When we think about insulating our lofts we only think about keeping them warm in the winter. However, with summers projected to get warmer, we also need to start thinking about keeping them cooler in the summer.
So insulation works both ways, yes?
Well yes and no. Never easy is it!?
In the same way that solid walls help to reduce overheating by absorbing heat during the day, similarly higher thermal mass insulations are better at keeping peak heat down in spaces like lofts. So effectively if you have a choice between two different insulation materials that have the same final insulation values (for winter heat retention) then the one with the great mass will be better at reducing summer heat and keeping a more constant temperature in the room.
This means that one might be comparing a modern phenolic board (which will be more efficient per cm depth) and a deeper natural insulation. The obvious comparison is between these modern boards and a more natural wood fibre board.
Wood fibre has a higher decrement value (how much it slows peak heat). So each 1 cm of wood fibre will delay peak heat by 1 hour. So during the longer summer days, even a 6-10cm board will remove the peak heat of the day from your loft space.
To illustrate this decrement effect a study from Delft University of Technology in the Netherlands examined two similar houses, each with a roof U-value of 0.21W/m2K – one insulated with lightweight fibreglass and the other with denser cellulose. Both were oriented to the south and remained unheated during the study period, throughout which outdoor temperatures fluctuated by 25C. While the temperature in the fibreglass-insulated house varied by 13C, it fluctuated by just 3C in the cellulose-insulated house. When the temperature outside dropped to 10C, the inside of the fiberglass-insulated house measured 14C, yet it was 18C inside the cellulose-insulated house.
This flattening out of temperature fluctuations in a great advantage in their areas of houses that are most prone to high temperature variations. Worth thinking a bit differently then!
Last note, if you are concerned about head-height, you are correct, it might not be the best solution, but using an amount of higher density insulation would still be advisable.
If you are able to lift the roof level, then using wood fibre sarking boards can be a great way of getting the same effect.