Three in one

Warm pitched roofs have two apparently conflicting requirements – insulation and ventilation. Manufacturer YBS Insulation has made it easier to meet those with a three-in-one product that acts as a roofing felt and as insulation without compromising ventilation. The company’s Therma-Flect Roofboard is made up of an aluminium foil bubble insulation bonded with expanded polystyrene (EPS). It is available in varying thicknesses but at 125 mm it meets the 0.25 U value expected to come into effect next year with the revision of Part L of the Building Regulations.

Therma-Flect differs from the standard warm roof detail by enabling the EPS insulation to be slipped between the rafters rather than sitting above them. The foil bubble layer sits across the top of the rafters and additionally acts as a waterproof membrane. Conventionally, insulation placed between the rafters must maintain a 50 mm air gap between it and the felt layer above but Therma-Flect by-passes this requirement by virtue of being a loose-fit design that allows sufficient air permeability to avoid the risk of condensation. The joints between the sheets are overlapped: this is sufficient to make the whole roof cover waterproof but also allows any internal moisture to filter out into the counter-batten void. Roofs still need ventilation but the ventilation channel sits above the Therma-Flect Roofboard layer where it is formed by counter battens nailed in on top of the rafters.

YBS has built a reputation for developing innovative insulation products. In fact, they account for four of the 36 products which accreditation body Wimlas is currently certifying, the others being a cavity closer, a partial cavity fill and an insulating DPC. Therma-Flect Roofboard can also be used on refurbishment work, where it is installed the other way up so that the reflective foil layer is on the inside. However it is in new build applications that the advantages really show through. At a cost of around £8/m2 for the 125 mm-thick sheet, you are effectively getting an insulation layer, a roofing felt and a vapour barrier all fitted in one operation.

Warren Sim runs Diamond Roofing in Leicester and has worked with both the standard style of warm roof and with Therma-Flect board. “Therma-Flect is far easier to lay,” he says. “For a start the boards are only 1.2 m wide, which makes them much easier to handle. Provided the rafters are spaced accurately, it just slips into place. I estimate that the fitted price is working out at £2.50-£3/m2 less than other methods of warm roofing.”

Another user is Hampshire housebuilder Nick Robinson: “I’m surprised that it isn’t used by everyone who builds a warm roof - it’s just so much easier to erect - even when you have to cut down the insulation to fit awkward sizes. If you have a perfect roof with all of the trusses exactly positioned then the installation of Therma-Flect is incredibly quick.”