Bringing sunshine and energy

BEAUTIFUL HOMES

The right positioning of windows is important as it has a large impact on heating and cooling loads, lighting and even the house’s interior décor. The best designed windows are the ones which make the most of the sun’s natural light and heat, says Anuradha Muralidharan

Most of us, the moment we walk into a house, head straight to the windows.  A lovely view of green trees and the hillside with the early morning sun shining in is great, but if we live in the city and can only see neighboring buildings, then too, the view can play an important role.

One of my favorite pastimes is to stand near my window at dusk and watch vehicles speeding by below, their headlights streaming and cutting into the darkness. To me, my home’s safety and stability takes on added significance as I stand inside my house and watch the world speed by.

Why windows?

We all know that windows let the sunshine in, they change a house into a home and they let in the air and sometimes raindrops too, if it is a heavy downpour.  But some of us may not be aware of the fact that a modern home loses more than 25 percent of its heat through its windows.  In fact, modern windows play a major role in a home’s thermal envelope. The thermal envelope is really the topmost layer around a building which helps to control the movement of air through a building’s structure. Windows, because they are holes in the wall and structure, become a part of the thermal envelope.

As the first step, when designing an energy efficient home, it is important to remember that a good thermal envelope results in lower utility bills at enhanced comfort levels. But if the thermal envelope is flawed it can lead to excessive energy and hot and cold spots within one single house. Today, we do have the knowledge as well as the technology to ensure that our homes are energy efficient and windows, can, thus, become one of the first steps in that direction.

Here are some tips to ensure that you have energy efficient windows in your home.

Design

The size, placement and type of material become the first step in designing energy efficient windows. The right positioning of windows is important as it has a large impact on heating and cooling loads, lighting and even the house’s interior décor. The best designed windows are ones which make the most of the sun’s natural light and heat.

Depending on the climatic zone, the directions of the windows are also important. For example, if a house is situated in a hot climatic zone then the window space on the northern, eastern and western sides should be less than 9 per cent of the floor area. For south-facing rooms, windows should have overhangs above the window to prevent overheating in summer. North, east and west windows should have low Solar Heat Gain Coefficients (SHGC). SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. Overhangs over south facing windows should have a higher SHGC to enable winter sun and heat to enter the sun. 

U-Factor can be used to measure heat transfer. U-factor is a calculation which gives the rate of heat transfer through the window (from inside to outside when it is cold, and from outside to inside when it is hot) per unit area and per unit temperature difference. The lower the U-Factor, the greater will be the resistance to heat flow and thus result in better insulation. U-Factor values generally range from 0.25 to 1.25.

Selection of material is the second step in designing energy efficient windows. The frame material is important as some composite frame materials have a higher thermal resistance compared to metal frames. Vinyl, wood and fibreglass, for example, have a higher thermal resistance power compared to metal frames.

If you prefer metal frames, then ensure that you include a thermal break, i.e. insulating plastic strip placed between the inside and outside of the frame and sash to reduce heat.

Today most home owners prefer to use glass for windows. Windows with awning and casement styles are preferred to sliding types as they shut tight. Fixed panes block airflow when closed and interfere with ventilation and the same goes with double-hung, sliding and single-hung. Casement, awning and hopper windows, on the other hand, provide good ventilation when opened and also remain airtight when closed.

Insulated glazing is a technique where two or three glass panes are spaced apart and permit air space between each pane. The layered glass and air space help to resist heat flow.

Low-emittance coating

You could also consider a low-emittance coating which is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass in a window with insulated glazing. The low-e coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Low-e coatings also reduce the SHGC and VT or Visible Transmittance.

Heat-absorbing glass contains special tints that change the colour of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation, which reduces SHGC, VT, and glare.

Gray and bronze tints reduce the penetration of both visible light and heat into buildings in equal amounts and are the most common colours used.

Reflective glass is coated to reflect radiation striking the surface of the glass. The reflective coating usually consists of thin metallic layers and comes in various metallic colours (silver, gold and bronze). Reflective glass reduces the passage of solar radiation through the window, generally blocking more visible light than heat. It reduces SHGC and greatly reduces VT and glare. Reflective glass is most useful in hot climates in which solar control is critical and it is usually used just for special applications.

Installation

Finally, it is important to ensure that windows are property installed and do not interfere with energy efficiency. Sometimes windows, when not installed properly can contribute to moisture problems etc.

To conclude, there are many builders who are incorporating all these factors in their designs to ensure that they provide eco-friendly house options. Some of these buildings are so well constructed that artificial cooling and heating devices become redundant. But instead of leaving all decisions to your architect or builder, getting involved in the design stage will ensure that you not only end up with a beautiful home but one that is energy efficient.

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