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Insulation R value only relates to conducted heat. Foil-facing reflects heat, so it reduces heat transfer between inside and outside via another mechanism (stops radiant heat from becoming conductive heat). Comparing foil-faced and non-foil-faced insulation of the same R value, the foil-faced will reduce total heat transfer more.

I'm in a situation where I may need to replace foil-faced insulation with another type that is not foil-faced. I can match the R value, but would lose the foil's reduction in radiant heat transfer. That would translate to more conductive heat, which could be offset by increasing the R value.

Is there a way to figure out what increase in R value would yield equivalent reduction in heat transfer as the foil adds (i.e., maintain the same temperature difference between inside and outside as the foil-faced insulation)?

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    It will be a change in the radiant heat transfer, not conducted heat if the only change is a lack of foil... A good book about heat transfer is By Simonson. – Solar Mike Mar 13 '20 at 7:22
  • @SolarMike, my understanding is that radiant heat gets reflected or absorbed. If it's absorbed, it get converted to conductive heat. – fixer1234 Mar 13 '20 at 8:36
  • This might be a question that better fits in Engineering. engineering.stackexchange.com – Ack Mar 13 '20 at 16:35
  • @Ack, if this was a theoretical question, Engineering might be a better site to figure it out. But I'm hoping this has already been figured out and there are existing, practical solutions ready to be applied by contractors and DIYers. – fixer1234 Mar 13 '20 at 18:46
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The layer fields on the professional Ubakus-Site allows to input also metal foils, so it should take the reflections into account. But those reflections do only have a tiny effect if the subject is building insulation. Much more important is the fact that metal is a barrier for waterdamp. And in general, wet insulation material decreases the insulation effect. So in reality, removing a metal foil could improve the insulation.

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  • Disclaimer: I do not have any association with that web site, I do not benefit/profit in any way from my recommendation of that site. It is just by far the most professional and sophisticated site to calculate and visualize the relevant insulation parameters that I have found up to now. For DIY the free access seems to be ideal. But I can't tell anything about the commercial option since I do not have access to it. – xeeka Mar 14 '20 at 8:41
  • Thanks for the response. That web site was a challenge--so much is in German (even with the English setting), and is based on the European model (U vs. R values). It mentions radiant heat but didn't seem to factor it in (or I couldn't figure it out). That said, I suspect you're right that the foil doesn't add much in my case. It's in a basement, so radiant heat probably isn't a big factor. – fixer1234 Mar 14 '20 at 15:41
  • I did not check all input fields, but the most important seemed to be translated. Switching to R instead U value is done by clicking on the tool symbol next to the U value. If radiant heat is not taken into account for metal layers, I assume it does not play a role in buildings - only it does as water damp barrier. – xeeka Mar 14 '20 at 17:00

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