What is the ruling on my work in insulation contracting, knowing that most of its earnings are unlawful, and I fear that if I leave it, I will not be able to repay the bank loan? And is it permissible for me to use the remaining money in a lawful project?

Perhaps the questioner means by the dominance of the unlawful in his earnings in the field of insulation, that he relies on deceiving clients, lying to them, and consuming their wealth unjustly. If this is the reason, then he must repent to Allah Almighty and purify his work from these forbidden matters. If he is unable to do so, then he must abandon this work.

If the loan was an interest-bearing loan, then this is another sin that necessitates repentance, for dealing with usury is a major sin.

As for the remaining money, if the reason for its unlawfulness was deception, then the questioner must return the value of the deception to those he deceived if he knows them; otherwise, he should give it as charity on their behalf. Shaykh al-Islam Ibn Taymiyyah said: "Whoever sells a deceptive item, only the amount of the deception's price is forbidden to him from the price. So, he must give it to its owner, or give it as charity on their behalf if returning it is impossible."

As for profit resulting from investing unlawful money, there is a detailed explanation for that.