crawl space ventalation

Recently purchased a 30 year old one story 30'x70' ranch built over a 4 to 5 foot crawl space with dirt floor. The foundation has vents with opening/closing louvers all the way around. Should the louvers be closed during the winter months? The floor is not insulated. I plan on laying plastic over the dirt to help stop moisture, but have not done so yet.

Also, I think I would like to insulate, with foam, the forced air furnace vents as the heated air at furthest ends of the house is not very warm. Any opinions?
Pat

 

The sole purpose for ventilation is to prohibit moisture damage to the home. It aplies to areas in the home where moisture can be trapped, such as attics and crawl spaces. It does not apply to walls. Heat usually has more humidity in it than cool air and heat is attracted to cooler surfaces where it condenses. Insulation does not stop heat flow, rather it resist it. The heat transfer mechanisms for heat flow through your floor into the crawl space are diffusion and capilliary action. Once the heat enters the crawl space and since all heat has moisture in it, the cold air from the vents absorb the humidity from the heat because it possesses less humidity. If the vents were closed, you would trap that moisture in the crawl space and then you have a problem.

The confusion comes when you consider the vents during the summer. You are cooling the home during the summer and the same principals apply except that the dominant heat transfer mechanism in cooling is radiant. Since the hot humid air outside contains moisture and the cool surface it wants to condense on is the parts of the home which is cooler, closing the vents in the summer is advisable. Furthermore, this is where radiant barriers shine, literally. Though I'm not an expert on radiant barriers, I am aware of its applications. Therefore I will refrain from discussing it with you.

Insulating the floor and the ductwork in the crawl space is known as the thermal boundary of the home. This is where you physically separate the conditioned space and unconditioned space of the structure to prohibit heat flow or loss. Covering the dirt floor wth plastic prohibits evaporation and it requires heat for evaporation. The heat is usually extracted from the floor located above the dirt.

 

I live in NC, a much more temperate climate than NY, and I close my vents in the winter to prevent water pipes from freezing.
I have a 3' crawlspace, insulated floor, insulated ductwork, insulated hot water pipes, and no plastic on the dirt.
It is very humid here naturally, and I live on the water, so we have high ground and air moisture.
For years I had condensation dripping off of water pipes AND the ductwork insulation.
I finally solved it by installing a 10" louvered exhaust fan in the crawlspace door. It runs 24/7 with the vents wide open (except during the winter).
Stays bone dry now.
If I were you, I would insulate ALL of the ductwork for energy efficiency, and if you have condensation problems, install a vent fan.
Good Luck!
Mike

 

Freezing water pipe are a major concern and one of the reasons I enquired about this. Last winter I forgot to close a basement window at old house and had frozen water pipes.
Pat

 

Even when you wrap your ductwork in insulation, there still should be enough heat loss in the crawlspace to prevent your pipes from freezing, IF your vents are closed.
Mike

 

If you're worried about frozen pipes, at the bottom of this message click on the little house icon with www next to it and read topic "Water Heating". You'll find the phone # of the manufacturer there.