When it comes to cold roof insulation and ventilation, being properly informed could save you quite a bit of money in the long run.
The purpose of a cold roof is to prevent ice dam formation. Ice dams occur when heat comes into an attic and causes the snow on your roof to melt. The melted snow then runs down to the edge of your roof, where it freezes and cause ice dams. If you don't detect an ice dam quickly enough, water can form behind it, get under shingles, and make icicles. This causes ruined sheetrock or plaster, wet insulation, and rotting walls, which can ruin your paint. Luckily, ice dams can be prevented by maintaining a cold roof.
How to Achieve a Cold Roof
Adding insulation is one of the best ways to keep your home's roof cold. If you already have insulation, though, you should still make sure it is working as well as it should by finding and sealing any possible bypasses. Attic bypasses are any paths through which hot air flows from your living space into your attic. Your chimney, light fixtures, and attic hatch are all potential bypasses if they aren't seal well. Make sure these don't leak around the edges or joints. Regarding your chimney, specifically, it is a good idea to seal it with non-galvanized steel and silicone caulk.
Proper ventilation in the attic is also important. Try not to plug any fresh-air intakes or vents. The purpose of a vent in your attic is basically to let moisture out. You don't want water vapor to collect on the insulation or the rafters. This can drastically reduce the effectiveness of your insulation and do a lot of damage to your home.
The amount of ventilation you need depends on the size of your attic. It is usually a good idea to have a square foot of vent for every three hundred square feet of attic space. Double this if you have no vapor barrier. Having vents placed at varying heights in the attic will give you the best air flow and circulation, and lessen the risk of cold pockets, moisture, and other common attic issues. Keep in mind that the screens on vents can reduce their capacity, and the openings should be increased as needed.