Insulating a roof?

If you live in a northern climate that has snow, you need roof ventilation. The purpose of ventilation is to keep your roof deck as cold as possible. Snow does not melt as readily on a cold roof deck. The optimum condition is to have the space below the roof deck the same temperature as the outdoor temperature (few homes ever achieve this, BTW).

I don't care how well one insulates, some heat will always escape pass the insulation above the ceiling. This heat will warm the roof deck unless it is vented away. A warm roof deck causes snow to melt and ice dams to form on the eaves. Ice dams can cause snowmelt to leak through the roof and into the home.

Both the roof peak and the roof eaves must have vent openings. Continuous ventilation in both locations is the best. Make sure that whatever insulation you use does not block the airway between the eave vents and the roof vents.

There is no such thing as too much ventilation in a northern climate, only too little. VENT VENT VENT and make sure that the first six feet of the lower 6-feet of roof deck is protected with "Ice and Water Shield" underlayment (or similar) instead of plain tar paper.

__________________
This is the government the Founding Fathers warned us about.....

Thanks for the website cfabe. I mentioned the finished interior just because I imagined it could be a messy job and very hard to clean up. I really do need to do some more research.

Thanks for the feedback Cabin Fever. I do live in a very northern climate and understand why and how venting can help with ice dams due to heat loss through the roof. In my case though we have not had any significant ice daming problems in the ten years we've been here. This is why I thought I could go ahead and just fill the cavities with insulation and apply a good vapor barrier to keep moisture from migrating into the insulation from the interior.

Since we've had no problems in the past I'm thinking that just adding more insulation will only improve things.

I'm wondering why you've had no problem with ice dams with an unisulated ceiling in a snowy climate? Is it because the snow immediately melts when it hits the roof and drains off? I've known some folks with no insulation and that is what happens. Their roofs stay "snowless" all winter long because of the excessive heat loss where other roofs have a foot or two of snow on them because of good insulation.

__________________
This is the government the Founding Fathers warned us about.....

We do have some insulation. 2" of some type of wrapped foam that was put on when the house was built in the early 70's. We pretty much have snow on the roof from late november to april in a "normal" year. In a big snow year I have to go up and shovel it off. It doesn't seem to melt off any faster then most other places I see around. It's a relatively low pitched roof so it doesn't tend to slide off unless it really warms up outside. If you look at the roof when it is snow covered you can clearly see the one place where the plumbing vent comes through does leak heat leaving a big dimple in the snow.

The fact that I don't have siginficant melt is one reason why I have put off this project in favor of others. The existing system seems to be working OK. I look up and see those 10 inch deep cavities (rough cut rafters) though and I can't help think that insulating them will only make things better.