Multifoil insulation: the choice for timber frame walls?


U-Value requirements can vary dramatically

Although the construction of a timber frame wall is standard – a vapour barrier internally, the timber frame acting as the load-bearing component, a sheathing board on the outside for additional bracing covered with a breather membrane for waterproofing, then cladding - the U-value requirements can vary considerably depending on the project and the construction. An existing TF barn refurbishment may only require U=0.30 W/m²K, whereas a new build that has had an energy calculation done on the fabric may require 0.16 W/m²K and sometimes even less.  Sometimes if the required U value can’t be achieved on the roof (usually because of lack of headroom) then it can be compensated for on the walls, though with an existing wall there can be concerns about encroaching into the internal space.

Finding the right build up to suit your project

Our Approved Solutions Guide has a comprehensive set of tables for build ups based on using multifoil insulation with PIR or mineral wool intlx silver multifoil used in timber framed walls standard stud depths. The U value will depend not only on the type of insulation material used, but on the type of cladding used and whether a reflective breather membrane is used.

A timber cladding is ventilated, with cold outside air in contact with the breather membrane, whereas brick cladding, although the cavity is drained, is considered to be unventilated and so this air layer contributes to the U-value. Similarly a reflective breather membrane is of little benefit in contact with a ventilated space, but enhances the thermal value of it if it is unventilated.

We find that customers often underestimate the amount of insulation needed in a roof, yet with existing walls the amount of insulation required is overestimated.

Achieving U-0.30 W/m²k

For an existing timber frame wall, 0.30 can be achieved by fully filling a 90mm stud with low-grade (0.040 W/m²K) mineral wool, with TLX Silver on the inner face secured with a 38mm deep batten. Mineral wool products designed for use in TF walls are 600mm wide and take the form of rigid batts that will not slump when fitted vertically (unlike loft roll, say).

Lower values can be achieved if necessary

tlx silver multifoil wall solutionsAlthough U-values as low as 0.18 W/m²K can be achieved with mineral wool and a 140mm deep stud, for lower U values it is more common to use PIR board, which in a 140mm stud  - leaving an unventilated heat-reflecting air gap either side of the TLX Silver – can achieve 0.15 W/m²K.

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, and here using TLX Silver internally with the appropriate insulation between the studs can achieve the desired value, as well as having the double benefit of providing a useful service cavity and an excellent vapour barrier.

Air gaps are always important

Because TLX Silver multifoil compresses to only 2mm, this means that a 38mm deep batten provides space for both the expanded product and a heat-reflecting air gap, whereas this may not be true of competitor products made from less compressible foam sheets, especially if only a 25mm batten is used, which may not give space for any air gap at all!

View our LABC approved wall solutions HERE

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