Crawl Space Vents

[fireant911]

What is the proper position of the crawl space vents for the winter months (I live in Alabama)? We do have the plastic vapor/moisture barrier on top of the earth underneath our home. I have heard that these vents should be open in the summer and closed in the winter and have also read that they should be open year-round to prevent the formation of condensation... What is the appropriate position, and more importantly, why it that the case (please substantiate your answer)?

[IROC]

Daryl,

I'm in Huntsville (Madison, actually) and my rule of thumb is to leave them open for all except the couple of coldest months in the winter.

Mike

[fastpat]

I leave my vents open all year round. Too much moisture under my house, and it doesn't freeze there, rarely dropping close to 40F.

[LakeCleElum]

Close them when temps get down near freezing. Otherwise, leave them open. Your vapor barrier should deal with the mositure...

[kach22i]

The building code I use does not say anything about closing vents when it gets below freezing. Michigan is just too wet for that sort of thing.

Closing the vents when it is cold is not like zippering up your coat when it gets cold. It would more be like putting on a cheap orange vinyl rain coat over your regular coat. This would keep more wind out, but it would also hold in all your body moisture which would make the insulation of your coat less effective. You can do this experiment for very little money.

Let it breath.

Clarification (Dew Point): In cold climates there is warm moist air in your house when compared to the outside air. Imagine a nose bleed from dry air you may get when it's really cold out. That because the outside air is really, really dry. The colder air gets the drier it gets. This is because the amount of moisture the air can hold is in proportion to it's temperature. Think about hot humid rain forest for a moment, now you got it.

So now imagine your insulated wall or in this case your insulated floor in your crawl space. Nature loves everything to be in equalibrium (warm to cold energy transfer) so what you have is all the warm moist air being drawn towards the outside. The warm moist air hits the colder wall/floor and suddenly cannot hold as much moisture. This condition is really bad around windows and doors because the rate of change is even higher.

Anyway, the air all of a sudden the air cannot hold all it's moisture because of the temperature drop so it just lets it go........that's right, right inside of your wall/floor and into the insulation.

The point where this change/exchange happens is called the "DEW POINT". This will be on the test later.

ASIDE: In cold climates the vapor barrier is put on the inside of the exterior wall (Michigan). I am told that in warm climates (Florida) the vapor barrier is put on the outside of the exterior wall. The vapor barrier is ALWAYS put on the warm side of the wall.

I don't know where the vapor barrier goes if you have perfect 50/50 weather but could look into it. I guess you could use an insulation which requires no vapor barrier such as Cellulose as it needs to breath (even more) to release the moisture it absorbs. This will not be on the test.

Odd Thought: Your exterior wall is vented to the whole outside via it's surface area. The crawl space is a cavity exposed to moisture coming up from the ground despite any vapor barrier laid on the ground. The vapor barrier which only helps control the rate this moisture can migrate is based on perms.