gmp,
My concern with mixing HR and RPE is now we have two subjective measures, reminds me of the blind leading the blind. RPE and HR are good, but are both heavily influenced by environment and perception. You really need an objective “3rd-party” when training in extreme environments, like Pace or Power. So, my recommendation is “no” to RPE + HR in poor conditions, but I suspect that other reliable coaches would have a different opinion.
For sure, that extra 5-10 points in hot/humid conditions is different than elevated HR from actual intensity. Just like when I come up behind you and say “boo!” and your HR spikes to 140, that’s not a workout, no fitness is gained. Regrettably, a HR is not necessarily linked to output. Like the engine temperature gauge in your car, it can be sky high when you are sitting at a red light.
David