When looking for insulation it should be remembered that most loft conversions involve upgrading the existing roof space by fitting the insulation material above or sometimes below the rafters.
Because the building regulations are so strict on the U-values that roofs have to meet using the most efficient insulation material is essential.
As a note it should be stated that the U-value of the roof must be 0.20 W/m2k.
In recent years the process involved in insulating pitched roofs has changed in that creating a ‘warm roof’ is now sometimes used. A warm roof is created by installing the insulation material above the rafters. This allows extra thickness and creates a kind of insulated shell.
But often a warm roof isn’t practical or required and the insulation material can alternatively be placed below or between the rafters.
Sometimes circumstances dictate that a mixture of the two kinds of rafter insulation is used.
The disadvantage with having to install the insulation below the rafters is of course that the available headroom will be reduced.
The roof space will be further reduced by the need to include a vapour control layer to prevent condensation.
There are also some issues with placing the insulation boards between the rafters the major one being that a 50mm gap for ventilation must be left above the insulation material. The common solution is to use between and below insulation.
Probably the most difficult question to answer when looking at the best loft insulation for loft conversions is ‘how thick does the insulation need to be’?
The usual answer was always around 150mm but different building control officers will have different interpretations but most are open to being convinced that a lesser amount can sometimes be used.
Rafters are usually 100mm in depth but are then built up with a 20mm timber batten.
This allows the 50mm ventilation gap to be left along with the bulk of the insulation being fitted between the rafters with perhaps just 50mm below the rafters.
But, whichever method of installing the insulation is used we still need to find the best material for the job.
Well, in the opinion of many in the trade, the best loft insulation matrial is foil faced Polyisocyanurate boards.
These boards have a remarkable 0.017 heat conductivity value which makes it the most efficient material available. Not only that but the foil facing acts as a vapour control layer making them much more space efficient than other materials.
For more info see the related pages below
The Importance Of The Thermal Envelope
Vermiculite Loft Insulation is an echo-friendly alternative to traditional insulation materials
The old loft water tank will need to be moved if the roof space is being converted into a habitable room
Structural calculations for loft conversions are of course hugely important to ensure safety and that the proposed plans meet building regulations