1) Reconsider installing humidifier in the attic. You know what could happen.
2) Waiver of liability means absolutly nothing. They should provide you with one....
If it is not worded to the letter of your state law it is not worth much. Well it could work in the small claims, but not much beyond that.
Legal papers are not as difficult to write as one may think.
Waiver of Liability
I, FirstName LastName, owner of the house, address etc etc, requested company ZZZ to perform humidifier installation in the attic.
I understand risks inherent, do hereby waive, release, and forever discharge CompanyName, its installers, officers, owners, and employees from any liabilities associated with this job.
I acknowledge that I have read and understand this entire Waiver of Liability and I agree to be legally bound by it.
I acknowledge that the Waiver is contractual and is intended to be as
absolute and complete as the law permits, and if any portion is held invalid,
it is agreed that the balance shall, notwithstanding, continue in full force and effect.