I've been looking for some comparisions between the multi-layer foil insulation and foam; everything I had read before (cheifly advertising material from the foil insulation manufacturers) indicated that the foil stuff was much better than 'old fashioned' foam - however, the extract below (from a manufacturer of foam) totally contradicts this. I've copied it here just in case it is of interest to anyone else as background material before deciding to foam your bus (for instance, it suggests that you need a minimum of 4" of foam to meet the minimum standards for new-build houses in the UK, which puts the typical inch or two of foam put into a 'fully insulated' bus into context).
The full document is here: http://www.foam-insulation.co.uk/insulation/multi-foil-insulation.htm
An extract talking about foam verus foil (remember that this is from a foam manufacturer):
"For aged polyurethane foam (worst case scenario), a typical calulated value of thermal conductance by test method ASTM is k value = 0.026 W/mK, to achive a U value of 0.20 would require an R value (depth/k value) of 5 for the foam depth sprayed (1/u value or 1/0.20 = 5) and therefore calculating for depth is k * R or 0.26 * 5 = 130 mm . A 130 mm coating of high density polyurethane foam will hence do the job here to achieve a U value of 0.20. Deph of foam required to achieve a particular U value is given by the conductance value of the foam divided by the U value, k/U = depth. Polyurethane spray foams have typical thermal conductances in the range 0.015 to 0.030 W/mK with high density foams at the lower end of the scale.
Essentially, the research conducted by the BRE has looked at 'real world' applications and concluded that multi-foil insulation products significantly differ from the measured on site U values (the lower the U value the lower the heat loss will be through the product and therefore the better the insulator will be) to that claimed by the manufactures. Significant because the real world measurements indicate that for most applications foil based insulation products cannot come close to meeting the new standards for thermal insulation introduced by the April 2006 Building Regulations. Best case U value was 0.43 given favourable assumptions whilst worst case was a U value of 0.98 (the lower the U value the better the thermal insulation value) Given that the Building Regulations demand at least U values starting from 0.25 and below, it is clear that foil based insulation products cannot meet the new standards. This is against a background where foil based insulation product manufacturers have been claiming that U values as low as 0.18 are achievable with multi foil. This was not borne out by real world research by the BRE where actual U values measurement were significantly inferior. No wonder the ASA so fit to take action against the multi-foil insulation manufacturers.
Based upon the BRE results, as a comparison, a 40 mm coating of polyurethane foam produces a thermal U value of worst case 0.53. A multi-foil insulation product of 30 mm at best case as tested by the BRE gives a real world U value of 0.78, significantly inferior to polyurethane foam. A 100 mm coating of polyurethane falls in U value to below 0.25, the new minimum starting U value under the new April 2006 Building Regulations. A multi-foil system is stuck at 0.78, hopelessly outclassed and cannot comply with Building Regulations. Even going from one manufacturer's test claim of U = 0.533 as the best of the best this still falls hopelessly short of complying with Building Regs."