Are we able to deduct our insurance premiums as a business expense for the year since my husband was self-employed for five months in 2018?

Partially, you will only be able to enter them for the 5 months he was self-employed. Insurance is counted on a monthly basis so you cannot deduct it for the 7 months he was not self-employed.  

That would seem logical, but I cannot find any publication that actually says you should deduct premiums only for the months in which your self-employment had income.  I also could not find in Turbo Tax anywhere that asked for the number of months with income or anywhere on Schedule C either. Could you send your source for this answer so I can put it in my tax file?