Insulation effectiveness - not as straight forward as it seems.
Habitech wall and roof panels utilise panelised insulation - a much more effective solution than site-installed batts and blankets.
The construction industry talks about insulation installation as though it's a fairly simple business. The bag of insulation batts delivered to site has a large R value printed on the side, and when the batts have all disappeared inside your new wall or ceiling, the expectation is that your wall or ceiling now has that R value.
Unfortunately that is rarely the case. In a study conducted at the University of South Australia, it was shown that installations of R3.0 ceiling bats achieved an as-built R value of 1.37 - 45% of the expected value.
Even small gaps will greatly reduce the effectiveness of insulation. Setbacks around light fittings can also significantly compromise ceiling batts.
A second issue is thermal bridging. Thermal bridging is the name given to the transmission of heat through materials that span the depth of the insulation. A traditional stud wall has timber bridging from the back of the plasterboard to the back of the external cladding. A pine stud has an R value around 0.9, which is substantially less than your 2.5 wall batts. So around 10% of your wall has an R value less that half of the batts. Steel studs conduct heat more easily than timber studs and so significantly reduce the R value of a wall if installed in a way that creates a thermal bridge.
Using panellised insulation ensures there is no opportunity to leave out the insulation, substitute it with a lesser product, or for it to be installed with gaps. Habitech wall and roof panels get much closer to their theoretical R values when installed, because the system is manufactured with fine tolerances and factory based quality standards.