In the last post I covered off the wall and roof panels as one component in the creation of an efficient thermal envelope. Today I’ll share information about our triple glazed windows, which are an equally important part given the envelope needs to be continuous and without breaks.
Windows pose an interesting challenge in passive houses as they are needed for light, heat, aesthetics and comfort (physiological and psychological), yet the nature of the material “glass” is such that it is a thermal bridge and cannot be insulated in the traditional way. Even triple glazed windows do not have the same thermal properties as the panellised walls, and so to ensure that the overall results for insulation (and other factors I’ll cover in a moment) remain within the overall allowances and requirements of the Passive House standard modelled with PHPP, control has to be made of various aspects of the window design, construction, location and size.
As a reminder, PHPP is the computerised model used to assess whether the various performance measures of the house comply with the requirements of a passive house. This model takes into account multiple dimensions and metrics to determine comfort and temperature tolerances, and is used in the planning stages of a passive house project. In relation to the windows PHPP can assist in determining size and location and during our planning and design phase we made several subtle changes to size of windows so as to comply or remain complying when we made a change for aesthetic or usability reasons.
If you are interested in reading more about the PHPP model itself this webpage published by the PassivHaus Institute provides a very understandable overview.
The goal of a passive house is to have optimal comfort and thermal efficiency. Windows play a very significant role in this as they are one of the worst performing elements of a house on both these fronts; they create cool surfaces and cause heat loss. Glass loses more energy than the walls and therefore feel cooler. Sitting near a window in a non-Passive House, you will find the side near the glass is colder due to several reasons; body heat is transmitted to colder surfaces nearby, and the glass itself acts to cool down the air nearby which makes the cool air sink and create a draft. Additionally, the relative humidity on the surface of the glass when the glass is cold results in condensation forming, which is not healthy.
The house we are renting during our build has double height glass windows along the entire north elevation. These windows are around 5m high and although double glazed, they are unpleasantly cold to be near and cool down the room. Of course, on sunny days, they allow all the heat inside and this often results in overheating. Just living here, has made us realise that what is visually impressive design-wise, can also be a lame duck from a comfort perspective! (so it’s quite horrifying that it won an architecture award when built less than 10 years ago!)
A certified passive house has windows that are no more than 4.2 degrees C cooler than the operative indoor temperature. There are also requirements to maintain a healthy humidity level to prevent condensation and mould growing. In our house we expect that sitting by windows will be as comfortable as sitting away from them, the temperature at the windows will not be noticeably cooler and there will be no condensation or mould growth.
Windows in Passive Houses
Consideration of the windows is important for a number of reasons, and not just the glazing itself, but also the frames, materials used, size of each component, whether and how they open, shading outside, orientation and method of installation.
Orientation and Shading
A window facing west or on the north elevation (south in the northern hemisphere) might seem like a great idea due to thermal gain from low-hanging sunshine on sunny winter days, but often that heat is intense and short-lived and we have all spent time in such a sunny spot on a cold day and felt how cold it gets the moment the sun has gone. Such variance and extremes are not helpful in a passive house, and indeed must be avoided! Similarly huge windows in a sun-facing location can also lead to over heating and loss of comfort. As we worked through the design process we ended up with very few windows on the west side (which is great anyway as we don’t wish to overlook the neighbour!), and the process of locating and sizing windows was initially very counter-intuitive.
So accepting we all want windows in our houses, how do we have them and not diminish our passive house capability? Triple glazed panes are the starting point. As stated above, glass is a thermal conductor and therefore window glazing transports warmth from inside to the outside very quickly (or vice versa on a hot day). Double glazing has been around for a long time as this has been understood for decades. In multiple layer glazing windows the space in between each pane is filled under vacuum conditions with either dehydrated air or a gas, thus forming a barrier, or resistance, to heat transfer. Typically the gas used is argon, which is 34% less thermally conductive than dehydrated air, which is already less thermally conductive than regular air. The reason they are less thermally conductive is because their molecules are relatively immobile and therefore do not transfer heat so well. Another advantage is acoustic insulation and the evening out the of the glass temperature thus reducing chance of condensation.
Our windows are triple glazed units and argon filled. Each of the three panes of glass is 4mm thick, and each of the air gaps is 12mm wide, so the total thickness of the glazing units is 36mm. Here’s a photo of the glazing unit with my car key fob as a reference.
For additional efficiency we chose to have low-e coating on the glazing. Low-e is low emissivity and refers to a metallic coating that is applied to the cavity side of each outer pane (faces 2 and 5). This coating allows heat from the sun (solar gain) and light to enter the room while reflecting back into the room heat from inside and has the biggest impact on thermal performance.
The glass for our windows is made by St Gobain and then the glazing units are coated and assembled by Viridian in Nelson. The window frames are made of 70mm, 5 chamber, uPVC extrusion with internal steel reinforcing, and they are made and assembled here in Christchurch by NK Windows. NK Windows has also carried out the installation and we have been super impressed by their efficiency and professionalism and knowledge.
Going back to the other elements of windows that need to be taken into account when building a passive house, improved airtightness (and better sound insulation) is achieved by having internally opening European style windows. This allows a much tighter seal to occur. Ours are tilt and turn models, meaning the window can open in two directions, and in tilt mode is highly secure whilst still offering ventilation.
In fact, we have chosen to only have opening windows where really needed, rather than all windows able to open. Not only did this save money, but it also improves airtightness and our PHPP results. On the ground floor there are three sets of French doors (two of which are outward opening, which has lower airtightness) which open and all other windows are fixed. Upstairs, for safety, we have opening windows in each bedroom, and also in each bathroom for odour, although I am reliably informed that the ventilation system will deal to odour as well as provide fresh air circulation. Gary and I usually sleep with the window ajar and so we are waiting to see if the ventilation system means we no longer do that!
Position and Installation
The windows are recessed into the building frame. By aligning the glass panes nearer the centre of the insulation line of the entire wall, the thermal bridging effect is reduced and so the window’s performance is further improved. The walls of our house are achieving an R value of 4.9. A typical double glazed window at NZ building code standard has an R value of 0.26. Our triple glazed units have an average R value of 1.15 when installed.
The window must be an integral part of the thermal envelope of the entire construction. This is especially important as the junction point of wall and window comprises additional surfaces from which heat can be lost. Furthermore, there are additional components used in the installation and fixing which could create thermal bridges. For this reason the windows were surrounded with insulating foam and then the Intello wrap has been brought up and onto the window frame and taped down. The black tape will be hidden behind the architraves later on.
Other factors that help achieve thermal performance are the ratio of window glazing to window frame The glass in our windows performs better than the window frames themselves; this is typical for triple glazed windows. This means that smaller windows performa worse overall than larger windows beciase the window frame makes up a larger proportion of the total window area. Similarly, opening windows perform worse because the frame takes up more of the window area. Our window framesand the uPVC material used for the frames. Frames in timber are also excellent for mitigating thermal bridges. Needless to say, aluminium, no matter how much the manufacturer says they are “thermally broken”, are not suitable for a passive house due to the conductivity of the material.
This blog post by Elrond Burrell gives really interesting (and some quite technical!) information on passive house windows.