Any insulator is 'good' its just a matter of how much is required to achieve a certain insualtion value.
The foiled sheets are worth the money if there are no alternatives at a similar price. If time, effort and alterations are a factor then these sheets may well be worth it
I suppose any insulation is better than none, but not all insulation is the same.
Most insulation materials are put through a BS 'hot box' test which determines the amount of heat loss through a 1m thick section of the material for a given temperature difference on each side. This is known as the thermal conductivity (or 'k' value) of the material.
So if you know the k value of the product you can accurately compare one with the other. A typical mineral wool quilt, for example, has a k value of 0.04W/mK. A typical foam insulation has a k value of 0.02 W/mK. So this tells you that foam insulation (please bear in mind that there are different foams as well) is twice as good as mineral wool. So, where you might need 200mm of mineral wool you would only need 100mm of foam. Geddit!
So it's a straight question of cost/benefit. What is the cost of x material and what is the k value. Compare this with the cost of y material, and it's k value, and you can calculate which is best.
Incidentally, the manufacturers of the miracle foil insulations do not publish k values so it is impossible to calculate heat loss and compare with other materials. They had a licence for a year and but did not come up with the figures so the licence was withdrawn in February. If the material was as good as they say I would have thought they would have come up with those figures by now but they haven't - which doesn't fill me with confidence.
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