Engineered timber joists manufactured by Boise Cascade have been used to create a fully habitable roof space or third floor within an individual house design, achieving Level 5 of the Code for Sustainable Homes.

The house, which is near Nottingham, was designed by Andrew Banks of Banks Design Architects and consists of a frame supplied and erected by Frame Wise Ltd, using the latter’s new Twin Wall system. Boise’s BCI joists featured in the two upper floor levels and the roof.

“The purpose [of the Twin Wall system] is to make the internal skin completely independent and load bearing, so that the floor joists sit on it without there being any thermal bridge,” said Frame Wise’s managing director Simon Orrells. “It is then possible to fix the internal vapour barrier up the wall, wrap around the end of the joists and continue on up the inner skin of the first floor wall to achieve excellent airtightness.

“The BCI joists, puckered with Isover mineral wool insulation, also form the 365mm deep roof construction which creates the third floor level.”

The wall construction achieves a U-value of 0.1W/m²K, while the air leakage rate is 1m³/hr/m².