Wood is non conductive, and does not transfer hardly any heat. Phenolic material also does a good job at not transferring heat, but its kinda pricey. Aluminum is conductive, which transfers heat rapidly (making it a bad idea if you are having heat issues).
I listed these materials in order from best to worst for protection from heat transfer (wood being the best). The only bad thing about wood spacers is they can warp and wear out over time....that being said, I have had the same 1/2" wood spacer on my car for 13 years, no problems.
