Quick note - Celotex vs. Knauf Space Blanket

[David WE Roberts]

150mm thick Knauff Space Blanket is on sale in B&Q at £3.74 for 1.97 sq metres (to fit between joists at 40mm spacings). Roughly £1.90 per square metre including VAT. R value is given as 3.4 and the U value as 0.044. No units.

Celotex GA3080 (80mm thick) is on sale from

£8.06 per metre squared (+VAT).

The R value is given as 3.45 m.sq.K/W and the U value as 0.29 W/m.sq.K.

I have consulted Wikipedia at

try and understand R and U values.

So:

(1) the R values look about right from the Wikipedia examples - and the Celotex comes up as about twice as effective as the fibreglass per unit of thickness. (2) The fibreglass is about 1/5 the cost of Celotex for the same R value (and this is from a discount Celotex web site).

So my one remaining problem is the U value. If the U value is the reciprocal of the R value then for the same R value shouldn't you have the same U value? And if the R values are similar shouldn't the U values be similar? Quick check with the calculator for 1/R shows that the Knauff figures are wildly off. Should have an R value of about 22.7!

I have checked another site

this gives the same figures for the 150mm space blanket, and includes the units this time which are the standard SI units as shown for Celotex.

O.K. - dodgy (but consistent) figures from Knauff.

Strong cost justification for buying the space blanket which is no doubt subsidised by me paying my heating bills.

Longer note than I intended - got baffled by U values part way through!

Cheers

Dave R

[Cicero]

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Space blanket is probably much easier to install if it's going in a loft. It will conform automatically to the joist spacings (requiring no cutting) and it can have a second layer laid at right angles to the first layer. It's also very clean to lay as it's fully encased. At the prices you've seen (1:5) even two or three layers are more affordable than Celotex.

The downside is that you virtually abandon your loft as a storage space.

Cic.

[John Rumm]

That's 'kin expensive really - sounds like about £27 / sheet. Have you checked for other suppliers of PIR foam (rather than celotex branded)? Our local seconds place (does mostly ecotherm rather than celotex) would probably be something closer to £17 / sheet

so far so good.

I suspect that ratio can be squeezed a bit...

yes

The only obvious way to c*ck it up that I can think of (if you exclude just a plain 'ole typo), is they derived their U value from the R value expressed in imperial units. That would mean you need to multiply your

22.7 by 0.1761 to convert from ft^2 F h/BTU. Still gives a R value of about 4 though which sounds wrong (much closer, but still wrong)

Quite possibly in this case - especially if you are going for a cold deck. (as I said before, slapping a sheet of PIR over the firings and making it a warm deck is easier IME, but I appreciate you have your reasons)

Not sure its any clearer, but you could try:

[Bruce]

I used Celotex under the boarded storage area and Knauf space blanket for the rest. The best of both worlds, I think.

[Hugo Nebula]

Sounds about right.

No because the U-value is the reciprocal of the SUM of the R-values. In other words, you add the R-values of the inner and outer surface resistances, the resistance of the plasterboard ceiling, the slates, etc. And for added fun-filled jollity, you have to take cold bridging by, for example, rafters and ceiling joists into account.

To compare the relative values of insulation, with everything else being equal, it's safe to use the R-value as a guide.

Where you have the space (in a large open loft space, for instance), mineral fibre wins hands-down every time.

[no-one]

Could this be used under a floor to insulate it from a 6ft deep void underneath?

[Hugo Nebula]

[no-one]

Thanks.