Please do not heat your coop now that your flock has acclimated, or move them back and forth between a heated environment and a non-heated environment. 'Comfort' to the ISPA is most likely not comfort by our standards. You have to keep in mind that they are catering mostly to the factory farms of Indiana. 'Comfort' by their standard probably is equal to production. After all, they feel the need to mention with pride all over their site how Indiana is the third highest commercial egg producing state, and you don't get that kind of mass egg production by concerning yourself with the actual comfort of the birds.

I have to roll my eyes doubly at the 'no higher or you'll interfere with their ability to tolerate cold' part. You already have at that level.
Heating to that level now that your flock has acclimated will not make them comfortable. They have already grown extra down to cope with the winter cold, encouraged by the change in temperature through the fall. Heating now would be like forcing someone wearing a heavy winter coat to come inside out of the cold, but not take off their coat. How comfortable does that sound? The same applies to bringing them to a heated environment for the night and then putting them back outside for the day. They will be miserable!
Through the time I was studying the use of supplemental winter heat, I came across many stories of chickens being healthier, and even happier, without the use of heat--and by the way, the person who said her flock was happier without heat was in Alberta, Canada, with several other people in similar areas agreeing with that sentiment. I only came across 'I heat because I think they need it' stories on the other side, never 'my birds are happier because I heat' stories.
Heating to the level the ISPA is suggesting from the start will make your birds miserable as well. The problem with heating that much is that your birds will not want to come outside. They will be cooped up all winter, whether given snow-free space outside or not, huddling under the heat because they were not afforded the opportunity to acclimate properly. This can lead to a buildup in the coop of excess moisture, and manure as well if you don't adjust your cleaning habits to make up for their increased amount of time inside, both of which are very unhealthy conditions for chickens. I'm not sure how that sounds more comfortable than birds moving around freely, scratching and pecking in the snow, flying around like maniacs, dust bathing where they are able, just generally being chickens. But, there again, they are looking at the bottom line--light and heat keeps the birds more productive so that as many eggs as possible can be squeezed out of them.
I should also mention that if you heat your coop to that crazy level, what happens if the power shuts off as often happens in the wintertime? Just yesterday, I saw a post on Facebook about such a sudden temperature change and almost every bird dying as a result. The person said they took a bird in for a necropsy and was told by their vet that it had surprisingly less down than expected for the season. That is pretty telling to me. This danger is on top of the death toll from fires every winter, including a few human lives as a result of fires spreading to nearby houses, all caused by the heat lamps that the ISPA is suggesting. Whether you decide to heat or not, I will ALWAYS suggest against using a heat lamp--there are better and SAFER ways to heat your coop if you insist on doing so!
I also want to add, in the case of frostbite, my first inclination is to look at whether the coop has enough ventilation or not. Most people are inclined to shut off some of their ventilation, but I have found that my birds little if any frostbite when I leave their ventilation as open as possible, only closing off the tiny amount of ventilation that might blow on them while they roost at night. I have 5 windows on the south side of their coop. I close the most windward one, and the other four are left open. I have seen no frostbite strictly as a result of cold since I started doing this, and the only frostbite I've seen has been as a result of other factors (such as the crazy Legbar that likes to dip her comb in the water during freezing weather

).
I have compiled a list of reasons for and against supplemental heat, along with other information on winter chicken keeping, in an article here:
https://www.backyardchickens.com/a/...led-look-at-the-question-of-supplemental-heat It might be worth checking out if you really are concerned about whether to heat or not. You will see there that I do agree with taking action in 'severe' levels, though I imagine my perception of a 'severe' level (negative twenty and below) is much different than the one I'm sure the ISPA is implying.
Just for the fun of it, here is a video of my flock a few weeks ago. I believe the temp was just below zero (you can kind of see when I point the camera down that I'm wearing my coveralls--it takes a pretty low temp to convince me to get them out!).
[VIDEO]
My girls have also been known to take dust baths in the dirt floor of their unheated and uninsulated coop during really frigid temperatures. Can't imagine they would want to do that if they were uncomfortable as a result of the cold. (As a side note, since I've been accused of it before, I have nothing against insulating the coop as long as proper ventilation is maintained--I'm just pointing out that my coop happens to be uninsulated.

)
They don't stay cooped up, especially now that we've built a large, fenced in area for them (which is where they are in the above video), but before that was built, they still came out to wander the snow as long as it wasn't too deep.
And then there's this.

Yep, she was eating snow rather than walk a few feet to the waterer.
(P.S. yes, she had frostbite; this was before I figured out the optimal winter ventilation level in my coop.)
I can get fairly heated on the topic (no pun intended

), but I just genuinely want to make sure people go into heating their coops well informed. This is not something you can just change your mind on once you've committed to it without the potential for harming your chickens, so there's a lot of things that need to be considered beforehand.