Can I use Multifoil insulation with Mineral Wool?


The PIR board shortage is beginning to bite, with some retailers reportedly upping their prices to builders desperate to complete jobs whilst still satisfying their insulation specs. A serious explosion at BASF’s raw materials plant in Germany has meant that production will be down for several months.

What sort of alternative does Mineral Wool provide?

Judging by the enquiries we get on the Helpline, the perceptions of mineral wool vary from those who optimistically think 100mm of it should always be good enough to keep Building Control happy, to those who are aware of the differences in thermal conductivity between PIR board at 0.022 W/mK and mineral wool, where the conductivity or lambda value (λ) varies from 0.032 W/mK to 0.044 W/mK, depending on the grade. Unless the rafters are deep enough, they're unlikely to be able to achieve the current 0.18 W/m2K standard using mineral wool.

How does Multifoil insulation work in this scenario?

But add TLX Silver into the equation and you get added benefit, not only from its core insulation value but from the heat reflection back across the adjacent air gap that is created by using a batten of the right depth.

By fitting the highly reflective TLX Silver under mineral wool used between the rafters, then securing with a batten to provide an unventilated gap – ideally 20mm deep - the U value can be improved.

For Roofs:

For example, 150mm of 0.032 W/mK mineral wool with TLX Silver below secured with a 38mm x 38mm batten then plasterboard will achieve the current B Regs requirement of 0.18 W/m2K for rafters @ 400mm centres. If they are at 600mm centres then only 0.035 W/mK grade wool is needed. And adding TLX Gold above this (in 200mm deep rafters) achieves U = 0.13 W/m2K!

Using TLX Silver or TLX Gold with mineral wool as an alternative to PIR insulation

And for Walls:

For new builds with timber frame walls very often the ‘notional dwelling’ U value requirement of 0.18 W/m2K given in Part L1A of the B. Regs is used.

As an example, a 140mm stud, brick clad and with TF200 Thermo as the reflective breather membrane need only be filled with 0.035 mineral wool with TLX Silver stapled to the inner face of the studs and secured with a 38mm batten.

What values can be acheived?

If lesser values are needed then this generic table for 140mm studs gives an idea of the range of U values that can be achieved using either a reflective or standard breather membrane.

Where an extension or new wall is being built then the U value required is 0.28 W/m2K, and this can be achieved using a 90mm stud filled with 0.040 W/mK wool and TLX Silver fixed internally with a 38mm batten.

And for an existing solid masonry wall requiring U=0.30 W/m2K, 75mm battens @400mm centres filled with 0.035 W/mK wool and TLX Silver multifoil insulation is the answer. 

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