Hello sander,
Do you have a picture or link to the type of insulation foam your using? The foam insulation should be the same on both sides. There's only one that I can think of that isn't, and I wouldn't use that type because it's an open cell foam so it absorbs water.
As for light bouncing back up from the foam, that's desirable. Defused light (light bouncing around from different angles) helps give more even coverage. You just don't want high temperatures, and/or extreme light intensity. Like sitting in direct sunlight all day getting hot. Especially for lettuce plants that typically are low light requirement plants and prefer part shade. Covering with shade cloth can lower the light intensity and heat buildup if needed.
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