The total power of the signal of a wifi router is around 2W. (Should I multiply it by x2 or x3 to power the internal system, the blinking leds, heat loss, ...?)
For 2W you need ~20m² (~200sqft). That is the size of two rooms. Probably using the surface of your whole roof you can get 4W or 5W.
This could potentially power a low power beacon every so often. It doesn't seem practical for IOT wearables or for anything industrial at that power density. I'd be curious about the cost, in particular what manufacturing process size can be used for this. 1cm² of wafer can still be quite pricey at .000012 watts.
5°C Delta T generation is pretty impressive though. There are some commercial systems that can operate with 60°C temperature Delta (organic rankine cycle) and recover about 12% of that energy.
They'd need to adapt to something like aluminium foil or UHMWPE film (made like paper from the fiber-spinning solution) as a base and coat that with poly-Si. Even if it is only half as efficient, it should be a lot cheaper and possibly strong enough to be laminated into fabric like e.g. Gore-Tex membranes (extruded microporous PTFE film). A reverse electric blanket would be fancy...
Thanks.