Roof insulation and insulating when renovating your property in France

Insulation

Times have moved on since the days when 10 centimetres of glass fibre in the attic was considered sufficient insulation, and the subject is now rather complicated, with new materials available, and a greater knowledge of how heat is lost from a building. The focus is now on much greater levels of insulation, and the removal of weak points or ‘discontinuities’ as they are called. Thus wall insulation should be continuous with roof insulation and floor insulation for example.

Heat loss from an uninsulated building is very broadly analysed as:

Roof 25%

Doors and windows 15%

Walls 15%

Floor 15%

Joins between walls and floors and roofs and windows 15% (point thermique)

Normal air exchanges due to opening and closing doors etc 15%

 

General

A house in France does need heating, and efficient insulation can dramatically reduce the cost of this heating. If you have less than 20 centimetres of rockwool or equivalent in your attic, or if this is not fitted with meticulous attention to gaps and spaces between the sheets of insulation, you do not have good 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 stopping the draughts. Both rockwool and glasswool type products, end extruded mousse sheets, double in efficiency as their thickness is doubled (if installed carefully).

Insulation - products used during property renovation

Rockwool and glasswool are not always ideal for roof insulation because they have a tendency to sag over the years, and to become home to insects and vermin which further increase this sagging (although more rigid versions are available). Hence discontinuities (gaps between the individual pieces)appear. One popular possibility is to use two layers of 10cm thick rockwool, with the layers laid at 90 degrees to each other, since this helps reduce draughts between the rolls - it will take longer to install, of course.

These products are however lighter to handle, and often the most practical solution when insulating an existing roof from the interior. Usually small metal fittings  are attached to the beams of the roof and the insulation is held in place by these until the plasterboard is put in place. The insulation is usually in strips 60cm wide and often a 'perfect' fit for the spaces between the roof beams.

When an entire roof is being replaced opportunities arise to install a wider variety of insulators. The main alternatives available include sheets of polyethylene mousse, and aluminium foil based insulators, although 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.

Polyethylene mousse

The sheets of mousse can be purchase with plasterboard or wood panelling on one side and slats of wood on the other, hence the interior of the roof is laid as part of the same process, and tiles are attached to the wooden slats on the other side. Hence this is a very quick system to implement and provides efficient insulation. The 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 the cost of these sheets is quite high, they act essentially as a roof replacement rather than simply insulation.

 

Foil based insulators

The aluminium foil based insulators have the advantage of being very thin, typically only 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. Typically two centimetres of this type of insulation is claimed to offer insulation properties equivalent to 20 centimetres of glasswool or rockwool. Hence it is possible to use this insulation without having a significant impact on roof height. We used one of these products in our barn renovation to good effect. Most common in France include those produced by companies Actis and Airflex.

Actis insulation product being installed

Floors and walls

Floors and walls should also be insulated when they are being newly constructed or installed, perhaps with rigid polystyrene or a foil based insulator. There are a wide range of possibilities for insulating walls, typically being polystyrene or glassfibre type products preglued to plasterboard. Some of these products can be glued directly to the existing walls, providing both insulation and an interior finish at the same time.

Insulating stone walls - these are frequently insulated from the interior before a plasterboard type wall is glued or fitted. However best advice seems to be that you should not insulate in this way, for a couple of reasons. First, a stone wall is already naturally insulating and allows water vapour to pass, but only when air can reach the wall and circulate. Second, insulation is much more efficient when placed on the cold side of a surface rather than the warm side.

 

Point Thermique

The joints between walls and floors and roofs are very important from an insulation standpoint. Hence it is very 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 sometimes been to use a substance called extruded polystyrene – this is not the same as expanded polystyrene. This is a wonder product, coming in large sheets that are easy and clean to cut to shape, and even having tongue and groove edges for slotting the sheets together in larger areas. I have cut pieces of this to fit the gaps, then squirted mousse into any remaining gaps and around the edges. This provides continuity in insulation between the walls and the roof. The best product as always depends on the exact situation.