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The TLX Gold installation instructions describe how you can drape TLX Gold between the rafters or else pull it taut and counter batten. The diagrams make this clear:
If you drape TLX Gold 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. So, the amount of PIR board between the rafters is then the rafter depth minus 50mm.
If you pull it taut, then counter batten, the top 1/3 of the product expands upwards. The lower 2/3 sags downwards (BR 443 Conventions for U-Value Calculations). Since TLX Gold compresses to 3mm by the counter batten,10mm expands into the counter batten space. 20mm down into the rafter space. So, with a minimum unventilated air gap, you require a minimum 30 mm of rafter space.
By counter-battening TLX Gold, you can accommodate 20mm more PIR board within the rafters as compared with draping it. But it does mean raising the roof to be 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 go down to the gutter. 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 ¬Ëœair-open’ and to ventilate through themselves, without requiring eaves and ridge ventilation (unless the rafters are >10m long). If the external covering is not air-open (e.g. a metal roof, or fibre-cement slates) then counter batten and introduce eaves to ridge ventilation.
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 should I make?
Firstly, what is the external covering? If it is metal or fibre-cement slates, then you need to counter batten, plus have eaves to ridge ventilation.
When 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 insulating breather membrane 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. When 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.
