Wooden windows - their influence on the energy efficiency of the house

I wrote the other day about Energy efficient houses and about CLT panel walls to keep heat inside, used in the construction of passive houses. But energy efficiency is not only achieved by insulating walls. Heat loss can also occur at other points. By far the most sensitive areas are glazed areas. Movable or fixed glazed windows, glazed doors are all sensitive points. The problem can be solved by using windows that keep heat out in winter and keep it out in summer. Such windows are what are known as double-glazed windows and can have frames in a variety of materials. As they can also be wooden windows, they are obviously wooden thermopane windows, as we have come to call them.

The term thermopan is actually the Romanian version of a registered trademark. "Thermopane" is the name under which the first window with a double-glazed glass pane was registered in 1941 by the LOF Glass Company, Ohio, USA. The system had been invented in 1930, also in the USA, by C.D. Haven, but the need for the glass to be of uniform thickness and completely straight made it impossible to manufacture at that time, resulting in very high production costs.

Over time this system has proven to be very energy efficient. Now we find it hard to believe that a new house can be built without double-glazing, and owners of old houses are increasingly choosing to replace old wooden windows with double-glazed ones.

How to determine the efficiency of wooden thermopane windows

Very important in the case of double-glazed windows is the heat transfer coefficient which, as with thermal insulation, is measured in W/m²K. The lower it is, the more thermally efficient the window. For passive houses, for example, it should be less than 0.85.

The thermopane window is made up of the frame and the glass sandwich. The heat transfer coefficient is calculated for both components as heat can pass through both. In addition, heat can also escape from the house through the area where the window is installed in the wall, so the heat transfer coefficient for the installed window is equally important. For passive house all 3 coefficients must be below 0.85.

How to build wooden window frames

Window frames are made of timber, and different types of wood can be used: fir, spruce, pine, oak. Unlike traditional joinery, where the frame is made from a single piece of wood, double glazed windows are made of several layers of wood glued together. The layers are laid along the grain, not perpendicular to each other.

The most effective frames are those in which the fiber of the layers is laid in a mirror. Heat passes through the frame as a wave. When it encounters the reverse-facing layer of wood, the amplitude of the wave decreases, reducing the amount of heat transferred. The more layers (thicker) the frame, the more energy efficient it will be. It goes without saying that the thickness of the frame cannot be exaggerated for the sake of efficiency. Everything is calculated to get the best result, so the thickness is also chosen to be efficient, but also strong, lightweight, easy to use and aesthetically pleasing.

Many times the efficiency of the frame is improved by making profiles, openings inside one of the layers of wood. The air in these profiles acts as an insulator. Combinations of wood and composite materials are also used to increase performance.

A very efficient type of frame, commonly used in passive houses, is the aluminum clad exterior. Between wood and metal there is free space and the air in this space is a very good insulator. The advantage is that the metal is very resistant to the weather and there is no danger of rotting, as can happen with wood if the construction solution is not well chosen.

Requirements for energy efficient glazing

Window glazing is actually a panel made of 2 or 3 layers of glass. In between these layers is dry air or inert gases that reduce heat transfer, and the pane is sealed airtight. The gases used are argon or krypton. Krypton is more efficient than argon, but also more expensive. It is used when the pattern requires a smaller distance between the glass layers. Sometimes a mixture of the two gases is used, giving an increase in efficiency without a huge increase in price.

The glass used must be of very good quality, durable, of uniform thickness. It may contain UV absorbers, be lightly colored, and have very thin protective films applied to the surface to reflect the sun's rays or increase mechanical strength.

Strips are placed between the layers of glass to keep the distance between them constant even with variations in atmospheric pressure. The strips prevent the build-up of moisture - they are anti-condensation - or leakage of inert gas from inside.

Passive houses use windows made of 3 layers of glass. The 3-layered panes are 40-50 mm thick and are very energy efficient. The leaves covering the glass can accumulate heat and release it when the temperature drops inside.

How to install for maximum thermal efficiency

Window installation is very important and influences the thermal and sound efficiency of windows. Fitting is done directly into the gap left by the builder in the wall of the house or into a wooden frame which is then fixed into the window space. The windows are fixed in place with polyurethane foam which completely fills the gap between the wall and the window frame.

The walls into which the windows are fitted are much thicker than the window frame, which are load-bearing walls. Windows can be installed in the gap left by the builder for this purpose in 3 ways:

• on the outside, the window is flush with the facade,
• inside, with the window flush with the inside face of the wall, or
• midway between the above positions. Studies have shown that this way of mounting is most effective, or as close to it as possible.

When installed, small channels form inside the polyurethane foam, through which heat can escape. In passive houses, windows are fixed to the wall by means of narrow foils which are glued both inside and outside. These sheets, glued to the window outline (which will be covered with plaster), continue the insulation of the wall, preventing the formation of energy bridges. This prevents heat from escaping through the channels in the foam. The foils are in the form of scotch-tape rolls which are stuck to both the frame and the wall.

Hardware, fittings, finishing of wooden windows

The quality and efficiency of the window is also ensured by the hardware, the gaskets used and the finish chosen. The hardware must provide a perfect seal and the hinges must be able to support the weight of the window. A weak hinge will gradually give way, the window will not sit perfectly and heat loss can occur.

Gaskets must be of good quality and withstand both low winter and high summer temperatures. A poor quality material will become stiffer, even brittle, in winter and the seal will no longer be watertight. In summer, poor-quality material can become too soft, even sticky, losing its qualities.

Window finishing is made with exterior resistant materials. Paints are the most durable, glossy varnishes the least durable. A perfectly clear varnish will not protect your window against UV radiation. This is why exterior varnishes are lightly colored with pigments. If wood is stained, the stains used must be resistant to the exterior.

The energy efficiency of your home can be improved simply by replacing windows

When building passive houses, the performance of the windows, like that of the walls, is calculated according to the climatic conditions in the area. Although built according to strict rules and clear standards, each passive house built is unique in its own way. The whole house is designed down to the smallest detail and the choice of windows is made at the design stage.

In ready-built houses, replacing ordinary windows with double-glazed windows can increase the energy efficiency of your home, thus reducing energy costs. You can find lots of calculations on energy savings, some of them even made for 20 years since the windows were replaced.

And in this case the window fitting is just as important as the window itself. If you decide to take such a step, look very carefully at the possibilities and call in a specialized company to fit them. Don't try to cut corners on quality, don't cut corners at this stage. The savings will come later, but only on condition that the choice of window and its replacement are made correctly.