Is attic insulation replacement for a rental property considered an improvement? What is the depreciation class (#yrs)? Is it qualified for Section-179 deduction?

Insulation is considered an improvement. 

For an improvement, use the same recovery period as that of the property to which the addition or improvement is made, determined as if the property were placed in service at the same time as the addition or improvement.

A Section 179 deduction isn’t allowed for property used in connection with residential rental property.

If the total cost was less than $2500, you can elect to expense it (write it all off). If you bought the material and paid for the installation separately, and either or both was less than $2500, you may expense that part.

There's another possibility. You may expense it if the cost of all repairs, maintenance and improvements is less than or equal to the smallest of these limits:
-2% of the unadjusted basis of your building or
-$10,000

Thank you, Hal...Those are wonderful points.

Thanks for the answers. I have a follow up question.
If, after elected to expense this repair, causes the rental income to become negative, should I still expense it? Because if I depreciate the expense, with positive rental income, I can claim QBI deduction.
Would you expsense or depreciate ?

Thanks

The only way to be sure is do the returns both ways and compare.  I would think expensing is best for most people. It's the deducted amount times you tax bracket rate. The QBI deduction is 20%, of your QBI, so only 20% times your rate + the depreciation deduction.