Ventilation primary purpose concerning insulation is to dilute the heat that manages to get through the insulation so the heat cannot condense. Moisture is the number one pollutant in homes today and inadequate ventilation is one of the leading causes. There are only two places in homes today where ventilation is applied to insulation. They are crawl spaces and attics.
Most people think the vents in crawl spaces and attics are there for the summer. If fact the vents are there for winter. In crawl spaces, heat from the floor above the crawl space begins to go through the insulation. The heat should first go through the vapor barrier, which reduces the moisture in the heat. Then the heat continues to travel through the insulation and when dew point is reached, there is insignificant moisture in the heat due to the vapor barrier for the heat to condense. If the vents are closed or blocked during the winter, though the moisture content of the heat was reduced, as more heat with moisture enter the crawl space, dew point will be reached and the heat will condense. This same principal applies in the attic.
The vents provide for what is known as equilibrium relative humidity. Cold air usually contains less humidity than warm air. We also know that materials absorb and expel moisture. In fact most materials start out moist. Temperature and the type of the material determines the amount and rate of absorption and expelling (drying out process) of moisture in materials. We also know that heat is attracted to cooler surfaces and it is there where condensation usually occurs. This is illustrated below with black stains around nails. When it comes to crawl spaces, based on the aforementioned, it would be recommended to open the vents in the winter. In the summer because of the heat and humidity of the air outside the crawl space, coupled with the fact that the masonry (heat sink) in the crawl space would qualify as a cooler surface for the humidity to condense upon, it would be recommended that the vents be closed.
With attics, your climate would determine ventilation. There are those who argue that in areas where heat and humidity is present most of the year, there should be no ventilation for the same reason mentioned above. I agree somewhat, except for the fact that there are no cool surfaces in an attic that could compare to masonry. Years ago when they didn't use insulation in homes, ventilation was not installed, even in areas where they had cold winters. It wasn't until they started putting insulation in homes that ventilation needed to be installed in attics. The reason was equilibrium relative humidity. Cold air outside during the winter usually has a low humidity level. Any heat with humidity in it that managed to get through the insulation, the low humidity cold air would absorb the humidity in the warm air in the attic. Thereby reducing the ability of the warm air to condense.
In the attic there are usually telltale signs the indicate inadequate ventilation. The first sign is black stains around the nails on your roof in your attic. Another sign is you get ice damming in the winter. Though both of those signs could be a result of other things, ventilation would be the first thing you should look at. The rule with attic ventilation is that you need one square foot of free venting for every three hundred square feet of attic floor area. Free venting is when you have about the same amount of square footage of high vents, such as ridge and gable vents, and low vents, such as soffit vents. These vents must be unobstructed from each other. One way of illustrating free venting is putting a straw in a liquid, putting your finger on the top of the straw and removing it from the liquid. The liquid remains in the straw until you take away your finger from the top of the straw. If you tape up your gable vents or stuff your soffit vents to keep out squirrels, you do not have free venting. If you have adequate free venting in your attic, the temperature in your attic during the winter will be at or below the temperature outside. If you do not the temperature in the attic will be above the outside temperature. If this occurs the heat in your attic will condense against the coldest part of your attic and they are the nails in your roof. That is what causes the black stains around the nails. If you see golf ball size icicles hanging from these nails, make sure the attic entrance way closes properly, holes in the ceiling below get fixed or whole house fan louver in the hallway gets sealed during the winter. Too much heat and moisture in getting into the attic coupled with inadequate ventilation.
Snow on your roof usually melts from the top of the snow, goes down your roof to your gutter. Then to your leader and away from the home. Ice damming usually occurs when there is inadequate ventilation in your attic. This will cause the temperature in your attic to rise above 32 degrees F. This will cause the snow to melt from underneath it and not from the top. The melted snow goes down your roof and when it reaches your gutter, because there is snow on top of it, the melted snow refreezes and dams back up the roof. Hence the term ice damming. If you look at how shingles are installed, the first row is put at the very bottom of the roof and subsequent rows on top. This application of the shingles are design for moisture to go down your roof and not up it. When the ice damming occurs on your roof, the moisture goes underneath the shingle and enters your home.
In areas where you have cold winters, during the summer you should have attic ventilation and they should not be covered. The number of days and amount of humidity in this area pales in comparison to areas where they have it most of the year. It is unlikely, that a moisture problem would occur. Since insulation addresses diffusion, the insulation has the tendency to retain heat. This is good for heating but not so good for cooling. Even though the insulation resists the heat in the attic going into the home. This is the reason why attics in the summer will be hotter than the outside temperature. It also explains why the dominant heat transfer mechanism is not diffusion but is radiant during the cooling season. Most homes in areas where they experience cold winters will have thermostatically controlled ventilating fans to alleviate this problem during the summer. Other ways to alleviate radiant heat transfer are light colored or reflective roofing materials and radiant barriers.
Sometimes people insulate their roof and the insulation falls down. You should not insulate the roof of an unfinished attic. The reason the insulation falls down is the same reason why the insulation falls down in a crawl space when the vapor barrier is installed incorrectly. The difference is in the crawl space the heat comes from the floor above the crawl space. In the attic the heat comes from the heat of the roof during the summer. As the roof gets hot the heat goes through the insulation, the moisture get trapped due to the vapor barrier. It makes the insulation heavy and it falls down. The other reason for not insulating roofs is that instead of the heat dissipating from the roof the insulation will retain it and bake your shingles.
Finished attic, Cape cods, vaulted and cathedral ceilings usually have problems with their roof as a result of inadequate ventilation. Whenever a part of the room is part of a roof, the insulation must be kept at least two inches from the roof. In finished attics and Cape cods it is the slant wall. In vaulted and cathedral ceiling it is the ceiling. If the room already exists and you have problems with the roof and it needs to be replaced, consider installing a cold roof. The way to install a cold roof is to rip off old roof including the plywood exposing the rafters and insulation. Make sure there is no moisture damage to the rafter or insulation. This should be corrected before proceeding. Then install a perforated plywood usually a quarter inch thick to cover all the rafters. One by two-inch slats is nailed sixteen inches putting them over each rafter. Then the structural plywood is installed over this. This creates a one-inch gap between the two pieces of plywood from the bottom of the roof to the top. After the shingles are installed, a ridge vent is put at the top and a soffit vent at the bottom. Though the cold roof cost more to install, the difference is that of instead of your roof only lasting 15 years, it will last 30 to 40.
The rule of free venting applies to all roofs, regardless of the kind of roof or how many roofs your home may have. Let's assume you have a split-level home with an attached garage and an addition. One level has a cathedral ceiling and the other an unfinished attic. The room above the garage is finished and the addition has a flat roof. The rule of free venting applies to all four roofs separately.