I use the mylar they sell at Target in the 'camping' section. I have heard that aluminum foil can be hot and cause seedlings to fry. I set my mylar on the back of shelves to reflect the light back towards the seedlings. It is very lightweight and pretty easy to use. I think it is about $1.99 for a large blanket size sheet at my target.

Here is a link that might be useful: previous post about aluminun

hmmmm I just ran across a site that says aluminum foil cools down soil by reflecting the sunlight:

International Society for Horticultural Science

Now I don't know what to think!

Well, that could be because outdoors, the soil is underneath the foil.

In any case, I think a humidity dome would be a more effective way to retain both heat and moisture.

Are there light receptors on the bottom of the leaves? I think not...So, I don't see the benefit other than producing maybe alitlle more heat and that can be accomplished better using other methods. You could I suppose even do some harm in directing too much light to the underside of tender young plants?

I vaguely remember someone saying they harmed the undersides of some hibiscus plants by doing this, and they were larger than seedlings. I'm not sure I'd risk it, unless you have some disposable plants you are willing to experiment on.

I think it may vary from plant to plant, I used white plastic, black on one side white up, it keeps the green from growing on medium and also reflectivity is always needed in my garden, aluminum foil tarnishes,corrodes,and when you water it just looks weird,mylar get crackly and bumpy,white plastic wash and shine, discount hydroponics is one store and Hydro Harry or something harry, good growing