my 2c...

I don't think it makes a difference to the insulation properties which way it goes...*
It would probably be better shiny side down for lighting in the shed. (Better reflected light)


*I have some similar (gold rather than blue) and it was supposed to laid dark side up for when laying before the final roof so that there isn't glare etc for the guys doing the work on the roof.

Ive got it on walls silver side facing in for reflected light but mine covers polystyrene.

That wrap has shidt house r value in that application, but better than none, venting in paramount as heat will be sandwiched and vent through overlaps unless taped, the blue is just anti glare, same as aircell or 2l2 has

All heat loss and gain is by one of the following:

1) conduction- eg heat transferring through a material like a brick wall. This is where bulk insulation (like glasswool or polystyrene is most effective)
2) convection - eg where a hot breeze brings warmth into an area. This is where sealing a building from random air flows in and out is most effective (eg seals around doors, windows etc)
3) radiant heat- eg feeling the warmth of the sun on a cold day. These silver foil products operate predominantly in the radiant heat loss/gain category (esp if their joins are not sealed).

Typically, these foil type products are very good at minimising SUMMER HEAT GAIN from radiant sun heat. Little or no value in minimising conducted heat loss (eg heat loss upwards in winter) as cauncy says.

Most of the RADIANT heat is transferred in the infrared IR spectrum (not the visible spectrum). This is especially the case where the roof is above it- as all the wavelengths of light (UV, visible and IR) and heat associated with them is absorbed by the roof sheet and re-radiated below as effectively only IR radiation. The blue dye on most of these wraps is a food type dye that acts in the VISIBLE spectrum, and has little effect on the IR spectrum. So, the way it is laid should make negligible difference.

By the way, TYPICALLY, the body experiences heat in different ways, and radiant heat is typically the most dominant factor in what the body perceives as thermal comfort (nominally 60% radiant heat, 20% convection/air flow, 20% actual air temperature (conduction to atmosphere)).

Clarence

Location/climate zone is critical- as what works in one climate area can be completely irrelevant in another.

As an example (and not that this relates directly to your specific case) in cold/cool climates if you use a foil backed insulation blanket the fluff should face the outside of the building, whereas in really hot climates the fluff should generally face this inside of the building.

This is more to do with how condensation forms than the effectiveness of the insulation, but interesting to note that what works in one situation does not mean it will work everywhere.

Clarence