The Financial impact of Multi Layer Inverted Roof Schemes

U-values for roofs have significantly increased over the past few years, and with the Government drive to Carbon Net Zero we will continue to see buildings becoming more thermally efficient.

This drive for thermal efficiency places high demands on the thermal insulation Lambda values, driving manufacturers to improve their products’ thermal performance per mm thickness. But what happens when you cannot use a single thickness warm or inverted insulation board to achieve the required U-value? Simple right? You just put a number of layers together to achieve the required U-value…

However, the outcome of adding additional layers impacts the installed cost through a number of factors:

• The number of layers of insulation boards.

• The hours required to handle and install the insulation boards.

• The number of vehicles required to deliver the insulation board.

• The number of crane or hoist lifts required to get the insulation boards to the roof.

• The storage space required on site for the insulation boards (if there is any).

• The number of pallets and volume of packaging to be disposed of.

• The carbon footprint of the delivery and the material, something increasingly being measured by clients and contractors.

There is also an impact on the design of the roof with regard to parapet heights, rooflights, detailing etc. These impacts can usually be dealt with in design for new construction but in refurbishment this can be a significant challenge.

All common sense you may say, but Dear Reader, are you aware of a significant additional impact for inverted roofing? Did you know that the method of calculating the Lambda value of an inverted roof system, ETAG 031, only accounts for up to 2 layers of insulation boards, any more than that and the calculation is not valid. This could mean that your thermal calculations and installation are not compliant and that would be a significant financial impact if the roof had to be removed.

This is something that QI see in the market on a daily basis; contractors tendering schemes with 3 or 4 layers of inverted roof insulation that cannot possibly be supported with a compliant thermal calculation because it is cheaper on a m3 rate (bear in mind our January 2021 article on the difference between buying in m2 rather
than m3).

When tendering your next multi-layer inverted roof scheme it makes sense to ensure no more than 2 layers are used as not only does this eliminate compliance risk it saves on all the costs related to the bullet points above.

Next month’s QI by QI – The impact of insulation fire performance reclassification.