Ventalate a crawlspace into the house

I don’t know if this is a good idea or not and this would be used in the summer only.

The crawlspace is at grade, not underground, and is concrete, not open to the earth. There is a ventilation fan that can be used to take air from the crawlspace and push it into the house, or reverse the process. The crawlspace is often damp and cold in the summer. The dampness is not a good thing down there.

I figured by using the fan to draw air from the crawlspace into the house it will help dry out the crawlspace while lowering the temp of the house, though increasing the humidity, which could be later removed by the a/c. Would this be a good idea? Any practical use to reverse the fan, blowing air from the house into the crawlspace?

Bad idea.

Your cellar is not manufacturing coolness. If you put a fan there you will quickly exhaust all the air in the crawl space which will progressively draw air from outside to replace it. Since the crawl space is cooler than the ambient, it may cool the air as it passes from outside, through the crawl space, and into your home (essentially making the crawl space into an incredible inefficient heat exchanger) but I doubt it.

The added humidity will have to be cooled. Humidity is “latent heat” and the thermostat doesn’t recognize latent—only “sensible heat”. So the house may end up very humid. (and the amount of cooling you’ll gain will be negligible)

Using the air in the crawl space for cooling is a logical idea (and many people have pondered this) but it will not work.

But I’m curious, who do you feel you need to dehumidify the crawlspace?

Another reason it is a bad idea: If there is mold growing in the damp crawlspace, which is pretty much guaranteed, you’d be blowing spores into your house.

It is a bad idea on many levels to allow air exchange between the crawl space and the house. (I’m presuming the crawl space is meant to be unheated, like most crawl spaces.) The last thing you want to do is to try to use your AC to dehumidify the crawl space, because the AC would overheat (AND you’d pay even more for electricity) and you probably would never get rid of the musty smell that would now be in your house.

A crawl space should be vented to the outside of the house. This is code in most areas of the country, but depending on how old your house is it might very well not be the case. If there is any way you can get ventilation grilles inserted into the walls of your house at crawlspace level, you should do this. It might seem overly expensive at first, but an unventilated crawl space will be damp (as yours is) and if that dampness doesn’t go anywhere at the very least your subfloor will rot out. There is also a very real chance of mold developing, which is another reason you don’ want that air coming up into the house.

Why not get a good general contractor or an HVAC professional to have a look at the situation?

Good ideas.

I’m am HVAC professional and I’ve never been asked to do this. I do not believe any mechanical ventilation needs to be done in most homes. (although many commercial basements that have heat generating equipment have exhaust fans)

Ventilation grilles are probably all that is needed. (and I’d be surprised they’re not already there and overlooked) But typically HVAC guys don’t do this, GCs do.

OK what about using it in reverse. In the evening when the outside temp drops, blowing the air from the house into the crawlspace to be replaced by outside air?

Let me answer your question and clarify my statement: “I do not believe any mechanical ventilation needs to be done in most homes.” It should have said that most basements, cellars and crawlspaces don’t need mechanical ventilation. OTOH, many homes have whole house fans that are wonderful.

As far as the house is concerned, opening a window will likely accomplish bringing cool air in. If you’re looking to create a draft, a whole house fan does a much better job.

What has me confused is why you’d like to ventilate the crawlspace. It often makes sense to ventilate attics during the summer days to cool the attic and reduce the cooling load. But I’m hard pressed to see a reason to mechanically ventilate the crawlspace. I can’t imagine any benefit, or reason to incur the cost. Typically a vent or grate is all that’s needed.

Did you bury someone down there? :wink:

Having read many articles over the years on the subject of crawlspace ventilation, I’ve concluded that the best thing to do is control exterior ground moisture via grading and diversion, and then seal crawl spaces with a poly barrier on the soil side. Once that is done, the crawl space need not have vents to the exterior (yes, I know this runs counter to some model codes), as the ground moisture stays in the ground, and there is no differential vapor pressure attempting to move into or out of the dwelling through the floor assembly. Not sure what articles I can link, but if you’re truly interested, I’ve got several decades of trade journals in the office to pick through, articles of which I can scan and send.

The reason is it’s here already, it was part of a old, now defunct central AC system. It’s powerful enough to create a draft inside the home, drawing outside air in to the house, blowing the warm house air into the crawl space.

dehumidifying the crawlspace seems like just a side benefit.