Here are the things to consider
Window Size and Placement
The honest truth is that is most of the houses we (Eco Vida Homes) design the window size and placement is determined by lifestyle choices such as capturing the best views, creating a seamless inside / outside space, allowing lots of natural light, and outside living spaces.
This is partly because for the most part we are dealing with a coastal Mediterranean climate in Marbella and the Costa del Sol.
In northern Europe or in inland Spain where it gets very cold in winter and very hot in summer then comfort and energy efficient considerations play a more important role. A window represents a weak point in the buildings thermal envelope. A window will let heat out (bad in the winter / good in the summer) and it will let heat from the sun in (good in the winter bad in the summer).
The bigger the window the greater the amount of heat loss / gain.
The lower specification the window the greater the amount of heat loss /gain. With high specification windows (triple glazed argon filled e-coated windows) the effect is minimised.
Glass walls: sliding or folding
Most of our clients are buying a view as well as a smart comfortable home. In fact they are buying a lifestyle and an important part of this is inside / outside spaces. The ideal is often an open plan living kitchen area where preparing the food is a social activity with family and guests, next to a wide shaded terrace, and beyond that on the same level the swimming pool and beyond that (over the infinity edge)the view of the sea, the mountains or both. Inside there are spaces for privacy such as a library or media room.
The inside open plan kitchen living room should connect seamlessly with the outside terrace where for most of the year you will be eating. The floor may be of the same material and the same level. However at night, and for a couple of cold months and hot months then you may want to close this space and for this use wall to ceiling windows so that you are connected to the outside even when these “French” windows are closed.
Folding glass walls have the advantage that you can open up the whole space s they “telescope” into a small space.. Sliding doors slide one against the other and so open up only half the space (unless they slide into a wall or layer on to each other more than once). So folding doors are best for the circumstances I have described however, they are more expensive, they can be heavy to move, and may require reinforcement above them to carry the weight.
Window Frame Material: wood, aluminium, pvc etc
Probably more important than the actual material is the quality of workmanship. There are good and bad examples of all material choices. You will clearly make a judgement based on the appearance of the material and whether it fits you design considerations.
You are also looking for durability. Part of this is whether, when it comes to maintenance, you can replace or repair parts, or whether you have to replace the whole unit.
You may be concerned about the sustainability of the materials. Clearly wood sourced from managed woodland is the kindest on the environment but the arguments are not straightforward. PVC, which generally considered to be costly to the environment to manufacture can be the most durable and for this reason ultimately more environmentally friendly. The arguments are complex. The carbon footprint of any material would need to include all transport costs of all the materials.
Double glazing is standard these days. In fact any window that isn’t at least double glazed is considered inadequate by today’s standards. Triple glazing for special situations such as large south facing windows in Spain where you want to capture the view but reduce solar gains in the summer are not uncommon.
An air gap between the glass of 16mm and 20 mm is consisered to be the optimum and the difference between them is negligible. Below 16 mm the direct heat transference between the panes reduces the effectiveness and over 20mm air convection between the panes has a similar effect.
Low e (low emissivity) coatings are a microscopically thin metal oxide or semiconductor film applied on one or more surfaces of the glass usually on a face between the panes of a double glazed or triple glazed unit. These coatings work by reflecting long wavelength heat generated within the room back intothe building while at the same time allowing short wavelength solar energy into the room. Low e coated glass looks identical to ordinary clear glass as the coating is invisible
Instead of air, argon, krypton or xenon gas can be injected between the panes. These gases have a better insulation than air and contribute to a much better overall insulation. For instance with an argon fill the U value would be reduced by over 30%. The gas escapes very slowly and the performance starts to degrade after about 20 years.
Thermal Bridging / Spacer Bar
The spacer bar is the material that separates the panes of glass in double glazing.
A thermal bridge is a break in the insulation envelope. If you think of your house as a sealed envelope with good insulation all the way around a thermal bridge is a break in that seal. Even the smallest thermal bridge, like a nail mistakenly hammered through the insulation, can ruin the thermal seal as all the heat escapes through the highly conductive metal nail.
If your spacer bar in your window is metal (aluminium for example) it represents a thermal bridge. This is very bad. The spacer bar should be glass fibre or stiff rubber or something. The words you are looking for in your window specification are thermal break (the opposite of a thermal bridge).
Author: John Wolfendale
Bio: John is a founder of Eco Vida Homes and is passionate about bringing modern design and construction practices to Spain. He believes a home which is warm in winter and cool in summer is largely a matter of design and selective use of materials. He is British and a Chartered Surveyor with over 24 years experience living and working in Spain.