Insulating finished attic

Question

I have half of a 100 year old twin in SE Pennsylvania. The house isn’t too big and I’d like to turn the attic into living space. It is a walk up and about 800sf. The roof is pitched quite steep but it has about 12’ high ceilings in the center so lots of useable space. The rafters are true 2x6’s. How should I insulate this for conditioned space? I was hoping to use rockwool because I don’t think I can afford spray foam. Is this possible or will it create moisture issues? Do I need an air gap? Also, worth mentioning there are furring strips between the decking and the rafters running perpendicular to the rafters with about an inch gap between rafters and decking. Thanks for any and all input!

Answer 1

Thing to consider before putting the first dollar towards this project:

1. Assuming that your attic is currently insulated, you will lose insulation value by expanding the building envelope to the attic. This will increase your heating bill.
2. Unless you have lots of space to add insulation plus space for air to circulate between the insulation an the roof deck, you will create a "hot roof" condition, which will decrease the efficiency of the house, increase your heating bill, and increase the probability of ice dams.
3. If you plan on using this as anything else other than storage, you will need to pull permits, and with that will come a lot of requirements, like expanding the heating system, fenestration, egress, etc. It can get expensive quick. If you do it without permits, you'll have to destroy the work before selling the house, as the buyer's mortgage company might not lend on a house with unpermitted work. Or sell to a cash buyer, the kind that will ask for hefty discount.

If you decide to go ahead:

1. Rock wool is a good choice, and in my opinion, better than spray foam.
2. As @crip659 mentioned on the comments, make sure that the ceiling joists can be used as floor joists, ie that they can support the live load.
3. If you need to leave an air gap between the deck and the insulation will depend on your current setup. For example, do you have vented soffits and a ridge vent?

Lot of people see attic space as wasted space the builder forgot about, but I've still yet to see a well-executed attic conversion. Actually, once in Germany, and the cost was eye-watering.