Insulation: performance equivalence table

[ToughButterCup]

Quoting from our architect's notes

 

'...Insulate fully between rafters with Kingspan K7 or equivalent, with 62.5mm of Kingspan K18 beneath rafters...'

 

That suggests that somewhere there is a performance equivalence table. But there isn't is there? There seem to be equivalence claims (often unverifiable, it seems)

 

This is the closest I got. Anyone help me do better, please?  Specifically, what is the equivalent of Kingspan K7?

 

[Ian]

@recoveringacademic

All he means by the word "equivalent" is a similar type of PIR  board with a thermal conductivity rating at least as good as the K7. The most important equivalency you need is the thermal conductivity rating because if you swop it for something with lower performance it will affect your 'U' values and therefore the SAP rating.

Edited February 7, 2018 by Ian

[ToughButterCup]

Thanks.

Is there a simple table somewhere which helps us choose board with similar characteristics?

[Ian]

1 minute ago, recoveringacademic said:

Thanks.

Is there a simple table somewhere which helps us choose board with similar characteristics?

Not that I know of but there's only a few major manufacturers of PIR board so it's quite easy to check for example the Celotex equivalent of Kingspan K7.

[Jeremy Harris]

as @Ian suggests, just look at the λ for the material and knock up a comparison table.  For example, the majority of PIR foam insulation is around 0.022 to 0.023 W/m.K, where the lower this figure the better it is as an insulator.

 

You can trade thickness for thermal resistance, too.  Say you want a U value equivalent to using 200mm of PIR with a λ of 0.022 W/m.K.  The R value (thermal resistance) of 200mm of this material would be the thickness (in metres)  divided by the λ value.  In this case that would be 0.2 / 0.022 = 9.091 m²K/W.  To get the U vale you add all the R values in the various layers and then take the reciprocal.  In this case there is just the one layer so the U value would be 1 / 9.091 = 0.11 W/m².K

 

Most PIR and PUR foams are pretty similar, with a λ of around 0.022 to 0.023 W/m.K.  EPS varies a bit but tends to have a λ of around 0.034 to 0.038 W/m.K.  If you know how much room you have for insulation, you can pretty quickly do some trade-off calcs to see where the best value is for you in terms of material and thickness, bearing in mind that insulation with a higher decrement delay factor will tend to make the house feel more comfortable, and that insulation materials like PIR have a very low decrement delay and are far from ideal in roof and wall structures, particularly those that are exposed to the sun.

[AliG]

I found a table that gives the different products for each application

 

https://www.just-insulation.com/downloads/celotex-rigid-insulation/celotex-promotion/celotex-comparison-matrix.pdf

 

Two products with the same thickness and the same conductivity are equivalent. So Kingspan K7 has conductivity of 0.020 W/m.K. 

 

K7 board is very overpriced for the extra insulation it gives IMO. Celotex GA4000 with conductivity of 0.022 W/m.k is around 1/3 cheaper. Other similar products are available.

 

To get the U- value -

 

First divide the thickness by the resistance, so for example if 100mm thick

 

0.1/0.02 = 5

 

This is the R-Value.

 

Then 1/R-Value is the U-value

 

1/5 = 0.2

 

So 100mm of K7 has a U Value of 0.2

 

If you are combining products, you can calculate the R-value for each product, add all the r-values together than put 1 over this to get the overall U-value.

 

100mm of Celotex GA4000 has a U-Value of 0.22

 

So a third extra money gets you 10% better insulation. K7 is only worth the cost if you have a height restriction and absolutely need the insulation to be as thin as possible. Thicker lower spec insulation may be better value or just modestly lowering the target insulation level.

 

Similarly K18 is top of the line board and very expensive.

 

My architect specified the absolute highest level of everything insulation wise and then I adjusted it all depending on value. You would think they are on commission from Kingspan.

 

Assuming your rafters are 200mm deep, that is a lot of insulation and will be very expensive. Also Kingspan does not come in 200mm thick boards which doubles the labour costs to install it.

Edited February 7, 2018 by AliG

[ToughButterCup]

@AliG and @JSHarris, you are both stars. 

That's what I love about this place: constructive, intelligent engagement with slightly complex problems that would take eons to work through for yourself.

 

Ian

[SteamyTea]

There is also vapour resistance and air permeability to take into account sometimes, as well as @JSHarris favourite, decrement delay.