FOIL FACED BOARDS

The first generation of boards were made of polystyrene and then polyurethane with foil faces to reflect radiated heat. The closed cell structure resists moisture both in vapour form and water ingress. Warm roof construction allows the sheets to be used on top of the rafters with counter battening to create an external air gap beneath the tiles. The roof line fascia board needs to be deeper to accommodate this increase in roof thickness and some consider this ugly.

The biggest drawback with this insulation is the installation cost when cutting 2400 x 1200 mm sheets to fit between rafters. It is also fairly wasteful on material because you end up with a skip load of off cuts. One answer to this is made to measure insulation such as Thermaflect from YBS Insulation. The insulation is fabricated to suit rafter spacing’s most of which are now standard but their bespoke service also allows for projects such as barn conversion. If there is sufficient rafter depth then the expanded polystyrene Silver product gives cost savings but if the rafters or timber frame walls aren't deep enough to achieve the required U value the company has a PUR Gold product. The best method of fitting the board is to lay it as each rafter set is fixed, so that each rafter is butted up to the panel to achieve the correct spacing and ensure a snug fit. This also weathers the building in as you go because the insulation overcomes the need for a sarking felt.

Another product designed for easy fitting between timbers is Rafterloc from Xtratherm which has wedge shaped cuts to allow it to squeeze together and lock in under tension. Xtratherm has also applied its innovative talents to the floor where a substantial proportion of heat is lost. Xtratherm's Hyfloor 75 enables housebuilders to achieve a U-value of between 0.20 and 0.22 W/K in a typical floor.

Hyfloor is manufactured from rigid Polyisocyanurate (PIR) core. It has a thermal conductivity of 0.023.The 75mm thickness not only achieves the required R value but 'comes course' with the brick work doing away with the need for makeup courses or 'soap bars' as they are colloquially known. The tongue and groove jointing on the long edges ensures a close fit for improved thermal performance.

Celotex has just introduced the first ever 200 mm thick board to their extensive range. Undoubtedly there are applications for this product but apart from floor slabs I am struggling to think where that might be. Normally a 200mm thick requirement is made up of 2 boards for the good reason that they can be laid counter to each other and overcome any thermal bridging of joists or rafters.