What's the consensus on crawl space vents? Do you leave them open all year around or do you close them in the winter months? One story is to leave them open so humidity doesn't build up. Another is to close them so the cold air doesn't get in. What's your take?

 

closed in winter to keep the floor warm and keep heat from escaping causing the boiler to cycle faster.
open in the summer for airflow to help prevent moisture buildup.

 

Current consensus in the construction community is to permanently close them--and meticulously seal off the crawl space from the ground, bringing the crawl space into the conditioned space of the house. This means laying and taping heavy (6 mil) plastic over the ground and up the sides of the foundation.

Jason

 

As well as at least 2" foam insulation panels on the walls, and insulating the rim joist, and sealing the mud sill. Roll foam insulation, sill sealer, is rolled out on top of the foundation blocks, before the sill, usually a treated 2X8, is installed, over the sealer. Where the rim joist and mud sill meet, caulking/sealer is applied a lot, or better yet, minimal expanding foam, then either 2" foam, or unfaced fibreglass installed over that. Theory more/less is that the heating mechanicals will warm the air in the crawlspace, and keep your floors warmer, it works. I've done it on customers houses and my own. My floors are warm, and crawlspace is bone dry.
Whatever you decide, NEVER insulate your floor in a crawl space, you'll just about guarantee cold floors.

 

That's interesting. I have the vents from home depot that open and close automatically as the temps change. They are closed in the winter and open at something like 65-70 degrees.

Not trying to sound like a SA, but if vents are not needed then why are new homes built with them?

It's also my understanding that the vents are placed in such a way that if your crawl space were to flood, the vents would basically allow the flood water to more easily escape from under the house and hopefully minimize damage to the foundation.

Clearly I'm not a builder nor am I in the trades, just curious.

 

What we have is a set of building codes, The International Building Code. Then, each state adds to that, which they think are a good practice. Then the municipality that the state code services, adds to it as well. Nothing can be removed, only added.
So, since everyone replying in this thread, is from a different state and municipality, differences in what is code is going to be slightly different.
At times, even the building inspectors throw things in there themselves, notice the "at times".
So what is required in Va., is not totally what is required in In, Oh, or Mi. The basics of the IBC, yes. But like I pointed out, it can be added to.
When I more/less retired from the building trades, these nonvented crawlspaces were an up and coming thing, I only maintain my license as we speak, just in case. I've never constructed the complete system, and always installed vents, but my vents were covered over, and therefore sealed.

Tim, I'm going to pick on you. How deep is your crawlspace, and where are the vents installed in relation to the height of your foundation. In your case, Hampton Va., the flow of water is an important consideration. Now with me, it isn't. That's just an example of why there are differences.

Jason pointed out the "consensus", and in some schools of thought he is correct. Good, bad? If it works, it's good, if it doesn't, it isn't.

There is a website, JLConline, I'm still a member, but not active. The OP may want to get over there and ask his question. It's all building professionals over there, inspectors included, and do answer questions from "Joe homeowner". There's a lot of construction knowledge to be had.
There is a magazine as well, if anyone is that interested, called The Journal of Light Construction, my sub is expired, but that mag, expensive, has some very very good info each month.

 

As usual I got sidetracked. Consensus is to close them in winter and open in spring through the rest of the year. It will tend to conserve some heat, but there is a better way as I tried to pointe out. Vents are there to provide crossflow ventilation to minimize moisture in the crawlspace. Moisture promotes moulds and rotting. Something you really don't want. In the winter months, with the colder air, when you add heat, the relative humidity drops, so in a lot of cases, you can shut your vents off. The vapour retarder, 6 mil sheet visqueen, should be installed so ground moisture will be slowed down.

 

My crawl space is quite tight and my vents are very close to the ground, I'm assuming to allow flood water to escape more easily should I be so affected.

 

Here in Mi , code now is to have a concrete floor in the crawl space ,(thats why I went with a full basement ) may help with moisture , but I doubt it will help with the temp ...