Tackling the age old question of ‘how much insulation’?

How you will tackle the problem of how much insulation to use and where to put it will depend on factors such as the rafter depth, whether there is an existing ceiling, the type of external roof covering and how much headroom is available. If you are doing an extension then it should have been designed with 0.18 in mind (0.15 if in Wales or Scotland), and Building Control will likely take a dim view if you say you can’t achieve this. In refurb situations, however, there may be limitations as to what is technically feasible, and a worse U-value may be permitted. For a new build an energy calculation (SAP) will have been carried out, and this would indicate the U value target to be achieved for each thermal element. These are often in the range 0.13 to 0.18 W/m²K, although if there is a lot of glazing it could be as low as 0.10.

When to drape the product?

TLX Gold can be used draped between the rafters (with the usual 10mm breather membrane sag), and since it is 33mm thick but compresses to 3mm under the tile battens, it will therefore occupy the topmost 40mm of rafter space. It needs an unventilated air gap underneath – a minimum of 10mm but ideally 20mm – to reflect heat back from its shiny yet fully breathable underside.

indicates the amount of PIR board needed to achieve U values from 0.18 down to 0.13, and the minimum rafter depth required to accommodate it.

There's also a pulled-taut solution

Alternatively TLX Gold can be used stretched taut and counter battened, which does not take up as much rafter space, allowing an additional 20mm more PIR board to be accommodated. To achieve U=0.18 W/m²K 125mm of PIR is needed, requiring rafters that are 180mm deep @ 400mm centres if used draped or 165mm deep if used taut and counter battened.  If the rafters are at 600mm centres then 110mm PIR in 165mm deep rafters (draped) or 150mm deep rafters (taut) is needed.

Using in combination with PIR

With deeper rafters it may be possible to make use of a second unventilated air gap by placing the PIR board centrally, enabling a reduced thickness of PIR to be used.

Although it is possible to use mineral wool rather than PIR, this has a lower thermal conductivity so is only worth considering using it as a between-rafter solution if the rafters are >200mm deep.

What if you have a vaulted ceiling?

If there is a vaulted ceiling then loss of headroom is generally not a problem, so draping TLX Gold between the rafters and running 85mm PIR board underneath is an option that achieves U=0.18 W/m²K. If there are purlins and the PIR is set between them, then there is a small adjustment to the bridging factor used in the calculation.

And if the rafters are <150mm deep?

A very common scenario is where the rafters are <150mm deep, when it becomes necessary to use PIR underneath the rafters as well as in between (though using TLX Gold taut with 110mm K107 phenolic board does not require this underdraw).

It is possible to use TLX Gold insulating breather membrane with mineral wool between the rafters, and a PIR underdraw, but this needs calculating depending on individual circumstances.

If there is an existing ceiling, then it will not be possible to fit PIR underneath and achieve U=0.18 W/m²K, but using TLX Gold can be particularly valuable here if there is no vapour barrier present, as there is no risk of interstitial condensation when it is used.

So although certain standard solutions apply to particular situations, where there are constraints such as not being able to raise the roof, or no headroom, or shallow rafters, then it is worth contacting the TLX Hotline on 01204 674730 to discuss the most appropriate option.

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