Insulation Ratings

[Greenside]

I see microfibre ceiling insulation in rolls for use above ceilings in a wide range of brands and prices. The principle of R ratings I understand, but the variation seems very wide for the same kind of insulation (fibregalss or microfibre in a mylar bag) that I wonder if you can place any faith in them at all. I also notice that some are giving an R-t rating, rather than simply R; anyone know what that's about? I can't find a reference on Google.

Two products from Global House that I recently bought seem virtually identical and yet one claims an R-t value of 21 and the other an R value of 12, the former being about 30% more expensive.

[John]

R = Thermal resistance

RT = Total thermal resistance

RT would be used where a roof has several layers of insulation with RT being the total.

[Greenside]

So quoting Rt for a single layer product means that somewhere in the small print they'll say how many layers the figure is based on?