TLX Gold: should I use it draped or counter battened?

12/09/2019

The TLX Gold installation instructions describe how TLX Gold can be used either draped between the rafters or else used taut and counter battened. The diagrams make this clear:

do i need to counter batten with tlx gold

If TLX Gold is fitted draped between the rafters, then the drape or sag (a nominal 10mm), the product thickness plus an unventilated air gap requires a minimum of the top 50mm of rafter space to accommodate it. So, the amount of PIR board that can be fitted between the rafters is then the rafter depth minus 50mm.

If it is used stretched taut then counter battened, it is considered that the top 1/3 of the product expands upwards and the lower 2/3 downwards (BR 443 Conventions for U-Value Calculations). Since TLX Gold is compressed to 3mm by the counter batten, this then means that 10mm expands into the counter batten space and 20mm down into the rafter space. So, with a minimum unventilated air gap of 10 mm this means that a minimum of the top 30 mm of rafter space is required.

By using TLX Gold taut and counter battened, 20mm more PIR board can be accommodated within the rafters as compared with using it draped, but it does mean that the roof needs to be raised by the height of the counter batten.

What governs the dimensions of the drape and the counter batten?

The drape is there for two reasons: firstly, to allow any water coming through the tiles to be ducted down to the gutter and not be trapped by the tile battens, and secondly to contribute to the ventilated space required in the tile battens. If the external covering is of natural slate or clay, or concrete, these are considered to be ‘air-open’ and to ventilate through themselves, without requiring eaves and ridge ventilation (unless the rafters are >10m long). Note, however, that if the external covering is not air-open (e.g. a metal roof, or fibre-cement slates) then counter battening and eaves to ridge ventilation is required.

When TLX Gold is used with a counter batten, the counter batten provides sufficient space for drainage and also ventilation. We recommend using a 38 mm deep counter batten if possible, to compensate for the upwards expansion of the product.

So what decisions are to be made?

Firstly, what is the external covering? If it is metal or fibre-cement slates, then counter battening (and eaves to ridge ventilation) is required.

If the rafters are sufficiently deep to accommodate the insulation required to achieve the target U-value using TLX Gold draped, then either choice is appropriate.

If the roof cannot be raised, for example if there is an adjacent property, then TLX Gold would need to be used draped, and possibly using an underdraw of PIR board to achieve the target U-value.

If on the other hand there is no headroom and PIR board cannot be used underneath the rafters, then either TLX Gold could be used taut and counter battened (if this allows sufficient depth to achieve the desired U-value) or else battens could be installed over the rafters to increase the effective depth. If neither are possible, then using a phenolic board (with a slightly better insulation value) would save a few millimetres.

If you need advice on your particular situation, or require a U-value to be calculated for a given buildup, then contact our technical department for advice on 01204 674730.

View our case study detailing an example of TLX Gold draped

View our case study detailing an example of TLX Gold pulled taut

View all LABC approved Gold solutions HERE

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