When we design and build an energy-efficient house, after the walls, the roof, the insulation layers, then it’s the turn of the windows, just before we start concentrating on the interior. If we don’t want to waste what we have achieved with our insulated walls and ventilated roof, it is extremely important we choose the right window frames, regardless of the material, than can be either wood, metal or PVC. The frame would need to be with thermal break, a type of double paned which splits the frame components into interior and exterior pieces. Double or triple glazing would then ensure that the whole window has the thermal performance required. The U-value is the overall heat transfer coefficient: the lower the U-value, the greater a window’s resistance to heat flow and the better its insulating properties. In our recent case, a wooden house in Umbria we have installed Finstral windows, high performing windows, with a certified Ug (referred to the glass) of 1.1, much lower than the 1.9 required by law for our climatic zone. This is especially important in the ‘conservatory’ corner of the house, where two sides are completely glazed. In this case we have also used low-e (e for emissivity) glass, a special glass with coatings that reflect radiant infrared energy, thus tending to keep radiant heat on the same side of the glass from which it originated, while letting visible light pass. This often results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside, while infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.

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