Crawl space vents

BrianGreen

BrianGreen

SSG Brian Green
Joined
Apr 5, 2008
Location
Kings Mountain
I know there has been a lot of talk lately about crawl space stuff. I have the automatic crawl space vents on my house (like most everybody). Yesterday evening I noticed they were open. It was only 20*. Is this a normal operation or are they too old and not working properly? No frozen pipes yet. I may cut some pieces of plywood to cover them for right now.

Thoughts?
 
Don't close your vents (ever). They are for air movement and moisture control, not temperature control. Ventilation through your crawlspace helps to keep it dry so you don't end up with mold and other moisture related issues.
 
Kickdeez said:
Don't close your vents (ever). They are for air movement and moisture control, not temperature control. Ventilation through your crawlspace helps to keep it dry so you don't end up with mold and other moisture related issues.
Click to expand...


That’s not accurate at all. This time of year is great for crawlspace moisture control sure, but it sure is hard on the water pipes lol.

Vents open in the summer time doesn’t control crawlspace moisture what so ever, as long as your thermostat for the cooling season is below 78ish. And then it’s only a maybe but probably not, based on several factors such as crawlspace height and width, House design (front and rear porch) etc.

In theory you want your vents open when the dew point is less than the typical duct temperature, but above freezing.

You want them closed when the Dew point is above the duct temperature.

So when it’s humid and hot in summer, you want them closed, and you want them closed when it gets below freezing.

But you want them open some in the spring and fall when it’s not humid.

With our climate that can be a day to day change.
 
Interested to hear more from others. Mine are not automatic, just good ole fashion manual. I have always left them open because I was told it is for moisture but common sense tells me it is also to help keep my pipes from freezing, so can't wait to hear more.
 
I'd buy a sheet of 1-2" XPS board, cut a bunch of 8x16" squares out of it, and foam them in place with expanding foam. Make sure there's a vapor barrier covering the entire floor, and plan to get an automatic dehumidifier installed in the crawl space before spring.

Some people will recommend foil-faced polyiso because of rodents, but it's not rated for ground contact (because hydrophilic), and its U-value at lower temperatures sucks (again, hydrophilic). I don't like it for those reasons.

Your crawl space should be 55-65F and 35% RH or lower this time of year.
 
shawn said:
Your crawl space should be 55-65F and 35% RH or lower this time of year.
Click to expand...

That's exactly what mine says at the moment. And that the battery in the remote sensor needs replaced.

Kickdeez said:
Don't close your vents (ever). They are for air movement and moisture control, not temperature control. Ventilation through your crawlspace helps to keep it dry so you don't end up with mold and other moisture related issues.
Click to expand...

The whole premise of vents in our (Charlotte) climate zone is flawed. In the summer you've got an insulated floor with some sort of vapor retardant flooring above it, a crawlspace colder than the outside air, and a bunch of nicely distributed sources for that air to get into the crawlspace and condense its vapor. Moisture control ends up keeping the crawlspace nice and moist, not removing moisture from it.
In the winter you have a big energy sink that freezes your pipes and gives you cold floors unless you close them. But at least condensing moisture isn't a problem.
 
Last edited:
shawn said:
I'd buy a sheet of 1-2" XPS board, cut a bunch of 8x16" squares out of it, and foam them in place with expanding foam. Make sure there's a vapor barrier covering the entire floor, and plan to get an automatic dehumidifier installed in the crawl space before spring.

Some people will recommend foil-faced polyiso because of rodents, but it's not rated for ground contact (because hydrophilic), and its U-value at lower temperatures sucks (again, hydrophilic). I don't like it for those reasons.

Your crawl space should be 55-65F and 35% RH or lower this time of year.
Click to expand...

House is about 2k sqft ranch built in 82. Existing vapor barrier sucks and is on the to do list. Sealing the CS isn’t on the radar on this house mainly because it would be a total PITA. Bottom end is about 4’ headroom and top end is about 18”. Just trying to figure out if my vents are fawked and I just need to block them temporarily to keep the CS temp above freezing. Haven’t had a pipe freeze yet. Our temp has barely gotten above freezing for over a week. Figure it’s only a matter of time before the CS temp drops enough with open vents.
 
The only positive thing an automatic vent can do is close at below freezing temperatures. This will keep pipes from freezing and make things better for your heating ducts. A warmer crawl space will make your house warmer and actually some energy. Go out and seal your vents! Use anything to stop the air flow such as cardboard or duct tape. You can open them back up in March.
 
Close off vents, add a small radiant heater as needed. Some folks like to run a higher wattage flood light. We did this in a well house works well until the bulbs goes or it stays this bloody cold. I tend to cycle our vents by season. In my rental the vapor barrier I replaced it and tried closing it off tight for a season. Exceptionally bad idea without the dehumidifier...in our summers with chilled air running through the crawl space no matter what its insulated in will be a swamp. This is something I should address.

I cannot verify by scientific data or done my own research on the next bit:
By dehumidifying the crawl space in a less than "tight" construction you can also save on the cooling cost of said dwelling.

A Hvac contractor told me about this hypothesis. My mind says its got some value to it since a ac unit is really trying to dehumidify the air as it circulates. This is do to the nature of the basis of the systems function. ME thinks less humid air is also less dense thus easier to thermally cool. I do know at least humid ball sweat is worse than dry heat!
 
WARRIORWELDING said:
Close off vents, add a small radiant heater as needed. Some folks like to run a higher wattage flood light. We did this in a well house works well until the bulbs goes or it stays this bloody cold. I tend to cycle our vents by season. In my rental the vapor barrier I replaced it and tried closing it off tight for a season. Exceptionally bad idea without the dehumidifier...in our summers with chilled air running through the crawl space no matter what its insulated in will be a swamp. This is something I should address.

I cannot verify by scientific data or done my own research on the next bit:
By dehumidifying the crawl space in a less than "tight" construction you can also save on the cooling cost of said dwelling.

A Hvac contractor told me about this hypothesis. My mind says its got some value to it since a ac unit is really trying to dehumidify the air as it circulates. This is do to the nature of the basis of the systems function. ME thinks less humid air is also less dense thus easier to thermally cool. I do know at least humid ball sweat is worse than dry heat!
Click to expand...
rather than an edit can anyone verify said claim?? @Mac5005 or @moldman05
 
Last edited:
Fabrik8 said:
That's exactly what mine says at the moment. And that the battery in the remote sensor needs replaced.



The whole premise of vents in our (Charlotte) climate zone is flawed. In the summer you've got an insulated floor with some sort of vapor retardant flooring above it, a crawlspace colder than the outside air, and a bunch of nicely distributed sources for that air to get into the crawlspace and condense its vapor. Moisture control ends up keeping the crawlspace nice and moist, not removing moisture from it.
In the winter you have a big energy sink that freezes your pipes and gives you cold floors unless you close them. But at least condensing moisture isn't a problem.
Click to expand...


Condensation still occurs in a vented crawlspace in the winter, it’s just not on the ducts. Typically on the foundation brick walls and band/rim joist.

The temp drops faster than the drying capacity/Air exchange rate of the cold dry air through the vents.


If you want to seal automatic vents it’s a bit more involved that standard mortared in vents. Cut 7x14 pieces and glue/tape these to the back of the vents. This can be done from the outside. Then crawl under and use expanding foam around the vent and cover.

Also serves to hold the vent in place better than spring clips and the factory screws.

If sealing then some type of means to dry and keep the crawlspace dry is necessary in conjunction with a minimum 100% vapor barrier.
 
The house was built in the 80s, brick/cinder block foundation and has the old school, tin metal prop up/lock shut vents. I shut them in the winter. A thermometer in the crawlspace reads 50* every time I've checked it when it is well below freezing.
 
My dad installed those vents forever and tried to get me too. I always saw them open in the coldest weather so I thought, whats the point? I defer to common sense on this one. Winter air is much drier, pipes can freeze if that cold air isn't stopped or slowed just a little, and it is easier for your HVAC to do its job in freezing temps if it's not surrounded by biting cold air all the time. Close all but two on opposing sides of the house and feel good about it.
 
Back
Top