I saw some Knauff loft insulation "slabs" in a well known DIY shop. I thought the advantage of using them was that a thin slab gave the same amount of insulation as a thicker roll, however reading the packaging closer two slabs (~10cm IIRC) give the same insulation as a 17cm roll. I suppose that 10cm is an improvement on 17cm but I was expecting a more dramatic difference. I hear Kingspan is supposed to be wonderful; how does this compare?
Also I have seen sheets of polystyrene in the same shop. How much insulation does that give (I was thinking of fitting some to the back of my loft hatch).
Is there a table anywhere showing u-values for these various materials? So I can compare like with like?
I've just increased the glass fibre insulation in our loft from 100mm to
270mm and used the boards to create an area where Management can store junk, erm 'heirlooms and things that might come in useful'.
The board was easy to cut and easy to manoeuvre and, importantly, much more pleasant to use than rolls of glass fibre. However, it's much more expensive than glass fibre and so the hard standing was kept down to a minimum. It appears, at this early stage, to do what it says on the label.
They're listed on the various manufacturers' websites.
I did that exercise too a couple of weekends ago when doing my loft so have a few suggestions. B&Q are selling packs of 4 spaceboard slabs for £20 IIRC. I forget the dimensions but something like 1000mm x 500mm x 52mm?
I priced up 2400x1200x50 sheets of celotex in the builders merchant. Roughly £17 inc if you buy a serious enough quantity. Wickes were selling the same product for £27 a sheet, so I'm glad I didn't impulse buy there! All in all, seems much better than the spaceboard option and celotex is foil backed and claims to have a strong core - its very fibrous in the middle. Cuts easily enough with a fine handsaw.
I bought the decent quality flooring packs from B&Q - £10.93.
The only trouble with this celotex lark is that once you have noticed the improved warmth in rooms, you then want to dryline all the ceilings, north facing walls etc to reduce heatloss even further :-(
On Wed, 19 Mar 2008 21:40:37 GMT, a particular chimpanzee, Fred randomly hit the keyboard and produced:
The way to compare the relative insulation values of materials is their conductivity values, expressed as their 'k-value' or 'lambda' (Greek letter looks like an upside-down lower case y), with the units Watts/metre.Celsius (or Kelvin) (W/mK). This is a measure of the material's insulation value for a given thickness, and the lower the better.
The Resistance value is the total amount of insulation a square metre of a particular thickness of a material gives, in metres^2.Kelvin/Watts (m^2K/W), and the higher the better.
Resistance can be calculated by dividing the thickness of a material by its conductivity; R = t/k. So 50mm thick of a material which has a k-value of 0.02W/mK has the same resistance as 100mm of a material with a k-value of 0.04W/mK.
A U-value is the reciprocal of the sum of all the resistances through a particular area of the construction, including surface resistances. So a roof would include the outside surface, the tiles and felt, the roof void, insulation, plasterboard and the internal surface. An allowance also has to be made for 'bridging elements' such as joists, etc. So be very, very wary of any material which claims that it meets any particular _U-value_. The only claim a material can make is its Resistance value.
Most insulation materials are in the range 0.023W/mK to 0.042W/mK, so the best still needs to be more than half as thick as the worst.
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