TOPIC: venting crawlspaces

my crawlspace has multiple vents with slides to close them or open.  When should I open them and when close them?

Welcome Cheryl,

Can you please let us know what area of the country you live in.  There is some debate on this subject depending on what type of climate you have.

Cheryl,

Stephen is correct in saying there is debate on this.  I always recommended that they stay open year round.  The purpose of this is to prevent structural settling, cracking, etc. The down side is that in the winter you take a risk of pipes freezing and higher utility costs.

Pipes and sub-floors can always be insulated if this is a concern in your area.

Hope this helps.

Chery

I would recommend a qualified home inspector that has a good knowledge of Building Science and provides energy audits.

First it needs to be determined if it is a heated or unheated crawlspace.

I agree, we need more details.

Your poll answers conflict. You might want to edit your answer options.


I will give you my opinion as an expert in psychrometrics rather than a home inspector.

As others posted, it depends where you live. However, keep this in mind;

It is a common belief that the ventilators should be opened in the summer time and closed in the wintertime.

Is always cooler in the crawl space than it is outdoors in the summer time.
If you have high humidity during the summer months and you put that hot humid air into the crawl space where the temperature is lower, you may go below the dew point temperature and cause it to rain in the crawl space.

Automatic crawlspace ventilators are designed to open in the summer time. In some of the Southeastern states, this is the worst possible time to open.

On the reverse side of the coin, in the wintertime the outdoor air is close to saturation (100% relative humidity). However, when this air enters the warmer crawlspace the relative humidity becomes extremely low and actually increases the dehumidification of the space.

You would think that you wouldn't want 100% relative humidity air to enter your crawlspace, but when you raise the temperature the relative humidity falls substantially.



On this psychrometric chart you can see that the dew point temperature (point at which water vapor will condense) of the outdoor air is 73.4 F. The dew point temp in the crawlspace is 60.4F which is 13 degrees below the outdoor air.

Once you reach the dew point temperature and remove the required btu's for a "change of state" (from vapor to liquid) it will rain/condense.

So as you can see, we can measure the conditions of your site and determine when the vents should be opened/closed and if ventilation volume  is adequate to dehydrate the space.