Insulation - are all types created equal?

[Jimothy]

Apologies if this question has been asked before, but I've tried the search function and didn't find the answer that way.

I find it strange that the "recommendations/regulations" about loft insulation always talk about a minimum depth for insulation, rather than specific thermal performance.

I understand that in most types of insulation, it's trapped air, not the material itself, that actually restricts the transfer of heat across it, BUT SURELY this can't mean that 270mm of fleece has the same thermal properties of 270mm of loose-fill vermiculite or 270mm of expanded polystyrene etc, etc...

Which materials offer more/less insulation per mm than standard fleece?
and what about silver-coated materials - is there any appreciable advantage? Can you get away with using up less space but still bring your loft up to spec?

 

[freddiemercurystwin]

The Building Regs specify a U Value not a depth. Mineral wool is less thermally resistant than rigid foam insulations. There are half a dozen or so different types of insulation generally available. Generally the better the insulation the more expensive.

Have a look at the chart on page 2: http://www.springvale.com/uploads/resource/tb2.pdf or http://www.greenspec.co.uk/insulation-introduction.php

The foil blanket insulations are a bit of a con tbh, the method of testing is considered dubious at best and you need an air gap each side so whilst the insulation may be 25mm thick the space required is 75mm. Even then they need to be used in conjunction with an additional layer of rigid insulation about 50mm thick to get them to work so a bit of as mentioned they're really a bit of a con, blooming pricey too.

Typically if you were doing your loft and you wanted to avoid the 3 layers of mineral wool you could use about 170mm polystyrene or about 120mm of Celotex or Kingspan.

 

[Jimothy]

Thanks for that, very helpful.

So essentially, in an attic that's been insulated with 100mm of fibreglass, then half boarded over, I could double the insulation if I lift the boards, replace the fibreglass with kingspan, replace the boards, and use the leftover fibreglass to double-layer the remainder of the roof? That sounds like a much easier way of doing things than having to increase the height of the joists just to lay the boards over twice the depth of blanket.

Have I got this about right?

 

[freddiemercurystwin]

Sounds about right to me.