I wouldn't bother with ventilating. As Canada Mortgage and Housing Corp. notes, "[a]ttic ventilation is overrated. In winter, the cold outside air cannot hold much humidity or carry moisture away from the attic. In summer, attic temperatures are more affected by the sun and shingle colour than by the amount of ventilation."

But I would add up to 10 inches insulation to the attic floor; be careful not to stir up the existing insulation if it is vermiculite, which has traces of asbestos in it. Before adding insulation, seal up all the cracks and holes that will leak out your expensive heat. (See attached link.)

Here is a link that might be useful: Attic Ventilation and Insulation

I think you were asking if you should insulate under the attic ceiling, right? The short answer is no, unless, you are up for the more complicated type of insulation such as might be used with a cathedral ceiling. This usually entails installing an overlayment ( i.e. actually above the insulation material and just under whatever is the lowest layer of your roof assembly) that has some ventilation channels. You can kind of get a picture of what is necessary by going to the Owens-Corning website and look at the diagrams for working on a cathedral ceiling.

This is different from regular attic ventilation which is what I think the pp was discussing.

However laying fiberglass batts (for example) on top of and at right angles to your attic floor joists and whatever insulation exists within them will help. However, it will render useless the floor for storage. Can you pull the attic floor and add some additional insulation (without compressing it) above what's there but under the floor?

I have the same situation you have and have opted instead for keeping my usuable floor space over adding additional insulation. I tell myself that covering virtually all the attic floor with boxes of stuff must have some R-value.

A good job of sealing up infiltration points all over the house and an extra specially good job sealing up/weatherstripping the door or stairs leading to the attic and caulking any ceiling penetrations in the top floor will make a big dent in energy loss, even if you wind up not adding any additional attic joist cavity insulation.

HTH,

Molly

You don't say where you're located. Ventilation becomes far more critical the farther south you go.

The amount of heat that can get trapped in an unventilated attic in a more southerly location can be staggering, which can increase your cooling costs in summer and also reduce the life of your roof considerably.

In Virginia an unventilate attic can reach 120 deg. F or more on a sunny summer day.

My attic in DC metro wasn't properly ventilated, and the upstairs was extremely uncomfortable in summer.

When I had work done on the upperworks I had the attic properly ventilated with ridge and soffit vents.

The three summers since then have been a LOT more comfortable on my second floor.

Putting boxes on the existing insulation and compressing it probably does more harm to the R value than good.

Proper attic ventilation, especially if you have inadequate levels of insulation, can help guard against ice daming on the roof during winter.

Ice dams can be extraordinarily destructive.

A number of years ago we had a bad snow that cause ice dams. One house had it really bad to the point where the water coming in caused nearly $30,000 worth of damage.

You DON'T want to insulate the attic ceiling - you want to do a good job of insulating the attic floor. That will keep heat in the living area of the house and not waste heat going up into the attic. Bad idea to add insulation to the attic ceiling - you will invite moisture problems which you are not having now. You might want to increase the amount of insulation on the floor. See what the R value is of what you've got now, and check if that's enough for your geography. Heat rises and you want plenty of insulation above the living area of your home.

If using the attic level for living space, then insulate the roof but not the floor. If it's just for storage, then insulate the floor but not the roof. It's only one or the other at a time. Because of the way attics "work" (for ventilation and temperature control), you generally never insulate both together. (I'm insulating my 250 year old attic for the first time now, myself.)

So if your roof is uninsulated, it's not supposed to be and you're fine. As folks have said, if it's cold and drafty in your house, add insulation to the existing stuff in the floor and that should help. (Wear a good respirator mask, if doing it yourself!)

Remember that how much heat is held is determined by the material's R-value, not its depth. Better R-value also gets more expensive... fiberglass and blown cellulose are two affordable but reasonable options. If you've got a deep budget for the project and want to keep more space, they make very effective insulation (like Icynene) that holds the same heat with a much thinner coating. (I covet Icynene... waaay out of my range!) :)

Best of luck with your attic, whatever you decide it needs!