Multifoil insulation: the choice for solid masonry walls?


Target U-Value 0.28 W/m²K?

Solid walls can be of variable construction and materials, but if your project is to be approved by Building Control they will be looking for a U-value of 0.28 W/m²K if it is a new wall or 0.30 W/m²K if it is an existing wall. If there is a cavity wall, the cavity is not of significant thermal value unless it is filled with insulating material. If the inner skin is a modern Thermalite block, this has far better insulating ability than the old dense concrete types and quite possibly very little additional insulation will be required.

The exact U-Value can depend on the bridging factor

For a brick, concrete or stone wall then the best way to insulate internally is to set studs against the wall. The exact U value obtained will be influenced by the bridging factor, i.e. the thickness of the studs / their spacing, but the examples below give approximate requirements. The 0.28 W/m²K example on our website is based on a 65mm deep stud with 40mm PIR set against the wall, TLX Silver stapled to the inner face and secured with a 38 mm deep batten, and makes use of unventilated heat-reflecting air gaps either side of the TLX Silver to contribute to the thermal performance.

TLX Silver multifoil used in a masonry wallAn alternative using more readily available batten sizes would be to fully fill with PIR between 50 mm deep studs, staple the TLX Silver on directly and secure with a 50 mm deep batten. The single 20 mm unventilated air space so created also serves as a useful service cavity to run cables up the wall.

Creating a barrier to vapour

TLX Silver is CE marked as a vapour barrier, preventing water vapour created inside the rooms from migrating to the cold side of the insulation, where it could condense against the cold outer wall. It is advisable to use a breather membrane against the wall, as this would allow any trapped water to permeate through to the outside – albeit slowly – when conditions are warmer, whereas a damp-proof membrane would seal it in. It is essential that any breach in the vapour barrier should be made good by patching and sealing, e.g. behind light fittings and electrical sockets.

Which stud depth?

Often an extension or refurb plans have been designed with a somewhat arbitrary choice of stud at 75mm or 100mm, without planning for the U value requirement. When the builder then wants to use mineral wool to save material and labour costs, he finds that fully filling the studs won’t meet the 0.28 W/m²K needed.

By adding TLX Silver internally this can be achieved with the added benefit of not requiring a vapour barrier nor insulated plasterboard. Although the precise buildup would depend on the grade of mineral wool used and the bridging factor, examples are:

100 mm stud, 0.044 W/mK wool, TLX Silver, 38 mm deep batten – 0.30 W/m²K

100 mm stud, 0.040 W/mK wool, TLX Silver, 38 mm deep batten – 0.28 W/m²K

75 mm stud, 0.038 W/mK wool, TLX Silver, 38 mm deep batten – 0.30 W/m²K

75 mm stud, 0.035 W/mK wool, TLX Silver, 38 mm deep batten – 0.28 W/m²K

Best batten size?

Where a result is required as quickly as possible at minimum cost, then simply sandwiching TLX Silver multifoil between two 25mm deep battens will achieve a U value of 0.47 W/m2K. This is particularly popular where there are walls with uneven surfaces, as the combination of battens and the flexible blanket enables the adjustments necessary to achieve a flat internal finish.

And if the studwork is Steel?

Where steel studwork has been used, however, it is not possible to either staple TLX Silver nor to screw through it (as the screws snag on the polyester fibres), so if it is planned to use the two together a batten should be used between the two, and the bridging factor of the steel sections needs to be accounted for in any U-value calculation.

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