Is the foil on rigid foam panels also reflective insulation?

[Stephen Tashi]

TL;DR Summary

Does the foil surface on panels of rigid insulation serve as reflective insulation?

Does the shiny foil surface found on some panels rigid insulation serve as reflective insulation? If so, does the extensive printing often found on the foil degrade its value as reflective insulation or does it have no significant effect?

(I think the main purpose of the foil is to be a vapor barrier.)

 

[DaveE]

Yes, it helps. Shiny surfaces don't absorb as much heat from EM radiation as "dark" surfaces because the radiation is reflected.

If you want to learn more, you can search for "Emissivity" or "Black Body Radiation".

 

[pbuk]

Does the shiny foil surface found on some panels rigid insulation serve as reflective insulation?

Yes, in the sense that reflectivity is equivalent to low emissivity. See example calculations (relevant to UK building standards) here:
https://sava.co.uk/news/calculating-u-values-reflective-foil-insulation/

If so, does the extensive printing often found on the foil degrade its value as reflective insulation

Yes, see the calculations linked above.

(I think the main purpose of the foil is to be a vapor barrier.)

This can also be true, although a vapour barrier is only effective on the warm side but foil is usually applied to both (and therefore must not be taped to provide a continuous barrier on the cold side). Also low emissivity is only relevant where there is an air gap and this would not normally be the case where a vapour barrier is relevant.

 

[berkeman]

although a vapour barrier is only effective on the warm side but foil is usually applied to both (and therefore must not be taped to provide a continuous barrier on the cold side).

Could you say more about this (especially the taping)? I'm not understanding it yet. Thanks!

 

[pbuk]

You tape the joints (normally with aluminium foil tape) on the warm side of a vapour barrier or Vapour Control Layer (VCL) so that the protection is continuous. If you tape both sides then moisture is trapped within the insulating layer which reduces its thermal performance and may lead to shortened lifespan.

This may help in general (found one for N America for y'all but you will have to delete some 'u's from the rest of this post ): https://www.iko.com/comm/introduction-to-vapour-barriers-and-vapour-retarders/

The idea is that any moisture present in the building envelope evaporates into the atmosphere due to the temperature gradient which is good. Put the VCL on the cold side and any moisture can only evaporate into the interior of the building which is bad, or condense on the inner layer of the VCL which is worse.

This all works the other way round for a refrigerated building like a cold store. I don't know anything about building standards in high temperature/high humidity environments.