Crawl space fan is causing smell coming from attic?

According to mold industry standards, I am required to have a crawlspace fan installed when there is mold in a crawlspace, unless for some strange reason the client wants the crawlspace clearance tested. Which cleaning a crawlspace to pass an air test is very labor intensive and expensive. All vents have to be closed so the fan creates a slight negative pressure and pulls conditioned air from the level up above.
I was talking to a waterproofing company and they claimed crawlspace fans cause what they called a reverse stack effect. A crawlspace fan only completely changes out the air in a crawlspace about every 15 minutes. I do not see how this could cause smells coming from the attic like they claim, unless possibly the attic is not properly vented.

Any thoughts would be greatly appreciated.

Yes I think it could cause Air from attic getting into the home .
Fan generates positive pressure in Crawl space and negative pressure in home .
Small holes from Crawl into Wall cavities like electric wires Plumbing vents and pipes.
Air gets to attic negative air in home pulls air from attic Via attic entrance or wire holes ect and you have smell if it is in attic .
The difference does not have to be much for this to happen .

Think of the many different ways that the air in crawlspace can communicate with the attic in a balloon framed home.

I have installed hundreds of fans systems in sub-structures and never had any condition occur like what you are talking about. What has happen is odors in the sub-structure go away.

See the PDF… I put this in my reports if we find poor ventilation. Cost of install is about $ 1,000.00



What is the monthly cost of fan operation…for ever and ever and ever…

Got to get away from the “on-the-street” knowledge (???) that the attic venting requirements are a tried and true carved-in-stone correct way to deal with attic issues. These #'s were developed by accident (they weren’t even researching attic venting but vapour barriers) in the 1940’s and have never been proven by any longterm testing. If anything, the #'s have been discredited by testing!!! I have a file of papers about 2" thick from various building journals and building science authors on attic venting and the misconceptions about it!!

A smell is usually caused by some type of moisture which causes some organic matter to grow, such as bacteria. So then what is causing the moisture?

I agree and when I say to people in your life time we will no longer be venting attics .
Many tell me how wrong I am and do not know what I am talking about .
Stay tuned its starting to happen now .

If an attic is properly vented with outside air, would the attic not have more upward pressure then downward pressure created by a crawlspace fan?

If there is a negative pressure in home the air will go to the least presure . Many things can cause lower pressure in home like Dryer, bath fan fire place with a fire, Power water heaters, Genair stove fan, window in home open on down wind side of home .

Keep in mind that the “upward pressure” of the attic air is what draws the air in behind it. The air leaving must be replaced.

Depending upon the conditions (outside wind, temperature, ht of the bldg, etc) a negative pressure in the attic could draw air from any opening to it…including bypasses that originate in the crawlspace.

It is odd to me that there would be a mold industry standard such as this since creating a positive pressure in the crawlspace (I’m assuming that the fan is drawing air from outside of the crawlspace, here) would force the air/spores through bypasses throughout the house.

Think of a laundry room directly over the crawlspace with penetrations in the wall that would connect to the crawlspace (electrical receptacles, plumbing for pipes and stacks, etc). Operating the dryer, itself, would create enough negative pressure to fill the entire room with crawlspace air under these conditions.

A fan inside the crawlspace that was simply circulating the air would not have this affect, IMO.

I can’t speak directly to your crawlspace question but two houses ago (an 1890 built balloon framed farmhouse colonial) my big-arsed whole house fan would pressurize the attic and the excess would go down to the basement through the wall cavities and back up into the house when I opened the basement door.

You could even feel air coming out of some of the electrical outlets on the living levels when the basement door was closed.

I ended up adding attic ventilation area, and would leave a basement window cracked open a little to be safe whenever we ran the fan.

Al in TN

No we draw negative air through a crawlspace, drawing the air from the conditioned level above to the outside. That way dehumidification is not necessary in the crawlspace and mold spore cleanup in crawlspace is not as big of a concern. We consider costs, when writing a mold remediation protocol. We want to be as gentle on the client’s pocketbook as necesary but still, at the same time, make the home liveable again.

These use less then a light bulb… Every home we have ever put one of these in the soil even standing water would dry out in a day or 2… These just about shut down mold condition and any oder’s… These don’t draft that mush air to have any effect of an attic problem…

I think almost every home should have on of these installed. there is always dead air spaces in all sub-structures. move foundation ventilation system are of poor layout or the front of the home is block by the garage the front porch/steps and then bushes. then if you look at the back of most home we see large decking systems then the sides of the home have a fence 5 feet away from the side walls. poor air flow.



We been having good results with crawlspace fans, just like Ron has.

S0 these fans run year round, sucking conditioned air (heated/cooled, etc) from the house…there can be a large cost for this!!

No go back and look at the PDF I posted… the cost is less the a light bulb but the benefits are very good.



Had a look at that!

Do you seal the house floor at all wiring, plumbing, heating duct/pipe, other service penetrations. If not you’re sucking conditioned house air out with the cross ventilation air.

To suck in the ventilation air as designed, the whole crawlspace must be under a negative pressure. This negative pressure will suck conditioned air from the house…there’s the energy loss.

Some of you guys are getting hung up on counting penny’s and that a wast of time that is a bigger dollar lost to the GDP Then any low volume vent fan is ever going to suck out of a home:mrgreen:… Just messing with ya… L.O.L.

But the energy loss is very limited to the benefits the fan will save in Mold, and fungus infection and Fungus damage a sub-structure would suffer over the life of a home…

But one could do all that you listed and then you may save a few Penny’s per year…

But The old farm house my grandfather built had gaps under the front and rear door. the old single glass windows had gaps and the joints in the floors had gaps. And even in the dead of winter I remember it snowing out side and my grandfather would have a window wide open and put a extra log in the fire place. then put a fresh pot of water hanging over it to put moisture back in the air. We were happy and to this day I sleep with the bedroom window wide open and the heater on. :mrgreen: Life is good…

And by the way the old farm house has no Mold problems to this day… People trying to plug up there home to save a few Penny’s will soon find walls full of mould and floors infected with fungus…

Better to vent then not to vent…:mrgreen: Fresh air Fresh air Fresh air…



Calling all Columbia, Missouri mold inspectors…Alert…Alert…