By Francesco Anselmo and John Mardaljevic

Throughout the centuries, anonymous builders from all parts of the world have increasingly fine-tuned their window designs to the context of climate and culture. Their creations offer a number of lessons to be learnt for contemporary buildings that aim to save energy and maximise the use of natural daylight.

Our impression of a building is largely defined by the detailing of the facade. Irrespective of the building size, our attention is immediately drawn to the windows. The shapes, proportions and sizes of windows, their materials and construction details, the glass types, the methods of opening, the finishes of windows all contribute to the character of our buildings, and so also our towns and cities.

From simple rectangular openings to arched or intricately ornamented screens, vernacular windows are carefully sized and located in buildings as part of a broader environmental and cultural strategy.

Vernacular window designs have been developed and transferred from generation to generation under the protective wing of tradition. One might think that tradition is immutable, but transmission is also the key to evolution. Genetic information is transferred from parent to offspring through inheritance, with the introduction of beneficial mutation that improves the fitness of the species. The idea of tradition as such is strongly intertwined with evolution. This means that tradition can change in time, adapting to changes in context, be it social, cultural or environmental.

An inspiration for sustainable design

In fact, the examination of vernacular windows offers many surprises, because the invisible, evolutionary architect who has created their designs is the local community. This ‘architect’ often responds to the environmental challenges and physical circumstances with inventiveness, uses simple tools and local materials, but also shapes his creations according to the culture, beliefs, customs and myths of the community itself.

In this article, we have attempted to ‘read’ the evolution of vernacular windows in the context of the local luminous climate for a few locations around the world. Our aim is to demonstrate how different environmental conditions influence the design and performance requirements of traditional windows.

These designs are the fruit of simple rules that have been shaped through time by the simple principle of maximising comfort using local resources as well as the geometry and orientation of windows. We hope that the forgotten skills of our ancestors can offer new inspiration for a more sustainable design of the building envelope.

How to read the diagrams below

The temporal luminous climate maps presented in the next pages use different colours to display the variation of exterior horizontal illuminance through one year. This metric can be easily related to daylight brightness: the more exterior illuminance, the more light outside.

These maps are like calendars or diaries that, for each hour, indicate the average solar (direct), sky (diffuse) or total (global) horizontal illuminance. Days flow horizontally, from 1 January on the left to 31 December on the right. Hours flow vertically, where midnight is both at the top and bottom of the plot and noon is the horizontal line at the centre. The lighting levels indicated in the maps are those of a typical year, which has been derived using statistical techniques that select the most relevant months from a database of many years.

Looking in more detail at the maps, it is possible to read the duration of the day in different seasons by looking at the length of the coloured vertical lines. The areas in black indicate night hours.

By looking at the diffuse and direct maps, it is also possible to see which days are cloudy (high diffuse illuminance and low or zero direct illuminance) and sunny days (low diffuse illuminance and higher direct illuminance). This observation enables us to identify cloudy/rainy seasons and tell how strong the direct sunlight is compared to the diffuse skylight.

The pattern of hourly values in a climate dataset is unique and, because of the random nature of weather, it will never be repeated in precisely that way. Climate datasets are, however, representative of the prevailing conditions measured at the locale, and they exhibit much of the full range in variation that typically occurs. Furthermore, these standard datasets provide definitive yardstick quantities for the evaluation and selection of competing design options – in other words, they can help us in the process of ‘evolving’ towards better building design solutions.

Stockholm

We start our journey around the world of vernacular windows from Scandinavia, located at the extreme north of Europe.

The direct and diffuse maps for Stockholm display clearly the seasonal difference in duration between day and night. This has a direct impact on the traditional window design: longer days mean that natural light can be present during sleeping hours, therefore the addition of exterior solid shutters helps to create a night environment inside the house during the summer months.

The low illuminance values (compare them with the illuminance in the Mediterranean, tropical and desert examples) require windows to let in as much light as possible. This means that privacy might be an issue if windows must maximise lighting.

A good approach to solve this issue whilst mitigating glare from the proverbial low sun of northern climates is the separation of view/privacy and light in windows. The top parts of the windows can be left clear to capture as much light as possible from the sky, but the lower parts can be screened with curtains or other translucent layers to avoid direct view from the outside in.

London

The London maps, with their seasonal variability and irregular cloudy and rainy patterns, represent the luminous climate of most of the European locations.

Before the sixteenth century, most European windows were built from stone or timber with unglazed openings, closed with wooden shutters, oiled cloth, paper, or thin sheets of horn. Only the wealthiest houses could afford glazed windows. By the end of the 17th century, larger glass pane sizes could be produced and timber sash windows and casement windows came into use. Since then, the different glass production methods resulted in a wide range of thicknesses, colours, and refractive and reflective qualities.
A relatively temperate climate allowed architects and vernacular builders to experiment with designs that maximise light. This resulted in a general trend for decreasing thicknesses of glass and increasing sizes of window openings. A large variety of glazing patterns became widespread in the early 19th century with ’lyingpane’ (landscape format) sashes becoming popular.

The shape of the bay-window is particularly connected to the need to maximise light penetration throughout the days and seasons. With their angled profile, they increase the flow of natural light into a building depending on their orientation. They also make a room appear larger and provide views to the outside which would not be available with a flat window.

The sills, reveals and most timberwork associated with European windows is traditionally painted in white colour to increase the reflection of daylight and its distribution to the inside.

Rome

Shading devices perform a triple function: they keep out the sun’s heat, block uncomfortable direct sunlight and soften harsh daylight contrasts. In the Mediterranean, the need to accommodate the conflicting requirements dictated by a climate that can have both cold, humid, dark winters and hot, dry and bright summers has influenced the development of a flexible window shading system.

Exterior, so-called Persian or Venetian blinds are the exterior shading layer of a complex system that also includes glazing and an interior shutter system for increased privacy and security.

The high degree of adjustability of this system allows the windows to cope with high variations in illumination conditions. The positioning of the adjustable shading device on the outside of the window is important to minimise solar gains in summer.

Osaka

With the notable exclusion of the northern island of Hokkaido, the climate of most of Japan is similar to the Mediterranean one, with more humid and hot summers. The temporal luminous climate maps for Osaka also show a particularly strong sun, especially in summer.

However, the traditional window design in this part of the world is distinctly different than the Mediterranean counterparts and can only be understood by looking at the natural, social and aesthetic context of Japan.

The destructive effects of earthquakes, tsunamis, volcano eruptions and typhoons, coupled with the temperate climate, have contributed to the development of houses that are lightweight enough to allow a quick escape and that can be rebuilt or refurbished more often than in other countries.

The characteristic thin walls of the traditional Japanese house include translucent ‘shoji’ screens that can be used both inside as partitions and at the building perimeter to protect from the outside environment. They also act as daylight diffusers, attenuating and gently distributing the strong sunlight. Exterior ‘shoji’ screens are combined with heavier ‘amado’ wooden panels, that are set into tracks running along the exterior walls, creating a verandah, or ‘engawa’. The ‘amado’ panels can be opened in good weather to bring in additional light and allow a view to the outside world.

The widespread use of translucent ‘washi’ paper for the window screens has created a peculiar aesthetic of soft light and soft shadows, which is in strong contrast with the more dramatic use of light and shading of the western world. Europe has created a complex sequence of adjustable shading layers based on the geometry of the sun path, that can still project the harsh contrast of light and shadow to the interior of a building. Japan resolves a similar daylighting problem with a single layer of diffusing paper and a sophisticated yet uncomplicated use of material. This approach both simplifies the window components and reduces their number.

Abu Dhabi

Mashrabiya screens are the most widespread form of window in Middle- Eastern architecture. They are projecting oriel windows enclosed with wooden latticework, but more generally the term Mashrabiya identifies the latticework itself, that can be made of wood or stone. In India, Mashrabiya are indicated with the term Jali, while the projecting balcony decorated with Jali is called Jharokha. Similar latticework is also widely present in vernacular Chinese architecture.

This type of window has originated in countries where the prevalent weather conditions are dry and sunny. The weather data plots for Abu Dhabi show this solar prevalence very clearly throughout the year. This implies a clear need for shading and ventilation.

The open latticework created by the wooden frame and pegs of the Mashrabiya screens is the best solution to this problem. Leaving openings in all the windows promotes
continuous air flow and cooling inside buildings, while the rounded shape of the pegs disperse the light, reducing the contrast and glare.

Another function of the Mashrabiya is related to the Arab culture and desire of privacy. The bright appearance of the exterior screens from the outside conceals completely the view of the darker interiors, while the dweller can observe the outside world clearly without being seen and without needing to open the window.

Port-au-Prince

In tropical climates, in this case represented by the temporal luminous map of Port au Prince in Haiti, days have similar length throughout the year and the sun spends most of its time overhead, in the highest part of the sky.

Deep solar penetration can still be a nuisance in the morning and afternoon, therefore local builders have developed window designs with awnings, jealousies as well as adjustable shading panels and shutters with higher and lower sections, that can be hung horizontally or vertically. A façade with some depth, like a porch or a covered balcony, creates a buffer zone that can contain shading elements to filter glare and block sun.

All these principles are embedded into the design of the Caribbean chattel house. These houses were occupied initially by the African slaves working in the British West Indies. They were designed to be movable, so that slaves living in them could build them on the margins of sugar plantations on land that they didn’t own and then dismantle and move them easily at the end of the growing season. For this reason the houses were built in wood and assembled without Mashranails. Their windows had two types of shutters: the top ones were hung with horizontal hinges and the lower ones were hung vertically to allow maximum flexibility against the wind and solar penetration. Their colloquial name – ‘jealousy shutters’ – reminds of the function to keep the inside of the house not visible to the eyes of the neighbours.

This article is featured in D/A magazine #19, for more information visit da.velux.com