0.11Wm2K and the floor a U-value of 0.08Wm2K.
The ‘futureproof’ triple glazed windows and doors that have been installed in the CREST pavilion have a whole unit U-value of 0.75Wm2K. The window and door units aren’t only triple glazed but they have low emissivity coatings and are 90% argon filled.
The timber frame came on site with each wall opening formed before the installation of the window and doors units. The window and door units were then screw fixed to this timber frame. All junctions were taped using airtight foil type adhesive tape to ensure air tightness is maintained. This overcomes any differential movement between different materials which may be problematic for achieving air tightness.
The above image shows the steel work frame that will be used to support the large area of glazing to the south west of the building. The steel work needs to be boxed in with insulation and plasterboard to ensure that it does not become a cold bridge.
The windows and doors are also a source of solar energy to the pavilion and constitute an important heat gain for the building. The aim is to gain the appropriate balance of heat loss and heat gain by having more south facing glazing and less north facing. The PHPP software will calculate the heat gain and warn against overheating in the summer months.
There is a larger area of continuous glazing on the south elevation, much more than on the north elevation to maximise the passive solar gain in the spring, autumn and winter months. With such a large extent of glazing, the concern in the summer is that the building could overheat. This risk is alleviated by the large overhanging eaves which are the most effective method of shading in the summer months when the sun is at its highest angle and the building least needs additional passive solar gain. These large overhangs are therefore a consequence of the shading required for passive solar gain.
The next diary entry will detail the air test and how the required test result will be achieved.