The trouble with insulation is it’s not all the same

  • Building Design Expert
  • 8 years ago

The title is at risk of stating the obvious. Of course, it’s not all the same. In fact, the last few years have seen an almost exponential rise in the choice of insulation types. Each has their own merits and de-merits, colours quirks, profiles and finishes. But it comes down to it we can split them quite easily into just two columns: ‘Compressible’ and ‘Rigid’**. The latter column is populated by the foam boards – Polyurethane, Polyisocyanurate and Phenolic, and of course the polystyrenes, both extruded and expanded. The compressible insulations used to be a very short list; it was either ‘glass wool’ or ‘rock wool’. We now have sheep’s wool and hemp. The majority can be formulated into semi-rigid slabs and most, due to their fibre composition, incorporate a high degree of breathability, allowing moisture vapour through from warm side to cold quite readily.

**There are the semi-rigid insulation bats, but for the purpose of this blog, we will include them under the ‘rigid’ heading.

Alas, as desirable and eco-friendly as most of the compressible insulation products are. They are noticeably less efficient than their rigid cousins. But that’s not to say they do not have their place. The breath-ability factor can be important if there are unusual quantities of  water vapour to dissipate, and the materials have a built-in fire resistance that the rigid boards do not. Additionally, if space is available to accommodate the increased thickness/volume of material, there are significant cost savings to be made, and always worth having.

There has been a recent spate of insulation give-aways. The government has used the fact that mineral wool insulation is so relatively cheap, to give it to householders in an effort to reduce the UK carbon output, which currently stands at just over a quarter of our total carbon emissions.

The gift will comprise approximately 300mm mineral wool insulation, installed in two or three layers at ceiling level. – Great, nothing wrong with that, and there really aren’t, in theory. The actual practice of increasing insulation is to be encouraged, but there is a checklist:

  • Compressibility – Remember this insulation is compressible. Take 100mm thickness mineral wool, and compress it to 50mm thick – the performance is reduced by half. – This is why whenever compressible insulation is installed, care must be taken neither to compress or pull out the material from its manufactured thickness.
  • Storage – Most of us use our roof spaces for storage of little-used household items such as boxes of Christmas decorations, suitcases etc. etc. – If the loft insulation has just been upgraded chances are that the ceiling joists that provided direct support before, are no longer visible. If the stored items are placed directly onto the insulation, it will compress and so reduce in performance. – There are also issues concerning health and safety. If you cannot see the ceiling joists slipping between and through the ceiling board becomes a lot more of a problem.
  • Ventilation – Many older properties may have been re-roofed using a bituminous sarking felt (membrane). The natural ventilation via the gaps in the old roof tiles has gone. Many roofing contractors do not retro-fit eaves and ridge ventilation, so the installation of more insulation just stores up the amount of water vapour that will eventually be released to condense on the roof structure and sarking and drip back down onto the insulation. – If there has been ventilation installed then this will bring with it dirt particles that carried in the air, and deposit them onto the insulation, also reducing its performance.

So we must exceed to the check-list. • Insulation must not be compressed, or otherwise altered from its manufactured state. • If we must store items in the loft then a platform should be constructed above the insulated level, although care should be taken to avoid construction of a ready insulation by-pass via the platform supports. • Ventilation is a must, and this is where ‘cold roof’ insulation is the least efficient.

The very essence of insulating a roof space in this manner is  a recipe, on its own, for increased heat loss over the same level of insulation installed in a ‘warm roof’ configuration, and this is why:

Mineral wool insulation Heat loss

The key incident is the movement of air. We cannot stop air moving, as it changes temperature it changes density. Just to complicate matters its properties are also altered by the degree of vapour saturation.

If we could keep air still, and dry, it would be a great insulator. The moment it starts to move, that very movement will extract heat from the surface of any object it comes into contact with; – remember how much cooler you feel standing in a breeze on a hot sunny day? – moving air over a warm body has a cooling effect. The same is true for any object, living or otherwise. Hence, ventilate a cold roof void and the moving air will assist the dissipation of the heat from the top surface of the insulation.

The effect will occur to varying degrees no matter what the insulation type. If you were to install a foam board insulation, it would retain the heat within it for longer due to the greater efficiency of its cellular construction, but the net effect of heat loss would be proportionately the same as the cold surface level.

Whilst any insulation is clearly an improvement on no insulation, the manner in which it is installed has a significant effect upon the net consequential gain. This is why the trouble with insulation is it’s not all the same.