Loft insulation U values. Where to find a chart?

[Mike Barnard]

Hi.

I need a basic chart showing the heat losses through a loft area if there is no insulation, 50mm, 100mm up to the 265mm nominal 'perfect' insulation. Anyone know where there is one or the facts (validated) so I can draw one up for myself?

Along the lines of...

One sq metre of uninsulated plasterboard has a U value of... ? Ditto with 50 mm standard rolled fibre insulation. Ditto with 100 mm, 150 mm etc.

It's so I can show someone a comparison so she can decide just when enough is enough compared with keeping spending more and more on more materiels.

I am googling, and it's ongoing, but all I find so far is detailed tech guides or company ads. I'll bet someone on here has a link already! :)

Thanks.

[The Natural Philosopher]

K value for plasterboard is 0.16. So U value of 16 for 10mm thick.. or

16 watts per square meter per degree centigrade.

k value of glass fibre around 0.034 so a U value for 50mm is 20 times that. 0.6 pr so? seems wrong..no..thats more or less right.

Divide by 2, 3 etc..

Note that when loose fiber insulation is used, the value of a WINDPROOF layer like plasterboard is really high. Its not the same with celotex, which is windproof to start with.

In terms of loft insulation around 100mm of rockwool, windproofed to the hilt, is probably the best cost-benefit compromise.

If you want a useable surface there , fill between joists totally with rockwool, then screw some 2x4s across the lot and fill again, then plate over with chipboard.

If its to be left unused, then lay that last layer across the joists instead.

[Andy Wade]

Nonsense - you've forgotten to include the surface boundary layer resistances.

For calculation principles and up-to-date data see BRE publication 443:

[The Natural Philosopher]

Who said I should?

That was straight out of te building regulations hand book.

Take it up with them.

I tend to trust them more than you.

[Hugo Nebula]

On Sun, 30 Dec 2007 01:04:01 +0000, a particular chimpanzee, The Natural Philosopher randomly hit the keyboard and produced:

But he is right. The Resistance value of all the elements is added together to give the total R-value; a U-value is the reciprocal of this. The R-value includes the surface resistances of the inside and outside surfaces (IIRC, between 0.35m²K/W to 0.04m²K/W depending on a few factors).

[The Natural Philosopher]

Highly dangerous factors. A ripping wind in the loft makes mockery of them for a start. And reduces rockwool insulation hugely.

However the OP did not ask for the total resistance of this or that construction, Merely the U values of the materials. That stands as written.

If you want the total U value of a construction do the maths and add in the boundaries.

BUT as I said, its dangerous to rely on them.

Anyone who has blown on his hands to warm or cool them can tell you that.

Judging by my boiler on times, `I lose 2-4 times more heat in a 20mph wind...

The difference when I fully boarded out 150mm rockwool in the loft was immense.

A fan blown heatsink will do about 5-20 times more heatloss than a convection cooled one. OH its DESIGNED to do that, which houses are not, but the U value calculations are VERY optimistic if they rely on ANY boundary layer outside the house, or in a 'cold' loft' whatsoever.

[Mike Barnard]

Interesting answers, thanks. All I really need to know is whether adding another 150mm of insulation will make a big difference to the existing 125mm. So If I had a chart as described I'd have an idea of a 'typical' heatloss for various depths. At what point does it become pointless (oops, bad english) to add more? Is it worth 300mm? 500MM? Where does one reasonably stop?

[Andy Wade]

OK, to save working out from scratch you could use Table S9 from the Standard Assessment Procedure (SAP 2005), here:

a bit of interpolation we can assume a U-value of around 0.35 W/m^2K for your existing 125 mm and 0.15 if you topped it up to 275 mm.

Here's a worked example, you can put your own details in. Say the roof area is 50 m^2, the difference in the U*A product between the present and topped-up cases is (0.35 - 0.15) * 50 W/K = 10 W/K (i.e. you'd save

10 watts for every degree of temperature difference between inside and outside). For a typical 2200 degree-day heating season the saving will be 10 * 2200 watt-days, i.e. 22 kW-days or around 500 kWh. If the net fuel cost is, say 4p/kWh then the annual saving would be a paltry GBP 20.

For new construction around 300 mm, but if you've already got 150 mm there's hardly any point in topping up at present fuel prices.

[Hugo Nebula]

On Sat, 29 Dec 2007 13:51:04 +0000, a particular chimpanzee, Mike Barnard randomly hit the keyboard and produced:

The 2002 Approved Document L1 (Dwellings) is still available on the Planning Portal

had some useful and not-so-useful tables in the back.

What you will hopefully see is that it's not a simple straight-line graph where doubling the insulation halves the heat loss. It isn't. It also depends on the proportion of heat loss through bridging elements such as ceiling joists. It is also subject to diminishing returns, where a small initial amount of insulation has seemingly dramatic effects but subsequent thicknesses have less and less effect. This is where other factors affecting heat loss and energy use such as airtightness come in.

[The Natural Philosopher]

Where the loses from other sources exceed the losses from the bits you are insulating.

In a house to modern standards, it is in fact the trickle ventilation that sets an upper limit on how much its worth insulating.

That and unavoidable 'cold bridges' around window and door frames.