When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Foil side goes down - this is the first layer that the underside heat will hit so you want it too do it's job of reflecting the heat as much as possible before heat rises up to the next layer of insulation. By the way I recently did some of this and in additoin the heat barrier I bought that was specifically cut to the car (which was very expensive and overpriced by the way) I added a layer of relflectix insulation on top of that. Purchased a 2ft. x 25ft. roll at Lowes for about $25. I actually think it's going to do more than the first layer for a fraction of the cost. If you do get this stuff I wouldn't use it near the gas pedal though cause limited clearance in that area already. HOwever you can put it every else - I even cut panels for the inner door skins. It's cheap so why not. Have fun.
Since the floor pan is solid, either solid fiberglass or metal depending upon the year, I'd think the first thing you need is an air gap, so I'd put the insulation face down with the aluminum foil on top. I'm not sure the aluminum foild is that good of a reflector if it's in contact with the floor pan. I'd think the aluminum foil needs a little air in front of it. However, this is just a notional answer. I don't think I have the expertise to make a definate recommendation.
Also, anyhow I don't think aluminum is a good reflector of heat. Hold a piece of polished aluminum, aluminum polished to a mirror like finish, facing the sun on a hot day, and the aluminum will soon be hot enough to cook the proverbial raw egg. Hold a mirror facing the sun, and the mirror will remain cool. Notice that both materials, polished to almost the same reflectance, have a dramatically different reaction to the sun's heat.
Foil side goes down. Saw a post by Wilcox--they also said down. That is the way I installed it. The only problem was you get the fiberglass on you when you put the carpet in--it itchs.