What you need to know to improve the energy efficiency of your home thanks to insulation

Insulation, energy efficiency, insulator, thermal bridges, fuel poverty

All buildings aren’t equal in terms of energy use: the most recent buildings fitted with reinforced insulation only use up 50kWh/m2 a year, whereas older constructions take up 200kWh/m2. New buildings must abide by increasingly strict regulations; however, most of our homes and workplaces haven’t drawn too much attention on insulation issues. That’s why it is advisable to get an insight into the strengths and weaknesses of your home’s insulation.

The construction of new buildings (whether homes or office space) has abided by thermal regulations[1] since 1975. The current RT 2005 will soon give place to the RT 2012, which targets more thoroughly the energy efficiency of buildings. The reason is that insulation is not only a matter of comfort anymore. Energy bills are a crippling financial burden to almost 4 million French households subject to fuel poverty, spending over 10% of their income on heating, as said Philippe Pelletier during the Rexel symposium on energy efficiency.

What is an “insulator”?

A thermal insulator is a material whose physical properties block thermal exchange between the inside and the outside. Thermal exchange is channelled through “thermal bridges” conveying the air. They are located at wall intersections, room angles, but also around window and door frames, electrical outlets as well as between roof tiles. According to the ADEME[2], 25% to 30% of heat losses occur through the roof, 20% to 25% through air renewal and air leaks, 20% to 25% through walls and 10 to 15% through windows. These four building components have their own characteristics, which key rules are presented below.

Some principles on building insulation methods

There are two ways to insulate a roof: the conventional way consists in unrolling insulation blankets against the roof structure and fixing them onto rafters. This insulating layer is then generally covered with plaster plates. Outside roof insulation is also possible, although more difficult. Rigid battened panels are fixed onto rafters and then covered with insulating material and with an outer layer attached to the battens. This technique sometimes requires raising walls, and, at any rate, erecting a scaffolding.

As for walls, the most common solutions are blanket insulations and rigid-board insulations. The more modern and efficient overpressure technique uses “loose-fill” insulation. Little pieces of insulating material (cellulose wadding, glass-wool, stone-wool…) are injected into the wall thanks to a specific machine. This way, overpressure injection fills the free space between the outside wall and the inside wall with the necessary amount of closely packed material to provide adequate protection. This method leaves you free to choose the thickness of the insulation layer and can also deal with uneven walls.

Finally, for windows and doors, the major difficulty lies in the frames. Low-quality or poorly installed frames entail significant thermal losses. Double-glazed windows (4/16/4 mm) have now become the norm for new buildings or renovation. More advanced triple-glazed windows will cost you an extra 150€ per square-meter.

Inside or outside insulation?

Inside insulation is the cheapest way but has a number of limitations: it makes dealing with thermal bridging more difficult, it shrinks room volume and, more importantly, it damages the thermal inertia of walls, as it is tantamount to heating up (or cooling down) the insulator, whereas the very role of insulation is not to store heat or cool, but, quite on the contrary, to block them. Laying out insulation panels on the outside is an adequate solution to this problem. However, this technique is expensive, particularly because it requires covering bigger surfaces.

Striking a balance between air renewal and air-tightness

To achieve efficient insulation, the inside air must be renewed on a regular basis without creating thermal bridges (for instance by opening a window). Indeed, moisture makes your house colder. Two kinds of Controlled Mechanical Ventilation are available today: simple-flow CMV and double-flow CMV. The first type merely expels the old air outside, creating a vacuum which sucks in fresh air through different openings (beware of thermal bridging!). The second type discharges the old air and brings in fresh air thanks to a double ventilation circuit including a cross-flow heat exchanger. Fresh air is heated up or cooled down by the old air and has the right temperature as soon as it enters the house.

Today new buildings are perfectly air-tight; including a double-flow CMV brings airflow to the minimum, significantly reducing thermal bridging. Besides, the materials used, called vapour-barriers or steam regulators, allow steam to flow out with no air leak; this way insulation doesn’t get moist and remains efficient.

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[1] In French, RT for Règlementations Thermiques
[2] The  French Agency for the Environment and Energy Management

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