Most houses use standard 6" x 9" soffit vents and the formula for the necessary number of soffit vents is based on the size of the attic. My house is just over 2700sft with about 500sft added for the garage so we will calculate off of the total of 3200sft attic space. The quick and dirty is one soffit vent for every 100sft of attic space so I should have at the very minimum 32 soffit vents...I had 12. It's time to add a LOT of soffit vents.
The first step for me was to create a template for the holes I would be cutting. I used a scrap piece of 1/4" veneer I had laying around but could have easily just used a piece of stiff cardboard. After removing one of my existing soffit vents I measured the distance from the wall so the hole in my template would space out the same and my new vents wouldn't be out of line with the old ones. Make sure to measure the center point of the hole as this will help in placement. The template is shown in action a few steps below.
NOTE: When measuring for the hole size to cut for your vent, measure the perforated portion of your vent, not the size of the whole vent. There has to be something for those screws to screw into.
Once the old vents are removed, take some measurements between existing vent holes.
Using a clip to hold the tape measure can be quite helpful if you don't have an assistant.
Do some math to figure the distance between center points of the new vents. I am placing three vents between each of the old vents so I mark the center points as 1/4, 2/4 and 3/4 of the total distance between vents.
Time to use that template! Just slide the template on to the soffit, line up your center point of the template with the spacing mark on the soffit and mark the outline of the hole in the template.
Use a nice dark marker so you can see where to cut later. Don't worry about aesthetics so much as the marks will be covered by the edge of the vent once it is installed.
Using a paddle bit or small hole saw, cut a hole in two of the corners to allow use of your saw.
If you hit a stud like I did in the picture above, just move your hole hole inside a bit and make a smaller hole for your vent. The vent will cover the stud but the stud will block some of the air flow....this is why you add more vents than you need. Whatever you do DO NOT CUT AWAY THE STUD. Studs are an important part of the house, all we want to remove is the thin wood that covers the soffit.
After the holes are cut in the corners just connect the dots with a jigsaw or reciprocating saw to make the hole for the vent. It is OK to skip a spot like I did to avoid things like utility hookups. Always be alert for what is on the other side of the soffit.
After you get those holes cut it is very important to get any insulation out of the way so that air that comes in the vent will be able to make it into the attic. Frequently the guy that puts in the insulation will spray it well into the soffit. I personally love using a leaf blower to make sure the soffit is clear.
...and really work that blower in the hole to make absolutely sure that air flow is not restricted.
Best to use a powered screw driver or a drill with a driver bit to put on the vents as there are six screws per vent and your forearms will look like Popeye's by the time you're done with a screw driver. Make sure to use galvanized wood screws to attach the vent.
Having addressed the first half of the equation (intake) we need to calculate exhaust. I personally prefer using whirlybirds or ridge vents for exhausting an attic since there is no electric bill and no motor to burn out like in a powered vent. For this house ridge vents were out since the house has a tiny top ridge. To calculate for how many whirlybirds you need there are complicated formulas but the simplified one I used was one whirlybird per 1000sft of attic space. In our calculations at the first I had 3200sft of attic space so I need about 3.2 whirlybirds...I have two effective ones placed high on the roof and one much less effective one on a lower ridge of the roof. I'll count the lower whirlybird as my ".2" and just add another whirlybird high up by the other two more effective ones. Unless you are a roofing professional i would very much say go hire a roofing professional to install your whirlybird. Not only is it a health risk falling off of a house but cutting a hole in your roof is a good way to ruin it if you don't know what you are doing. Here's the before pic of the two good whirlybirds.
...and the after with the third whirlybird in the middle. The less effective whirlybird is just out of camera shot to the right on that much lower ridge.
Now when the sun heats the attic, the hot air rises out of the whirlybird and actually creates a suction to pull cool air through the soffit vents. With the added soffit vents and additional whirlybird we now have a much more effective ventilation system for the attic. This was a relatively simple project and last summer since I completed this project, the energy savings have already paid for the materials (including the roofer's installation of the whirlybird).
