Times have moved on since the days when 10 centimetres of glass fibre in the attic was considered sufficient insulation, even for a house in France, and the subject is now a bit complicated. There are newer ‘high-tech’ materials available, and we have a much greater knowledge of how heat is lost from a building.
The focus is now on providing much greater levels of insulation, ultimately to save on heating costs and save the planet, and also on targetting weak points or discontinuities as they are called in the ‘insulation envelope’.
Thus your wall insulation should be continuous with the roof insulation and floor insulation for example.
Heat loss from an uninsulated or very inadequately insulated building is very broadly analysed as:
Doors and windows 15%
Joins between walls and floors and roofs and windows 15% (point thermique)
Normal air exchanges due to opening and closing doors etc 15%
Bad news: a house in France does need heating. Using modern and efficient insulation can reduce the cost of this heating dramatically. If you have less than 20 centimetres of rockwool or equivalent in your attic, or if your insulation is not fitted with great attention to any gaps and spaces between the sheets of insulation, you do not have good enough insulation. It is not the case that you get most of the benefits by having just a little insulation. There is more to insulation than simply keeping out the draughts. Both rockwool and glasswool type products, end extruded mousse sheets, double in efficiency as their thickness is doubled (if installed carefully). This does not apply to the ‘aluminium foil’ type products.
Insulation – products used during property renovation
Rockwool and glasswool are not always ideal for roof insulation because they have a tendency to absorb humidity, and to droop over the years. They also provide a warm home to vermin and insects, which further increases the sagging. This gives rise to discontinuities – gaps between the individual sheets of insulation. One popular option to reduce this is to use two layers of 10cm thick rockwool, with the layers laid at 90 degrees to each other. This significantly reduces the level of draughts, but does inevitably increase the time taken for installation.
These products are light to handle and install, and are often a practical solution when you are insulating an existing attic space / roof from the interior. Usual practice is to fit small metal fittings to the roof beams, with the insulation held in place by these until the plasterboard can be put in place. The insulation is bought in rolls 60cm wide and should be a perfect fit for the spaces between the roof beams, especially in newer buildings.
If you are replacing an entire roof, you have the opportunity to choose from a wider variety of insulation materials. The main options include sheets of polyethylene mousse, and aluminium foil based insulators. Both are fairly new products and hence long term stability has not been entirely proven. Other eco-friendly products, based on linen and other materials are also available.
These sheets of insulating mousse can be bought with plasterboard or wood panelling already attached on one side, and slats of wood on the other. This allows the roof (and ceilings) to be constructed quicly and more easily than with traditional ‘piece by piece’ type rooves. This also provides efficient insulation. The insulation panels, which can be several metres long, are simply attached to the existing wooden roof frame. We used this system for our cottage renovation. Although these mousse sheets are quite expensive, they act as a roof replacement rather than simply insulation.
Foil based insulators
The aluminium foil based insulators have the advantage of being very thin, typically about two centimetres . They consist of two sheets of aluminium foil encasing various other thin layers of material, typically bubble wrap type sheeting (the bubbles filled with dry air), further layers of foil, foam, and fibrous cloth. Two centimetres of these foil based insulators are claimed to offer the same insulation levels as 20 centimetres of glasswool or rockwool. Because they are so thin, it is possible to use this insulation without having a material impact on roof height. We used one of these products in our barn renovation to good effect. The most common of these products in France are those produced by the companies Actis and Airflex.
Floors and walls
Walls and floors should also be insulated if they are being newly constructed or installed, usually with the rigid polystyrene or a foil based insulator. There are a wide range of possibilities for insulating pre-existing walls – most common are polystyrene or glassfibre type products preglued to plasterboard. Some of these products can be glued directly to existing walls, providing both insulation and an interior finish at the same time.
Insulating stone walls – these are frequently insulated on the interior of the room, before a plasterboard wall is glued or fitted on top. However, be aware that best advice seems to be that you should not insulate in this way, because it can give rise to future problems. First, a stone wall is already naturally insulating and allows water vapour to pass, but this only happens when fresh air can reach the surface of the wall and circulate freely. Second, insulation is much more efficient when placed on the cold side of a surface rather than the warm side.
The joints between walls and floors and roofs are very important from an insulation standpoint. It is important that the wall insulation, for example, meets the roof insulation. When small, or difficult and inaccessible gaps arise, as they will in an old house, my solution has often been to use a substance called extruded polystyrene (this is not the same as expanded polystyrene, that breaks into hundreds of little bubbles when you cut it). This extruded polystyrene is a wonder product, and is bought in large sheets that are easy to manouvre with, and easy to cut to shape. The tongue and groove edges make it easy to slot together in larger areas. I have often cut pieces of this to fit gaps, then squirted expanding mousse into any remaining gaps around the edges. This provides continuity in insulation between the walls and the roof.