It should allow in as much air as it expels. This constant flow of air is meant to prevent condensation in your attic space. However at the same time, this air movement can cause what is known as wind wash. Wind wash can move insulation from one area of your attic to another and in some cases leave a section with no insulation at all. This is even more likely in a windy open area and is very common on acreages. Insulating your attic with
spray foam is a sure way to eliminate windwashing, and get the most out of your attic's R-value.