What I recommend is to use a down bag or synthetic bag over a normal sleeping pad. There are ways of dealing with the moisture. First of all, excess moisture tends to form at the center of the ceiling, because there is where the heat rises. If the ceiling is less than perfectly smooth, then moisture may drip off of those little points. If the ceiling is smooth enough, then the moisture starts to migrate toward the side walls and then move down. If your sleeping bag is laying up against the igloo wall when this moisture is moving, you will have a problem. Instead a simple plastic tarp or mylar space blanket can be placed, perhaps half under the sleeping bag and half against the igloo wall. Then, if the moisture is moving, it will go behind the tarp. If you leave the igloo, you can fold the tarp more directly over the sleeping bag just so nothing will drip on it.
When the igloo is constructed, it is nice to have sleeping benches made out of snow or ice. These will raise the sleeping bag off the floor by a foot or so. That is recommended since the coldest air will sink to the lowest point.
Normally you want to punch an air hole in the ceiling of the igloo, and then have another air vent somewhere low like at the door. That way, there will be a slight flow of air which will also decrease the humidity.
A lot depends on where this will be. California snow is a lot different from Colorado snow which is a lot different from Maine snow, etc.