Installing spray foam is certainly an option, but it is not a magic product that you just quickly install up there. You need to understand the product and make sure you use a green contractor that understands building science and the "house as a system" concept.
On retrofits, there have been some reports of problems with spray foam - outgassing, odors, moisture problems, etc. -- so you must do your homework to choose the best insulation for your situation. There are other options out there. There are many experienced weatherization companies that will be able to blow in cellulose even in a tight attic. This with a good air-sealing package will perform just as well as a foam job and may cost less.
If you decide home foam insulation is the way to go,
- you will first need to decide between open-cell and closed-cell, depending on your particular climate, etc.
- Are you going to spray it to your roof deck and create an unvented attic?
- You also need to make sure your furnace is sealed combustion. Are there any other combustion appliances in your attic? Water heater? These too will need to be closed combustion.
- In addition, you will want to investigate whether your system is the correct size, if it has a variable speed blower, two-stage compressor, if the thermostat can measure humidity, etc.
When you change the thermal envelope of the house and make the house tighter, you need to go through all these steps. Most HVAC units are already oversized; once you make your house tighter by spraying foam, it may then be really oversized, so you could be creating a situation where your unit will short-cycle. This will also depend on the humidity for your climate in how big of an issue this could become.
If your house has existing soffit and ridge vents, these will need to be sealed up. You will need a good installer who will make sure the mix is done properly and will pay attention to the details.
If you overlook any of these issues, you could create big problems in your home.