Window joinery options

When it comes to window joinery you might think the only big decision to be made is around somehow matching the colour to both your cladding AND your interior walls. Hands up if you've been there! Aside from that, it's a given that it will be aluminium, right? 

WRONG.

Other frame types such as uPVC and timber typically outperform aluminium thermally, while there are also many options in glazing -  clear or tinted, low-e, and gas infill - which all alter the thermal performance of the windows.

Lets have a look at some of the options available and the pros and cons of each.

Window frames

Aluminium

Aluminium is a great conductor, meaning that heat passes easily between inside and outside. This is not what we want for thermal efficiency. The recent addition of thermal breaks (you may have heard of "thermally broken") in the aluminium frame help to regulate the temperature difference and are a big step up from standard double glazing. Yet there are still problems with where the window is installed in the wall; Typically they are on the exterior face and therefore reduce the effect of the thermal break because there is a gap between the frame and the insulated wall. A better way of installing is by recessing the window into the wall, effectively creating a continuous line of insulation between the wall and the window. This has historically been more challenging in NZ with the need to ensure weathertightness is maintained, but it has become easier with the addition of proprietary flashing systems and an increased awareness of the detailing required. Yes recessed windows cost more, however it's something we like to push when the budget allows. 

The difference between the typical and recessed window installations

uPVC

PVC is a far better insulator than aluminium. It's essentially what the thermal break is in aluminium windows, so using it for the whole window frame makes the whole window perform better thermally. The frames are larger than aluminium to get the same structural performance and they can't span as far as aluminium. These can be drawbacks if you want entire glazed walls, however we do need a collective mindset shift to "picture windows". Big open expanses of glass reduce the thermal efficiency of the whole building.

uPVC windows are a great option for replacing old timber windows as the "chunky" frames are similar to timber aesthetically and help to retain the character of older houses. They are typically more expensive than aluminium, but provide better efficiency (lower operational cost on heating/cooling). They usually involve European style joinery which is far more airtight, and has the added bonus of reducing outside noise. Also tilt-turn openings which provide two types of ventilation - fixed secure ventilation at the top and full window side opening are available.

Tilt-turn window

Timber

Timber frames are the top solution. They not only look great but provide excellent thermal efficiency, and using naturally renewable material is better for the environment. The timber windows available nowadays far surpass the single glazed that we are all used to. Windows come with glazing seals rather than putty to hold the glass in place, European hardware is used, and tilt-turn options are available. All this combines for a high end result. There are also options to have aluminium cover plates on the outside which reduce maintenance and allow for a coloured finish.

Glazing

Nowadays there are different configurations available in addition to double or triple glazing. In general terms it's about balancing the need to let sun and light in, while also not overheating.

Tinted glazing

Tinted glazing works to reduce heat and light from the sun while also providing increased privacy for occupants. This can have the negative result of making inside spaces darker and reduced warmth from the sun in winter months, so it is important to consider the specific situation as to whether tinted or clear glazing is appropriate.

Low-e glazing

Low-e is a film that is installed on the glass that helps to reflect heat back to where it came from. That means retaining heat inside in winter and reflecting heat back outside in summer. This is appropriate in most cases. 

Gas infill

The gap between double or triple panes used to be filled with air. It's now far more common to use Argon or other gasses which are better insulators than air. Unfortunately the gasses tend to dissipate over time and therefore reduce performance, but we still see this as really good option to increase performance overall.

So this is all good in theory, what about some numbers to quantify the differences?

R-values stand for Thermal Resistance and are calculated by the thermal conductivity of a material and it's thickness. In general this means that the higher the R-value the better the material is thermally.

Typical window values:

Single glazed, aluminium frame: R0.15

Single glazed, timber frame: R0.19

Double glazed, aluminium frame: R0.26

Thermally broken, double glazed, aluminium frame with argon gas infill and low-e film: R0.46

Double glazed, uPVC frame with argon gas infill and low-e film: R0.7

Double glazed, timber frame with argon gas infill and low-e film: R0.77

Passive House quality window (uPVC or timber): R1.0+

Takeaways

As you can see there are lots of options which alter thermal performance. Look to buy the best windows you can afford. Aim for smaller, picturesque windows to reduce cost. The selection between frames and glazing can be complex, especially for high performance buildings - this is where thermal modelling becomes really important and is something we can help with.

There is no bulletproof solution for all situations. That's why we are here to advise on the right selection for your particular project. Please get in touch!