I am getting ready blow in cellulose insulation into the ceiling cavity below my attic floor. I have pulled many of teh T&G floor boards for access to blow the insulation. I have also blocked out the cavities (and around light fixtures) under the floor near the soffits so when I blow the insulation it won't get into the soffit vent area. Do I need any sort of vapor barrier? Not sure how I would apply one if I did. My ceilings are a mix of drywall and plaster&lathe depending on the room. I assume it's standard practice to not have a vapor barrier when blowing insulation in. Especially when it's eing blown in under the flooring.

Any suggestions would be greatly appreciated. I do intend on finsihing the attic in a couple years and will simply install rafter vents and place batt insulation in the roof rafters when I do that. Thanks.

Putting a vapor barrier in the ceiling below the attic is a iffy one. Some people do it and some don't. In your case, it probably wouldn't be worth the effort to do it because you would have to remove the ceiling below to ensure a tight seal. But on the other hand, if you plan on finishing it off in the future, then there shouldn't be one.

It is true that in the winter months hot humid air tries to equalize with the colder drier air outside the house. The reason for a vapor barrier is to keep the insulation from becoming saturated with this moisture. Since most insulations work by creating still air spaces between their fibers, the introduction of moisture, which conducts heat rapidly as opposed to still air, essentially short circuits the insulation.

My understanding is that two coats of good latex paint on the walls provides the preferred moisture permeability rating that most insulations require. So a good paint job, especially in the kitchen and bathrooms where ambient moisture levels are higher, will certainly provide your insulation with the protection that it needs.

Regards,

Tom Judt

Vapor barriers (or more accurately) vapor retarders prevent the moisture in the warm air of your home from permiating the insulation where it can have an adverse effect by reducing insulation efficiency or creating condensation issues. Warm air in a home has a higher vapor pressure than the cold air outside, so it will naturally want to move from higher pressure to lower. In your case the lower pressure area is the cold space of the attic, above the insulation. The moisture will penetrate the insulation and with some luck will not reach dew point somewhere within the insulation before it discipates to the attic airspace. The simplest fix for this is to paint the ceilings with a primer that has a low permiability rating. Contact a good paint store and look for a coating with a perm rating of 1.0 or less. There are primers speciffically manufactured for this purpose. This primer will becoime your vapor retarder and is about as efficient as you can get since it is directly in contact with the warm air and subsequent moisture that you are trying to capture. A properly placed vapor retarder will not create a moisture issue, but rather the opposite, it will prevent one. The proper place is as close as possible to the warm air.