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What do I need to know about insulation?

• Insulation works in both directions: if it is well insulated, a building is more comfortable whatever the season, warmer in winter and cooler in summer. It is therefore more economical as regards heating and air conditioning.

You can never insulate too much. Don’t skimp when carrying out insulation work: use a thickness of at least 20 cm (glass wool or equivalent in rigid panels) and even more if your budget and the layout of the building allow.

• “Insulation” always goes hand in hand with “ventilation”. The better insulated a building, the more efficient its ventilation needs to be.  Otherwise it becomes unhealthy and deteriorates because of accumulating damp.

• Insulation is the most profitable of all energy investments. This profitability is improved still further by the Rénolution grants (the old energy grants). Always start with the roof, which is where most heat is lost. Then turn your attention to the glazing and frames.

Insulating must be the priority


Homes and buildings in Belgium are among the most poorly insulated in Europe. On average, our houses are insulated like homes in Portugal. In the Scandinavian countries, the household heating budget is lower than that of Belgian households. so there is a great deal to be done in this field here in Belgium.

What insulation thickness should you install? The thickest possible. Every euro spent on adding a layer of insulation is reimbursed and then pays off in heating energy savings. In passive houses, the insulation is up to 40 centimetres thick. These houses no longer need a heating installation.

The materials can be compared on the basis of their “lambda” or thermal conductivity coefficient. This is always indicated on the packaging. The lower the lambda, the more efficient the insulation. So a thinner layer of an insulating material with a low lamba is needed to obtain the same insulation capacity.

A higher lamba does not mean that an insulating material is of less good quality. It’s just that you need a thicker layer to compensate.

Here are the lambda (λ) thermal conductivity coefficients, expressed in W/mK (watts per metre times Kelvin) for some insulating materials:

  • Polyurethane 0.023 to 0032 W/mK
  • Glass wool 0.032 to 0.040 W/mK
  • Extruded and expanded polystyrene 0.029 to 0.042 W/mK
  • Cork 0.038 to 0.040 W/mK
  • Paper flakes 0.039 W/mK
  • Perlite-vermiculite 0.051 to 0.053 W/mK

In practice, 7 to 9 cm of polyurethane insulates as much as

  • 9 to 12 cm of glass wool or polystyrene
  • 13.5 cm of cork
  • 42 to 70 cm of cellular concrete
  • 60 cm of wood
  • 300 cm of solid bricks

So a solid but uninsulated wall, even if it is very thick, does not provide any insulation benefit.

Insulating costs less than any other energy investment and starts paying off immediately. What’s more, it is something you can do yourself, bringing the cost down still further. Finally, insulation is subsidised by means of grants that improve its economic profitability still further.

Ventilation is essential with insulation. In the past, houses were ventilated naturally by cracks in doors and windows.  But a house insulated in accordance with current criteria is airtight. However, all human activities give off humidity. Without ventilation, a very well insulated house will inevitably suffer from damp problems:

  • condensation
  • the appearance of mould
  • deterioration of wallpaper, plastering and paintwork
  • appearance of allergies and respiratory problems among the occupants.

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