I am going to reply to this one with some advice very different than the usual tact.
While it is comendable for owners to attempt to meet a animals conditions as closely as possible.It is not always necesarry to be so exacting.
These creatures have developed in the wild for millions? of years entirely without our assistance.
We as humans love averages ..average humidity 75% average temp average rainfall ,
We try to match that in our cages..well I dont want average humidity I want to mist everything down till its soaked 100% humidity , then dry out so everything isnt molding .Nature calls that rain .
We try to incubate eggs by putting in a presise amount of water and then replacing by weight .Well I use the scientific method of hmmm feels slightly damp ..if I squeeze it hard water will come out. at this point my hatch rate is at about 98% for over 200 eggs. Im thinking nature doesnt have a scale when it rains...
Temps.. Cresteds do not fall over dead at 86 degrees.. and thrive wonderfully at 84 degrees. I was vacationing in Florida last year and was nearly in tears when my son still in NY called to ask if the house should realy be down to 45 degrees with all the reptiles in it. I figured they were all dead
I had a repair man there imediatly and when I arrived home was delighted to learn I lost not a single animal. They survived at least 24 hours in those temps. I have since made back up arrangements to avoid that experiment again.
Provide plenty of water dark shaded areas to bury themselves in and a large moist hide and youll find your geckos doing what geckos do..will surivie the occasional spike to 90 degrees just fine.Do I suggest you keep them at 90 or above?No.. if the temps are going to be insane try your cellar perhaps..
I have found cresteds to be amazingly hardy animals that require very little fuss.I suspect more cresteds have died from overly concerned owners and over zealous solutions than harsh conditions ..but thats only a theory.

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