This is a great idea as buildings account for 39% of US energy consumption every year and 38% of US CO2 emissions, more than any sector. You can read an article from the Center for Climate and Energy Solutions HERE. Meghan what you are doing is not just about ROI it is also about being responsible. I understand when the money is not there to do this but when you have the means to do this you should.
I'm an Architect in the DC region and all my clients bring up the question on how to make their homes/building energy efficient. Go to Green Building Advisor if you really want to read up on how to be energy efficient. They have great articles which I have spent hours and hours reading.
Here are some tips on becoming energy efficient:
1. Insulation, insulation, insulation. This is the cheapest form of making an efficient building. You can use spray in cellulose insulation (environmentally and cost friendly) which will be easy to install in an existing building. Definitely insulate your basement by furring your walls. However, in your basement you cannot use cellulose. You will have to use closed cell spray foam (more efficient but messy and bad for the environment) or rigid insulation boards (easier to install but you will need it thicker to match closed cell R-values).
2. Windows. Get better windows. However, be sure your installer uses spray foam insulation to cover the gaps between the window and the wood studs. In most old building you can put your hand around the window and the draft that is coming in is probably because there is no insulation in those cracks.
3. Use Water Sense labeled plumbing fixtures (read HERE)
4. Use Energy Star labeled electric fixtures
5. I don't know your heating/cooling situation but on remodels you could use a mini-split system. They are more efficient than traditional system and provide healthier air for breathing (dust is pushed around everywhere). This will also look much better than units installed in the window!
6. Solar panels are dead last on the list unless your state gives some good rebates or allows for leasing. Same goes for solar water heating.
In terms of your roof top deck here are a few things to be aware:
1. Go to your municipalities zoning department and make sure you can do this!
2. Will this be an issue with your insurance?
3. If you do rent the rooftop to outside parties be sure your tenants know that this is a possibility. They might get pissed with people entering and exiting the building that don't live there. This could be a security issue.
4. Check with the building department on how many people can be up there before you have to provide a second means of egress. Usually it is up to 49 people. 50 people requires two or more exits. Here is the International Building Code, go to page 4. This equals to to 343 SF of useable roof top space. This is calculated by 49 people multiplied by 7 which is how much square feet is required for each person.
If you have any questions just ask away on the forum or PM me.
Good luck and I appreciate the fact that you are pursuing this