An R Value isn't everything?

13/10/2019

"Why should I use your product when manufacturer X’s product has a better R-value than yours?”  

What’s in an R-value?

The R-value (thermal resistance) is a measure of the amount of thermal protection the product provides. It will depend on how thick it ismultifoil r value - a 100mm Kingspan board, for example, will have twice the R-value of a 50mm board – and also on the thermal conductivity of the material (i.e. how easily heat can pass through the material). So 100mm of glass fibre used on loft floors with a thermal conductivity (λ) of 0.044 W/mK will have the same R value as 50mm of 0.022 W/mK PIR board such as Celotex.  A thicker multifoil will have a greater core R-value than a thinner one – but is this necessarily desirable?

Still air is important too?

The whole point of using a multifoil is to take advantage of a layer of static air next to it, since air – which is a good insulator to start with – has an enhanced R-value when heat gets reflected back across it from the shiny outer layer of the multifoil. So the total R-value consists of the core value of the multifoil itself, plus those of 1 or 2 air gaps adjacent to it.

So that’s why many manufacturers quote a combined R-value with two air gaps. But that’s OK, right?

Well, no, because you really need to ensure that in the buildup you’re proposing to use, that you have two unventilated air gaps at least 13mm wide either side. If your buildup is under a bitumen felt that requires a ventilated space under it then this won’t apply, as cold outside air is being introduced, so you’d only benefit from one unventilated air gap.

So you staple the multifoil in place and the batten creates the air gap, right?

It depends how deep the batten is and what the multifoil compresses to. TLX Silver compresses to 2mm, so by leaving a 30mm gap at the bottom of the rafter and then using a 38mm deep batten under the product you create 70mm of space that can accommodate the 30mm TLX Silver and 2 x 20mm air gaps. The rules for calculations state that 1/3 the product ends up in the rafter space and 2/3 in the batten space beneath.

But other manufacturers imply you only need a 25mm batten?

Then it’s unlikely that you’re getting the benefit of the air gap. But work it out – if their product is, say, 40mm thick and compresses to 7mm between the rafter and the batten, then 11mm will end up in the rafter space and 22mm in the batten space, meaning that a 25mm deep batten will only give a 3mm air gap. You’d need a 38mm deep one to get a 15mm air gap, and would also need to leave sufficient space at the bottom of the rafter.

So having a thicker product is of questionable benefit, as there may be more loss of headroom and inadequate air gaps, and where rafter space is limited it may be better filled with PIR board.

I see buildups where silver multifoil has been used over the rafters or in double layers. Why don’t you show any like this?

You can use TLX Silver over rafters, BUT since it is a vapour barrier (as are all the silver multifoils), it is essential to use a really good vapour barrier under the plasterboard, and ensure that it is never cut into, e.g. when downlighters are fitted. If not, a severe condensation problem can result, with water vapour migrating into the rafter space then condensing when it gets to the cold side. Why even risk it when you can use TLX Gold instead? Also you may well find that Building Control considers such buildups unacceptable because of this condensation risk. Really a vapour barrier should only be used on the warm side of the insulation buildup.

When 1+1 doesn’t = 2

If you use a double layer of a silver multifoil you will be using two vapour barriers sharing the same air gap between them, so the thermal resistance is not double that of a single layer + 2 air gaps. Even worse, unless you use a separating batten - usually 50mm – that can accommodate both parts of the multifoils and create an air gap between – there will not be any air gap at all. If there is no other insulation on the cold side there will be a condensation risk if the inner layer is breached in any way. Products that are stitched rather than having the ultrasonic weld of TLX Silver will allow water vapour to pass through the stitch holes.

Wouldn’t it be easier to say what it is equivalent to in terms of mineral wool or PIR board?

No, because so much depends on having an air gap of the right dimensions adjacent. And for one product the equivalent mineral wool value quoted has no relationship whatsoever to the core R-value measured in a lab, but rather reflects the benefit of having an airtight layer on the inside of a buildup – which won’t wash when Building Control want to see the U-value calculation.

But at least I can go on the quoted core R-value?

Not exactly. You might think that since a British Standard exists there would be a level playing field – but in fact some test houses test in a slightly different way to give improved values. And a ‘one-off’ measurement of a manufacturer’s best offering is different from the value required by a BBA Certificate, for which multiple samples have to be measured, then that of the worst 5% is taken and rounded down (the R90/90 value). Some manufacturers have obtained a BBA Certificate but don’t then use the BBA-measured value, continuing to use a previous better value instead.

TLX Silver has had a BBA Certificate since 2006 and the R-value we use is that measured by them. Our build-ups and fitting instructions are carefully calculated so that you will get the U-value we state. It is an excellent vapour barrier and has ultrasonically welded seams – no stitching or tags through the material. It compresses to 2mm, so there is minimal loss of headroom when used with a 38mm deep batten appropriate to give an air gap.

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