roof insulation advice

[namnok]

i've finally got cash to insulate under the roof with insulation boards.

i've just got 4 sheets of xtratherm so far but looking at the entire roof area its gonna cost me another £200 on top of the £300 i've put aside if i continue with this method.

Now can i simply just get some cheaper non foiled version then stick up foil over the top? same trick?

 

[^woody^]

Not unless you glue the foil flat on the board

Why not use a different product or thinner board?