"U" Value for Sterling Board?

[aesmith]

Hi,

Still not sure what to do with my roof. Anyone know where I could find a rough U value for Sterlingboard? I know it won't be much, probably significantly worse than wood for the same thickness - but I'm comparing with thin steel sheet here so its got to be an improvement on that!

Thanks, Tony S

 

[srp]

You really mean K-value (U value takes into account the convective and radiation heat losses, as well as the thermal conductivity of the material). So if you search for 'K Value table' or similar you might find something that will give you a rough guide and enable you to compare it with other materials.

Having just insulated my workshop, I have to say that if you're going to the trouble of lining it internally, it would be a shame not to put something in at the same time. I bought a couple of packs of 100mm blanket (made from recycled plastic bottles) from B+Q to line the pitched roof on mine, plus a roll of thin polythene vapour barrier from Screwfix - total cost £38 for a building just a bit bigger than a single garage. Should have done it years ago, it's made an incredible difference. Currently, most of the big diy stores are doing a 2 for 1 offer on insulation.

 

[aesmith]

So is "U" more of a rating for the installation, rather than the materials?

I found a "k" of 0.13 for Sterling Board. Units are given as Watts per metre per degree.

So heat transfer per degree will be k x area / thickness - does that sound right? That would make it 11.8W / sq.m per degree if I work that out correctly, for 11mm board. That doesn't sound right, coming out at not much less than the metal sheet.

 

[srp]

The k value given means it will conduct 0.13 watts per metre of thickness for every degree C difference between inside and outside temperatures. If you're thinking of materials in terms of how well they insulate (as opposed to how well they conduct) it is probably easier if you convert the k value to an R value. R is the reciprocal of k, so:
k = 1/R, so the R value of sterling board is 1/0.13, or 7.7 m degC/W.
Steel is approx 0.08, Concrete is around 0.6, and still air is around 40. In other words, a low number is bad and a high number is good, and these numbers explain why it's better to fill a sealed double-glazed unit with air rather than concrete or steel

You're right in thinking of the U value as a kind of overall figure for the complete construction, rather than for the individual materials, and it is calculated by adding the R values for all the materials in the 'sandwich', and at the same time allowing for their thickness (thick layers insulate better than thin layers). You should also add in the figures for convection and radiation - a building loses more heat if it's windy(convection), and more if it is painted matt black (radiation).

So, a steel roofing sheet will provide hardly any insulation, sterling board will not be much better, but an air gap is good, provided the air is still (moving air will also move heat). If you want a big air gap then loft insulation will provide it.

Putting all this into real life, adding a layer of sterling board will do little to conserve heat, you'll probably find that it will still suffer from condensation (not so noticeable because the board will soak it up until it becomes saturated) and within a year or two you'll have to rip it all down and start again. I'm afraid the only way to make a workshop warm is to exclude damp, insulate, prevent unwanted draughts and provide heat. Missing out any one of those four will cost you money.

 

[aesmith]

Cheers. I'm working my way through those magic four. Damp was the biggest problem, so making the roof waterproof is a step in the right direction. I still have to sort out gutters and drainage which will hopefully help dry out the masonry. Heat and insulation sort of go together .. there's not much point in adding heat when there's an uninsulated roof!