If you are increasing the amount of insulation in your home then it is important that you consider your attic venting system before you install your insulation. Attic insulation will keep the interior of your home warm in the winter and cool in the summer but for your attic to remain cool it needs proper air flow. In most new homes that means that under the overhang of your roof you have soffit vents and at the peak of your roof you have a ridge vent. These two items must be free to allow good flow of air through convection. In older homes you might not have a ridge vent and you would most likely still have venting of some type to allow your attic to cool its self. This means you may not have soffit openings for venting you may however have an automatic fan that will cool your attic when temperatures get above 95F. In homes without soffit vents a powered fan system is required most of the time.
If you are adding insulation to your home you may already have soffit vents installed with a rafter duct. You should inspect the ducts for length and to make sure that they have not been damaged or crushed.
If you are converting an old home to a passive system with soffit vents and a ridge vent then you will have to make sure that the overhang of your roof can support the soffit vents and then you will need to install Rafter Ducts between each roof Joist. This is not an easy process especially if you have blown in insulation already.
Actually because of the work involved I would probably want to hire this job out to a professional but if you are attempting this yourself you should do the following.
Use a small rake to pull the insulation away from the soffits and then place a piece of plywood in the area so you can get to the underside of your roof. You will need to be laying down to do this.
Now staple the rafter duct to your roof rafters at the top and as far down as you can get. You don’t need to staple the bottom to the rafter as they are made of semi ridged foam and will stay in place until you can force a piece of bat insulation up against the bottom.
The Rafter Duct needs to hold firmly against the bottom of your roof rafter so the flow of air will pass from your soffit and through the duct and then into your attic.
Each Rafter should have a duct placed. The ducts are normally 4 feet long and should give you plenty of room for even an R60 application of insulation.
This really is a tough job especially if you have insulation in your attic already. However the materials are very inexpensive and I would expect a lot of people will try this on their own. If you do remember to wear a good dust mask with a valve so you have good easy breathing in a hot condition. If your mask does not have a valve then the moisture of your breathing will block your dust mask and you are likely to remove it in frustration. Pay extra and get the valve for this job you will need it.
Also be very careful you don’t cause problems with your ceiling drywall. Sure you can put a foot through it but you can also crack it much easier.
Finally Work safe and take breaks. When you work in hard conditions that you aren’t use to working in you have to stop more often and that will give you time to assess your work and your approach to the work. You might think of something that could make things easier.. but honestly this is not one of the easy jobs in your home. It is not technically difficult it is just frustrating, hot and tiring. Good Luck.