Foil under UTH question

[Bogertophis]

I'd wouldn't use foil, not because it would cause a ventilation issue, but because of my concern that it may short out my heat mat and cause a fire.

I agree. Installation instructions that I've always gone by say do not install on metal (& I've been using Flexwatt UTH literally for decades- safely with no issues). Most of these UTH products (Flexwatt & similar knock-offs) are not heavily insulated- it wouldn't take much (like the plastic being abraded & left unnoticed) to be a safety issue when used on metal. I'd say don't do it.

As far as "ventilation", Flexwatt instructs users to leave a few gaps- that helps to guard against overheating when used on high. I've ALWAYS done that too.

What I'd suggest- rather than putting foil under it, is to put ceramic tiles under it- as they absorb & help retain heat like rocks do. My installations are on wood furniture, & the ceramic tiles help protect the wood also, but if you lose power, tile helps to hold the heat- whereas foil conducts it without preserving it.