# Crawl Space vents -open or closed??



## mgrclicket (Jun 21, 2010)

First let me say:
A basement house has hvac in basement,with condensate line ran outside or in drain pipe. This house has windows,
doors,etc regardless, the basement is either finished or not, it is not a crawl space,but buttoned up, and you dont get condensate on the ductwork..
Now... on the otherhand, a crawl space has vents that can be open or closed.
When warm air from the outside enters the crawl space,
condensate collects on the ductwork and drips to plastic on ground from temperature change.
 Should'nt the vents be closed in summer to help eliminate the condesation?? If the plastic is wet and causes humidity to rise in the crawl space how is it ever going to dry out? Conversely, if you put a blower fan at the crawl space door,close all the vents except the opposite side, to draw air from one side of the crawl space to the access
door the hot air drawn into crawl space keeps the 
condensation on the ductwork,allowing to keep on dripping. Have I missed something here? it don't make
sense to me. If the duct insulation is wet from the outer edge of the ductwork to the outer foil skin of the ductwork insulation...
How can you possibly dry it out???? Just seems to me
you are defeating the purpose by installing a blower to 
 dry things out.
Any Heating/Air, Ductwork Pro's... please help me understand this....Thanks


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## BASSCAT1 (Jun 21, 2010)

*vent*

pull the air from the floor of the basement/crawl using a small duct fan and vent the air out the window or outside some how. Alot of those companies that promise you a dry basement /crawl space use this same theory.


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## jonkayak (Jun 21, 2010)

I do this for sort of thing for a living. While use to they told you to open the vents the new way is to close the vents and seal it up the best you can. If you have a gas furnace you need a fresh air line ran for it but other wise seal it off. If you can install a layer of plastic down and seal it to the walls and all the seams. Then install a fan that draws the air from under neath the plastic and vent it to the exterior of the home. The area between the plastic and the floor above needs no venting. This technique that we install in homes everyday helps to eliminates most moisture related issue in homes with crawl spaces such as wrapping floors, rotting beams, and humidity. Also it has been proven to lower your HVAC cost but I'm not sure to what degree.


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## Harlee (Jun 21, 2010)

The vents should be open.  The duct work should be insulated well enough to eliminate condensation.  If the duct work is condensating, then the insulation is either compromised somewhere or not sufficient enough.  Over time the insulation can break down, become torn or simple fall loose.  I am assuming that the duct work is galvanized pipe and not flex duct.  To eliminate the condensation the duct work should be inspected and any loose parts re-taped or stapled.  If the insulation is in very bad shape the duct work can be totally re-wrapped.  The plastic on the ground is a moisture barrier to keep the moisture in the ground and away from the floor joists and sub-floor of the home.


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## jimbo4116 (Jun 21, 2010)

mgrclicket said:


> First let me say:
> A basement house has hvac in basement,with condensate line ran outside or in drain pipe. This house has windows,
> doors,etc regardless, the basement is either finished or not, it is not a crawl space,but buttoned up, and you dont get condensate on the ductwork..
> Now... on the otherhand, a crawl space has vents that can be open or closed.
> ...



If you have condensation on the ductwork falling onto the vapor barrier, you will soon have rotted joists and seals, plus you are providing moisture for termites to thrive.

I would leave the reason for your condensation to an expert. Have it inspected by a HVAC expert.

I had the same thing. Water puddled on the vapor barrier and rotted out part of a seal and 8 joists under our MBR and bath.

Had the ductwork redone and double insulated after taking floor out and before replacing.  Installed a exhaust fan with thermostat. 9 years ago no more problems.


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## Trigabby (Jun 21, 2010)

jonkayak said:


> I do this for sort of thing for a living. While use to they told you to open the vents the new way is to close the vents and seal it up the best you can. If you have a gas furnace you need a fresh air line ran for it but other wise seal it off. If you can install a layer of plastic down and seal it to the walls and all the seams. Then install a fan that draws the air from under neath the plastic and vent it to the exterior of the home. The area between the plastic and the floor above needs no venting. This technique that we install in homes everyday helps to eliminates most moisture related issue in homes with crawl spaces such as wrapping floors, rotting beams, and humidity. Also it has been proven to lower your HVAC cost but I'm not sure to what degree.




Any chance you can elaborate?  What kind of fan, what do you use to draw the air from under the plastic, etc?


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## jonkayak (Jun 21, 2010)

Trigabby said:


> Any chance you can elaborate?  What kind of fan, what do you use to draw the air from under the plastic, etc?



http://www.radonaway.com/radon-fan-gp-series.php

The link above is to the fans that we use. They consume about the same amount of electricity as a 75 watts light bulb does which relates on average to about $5 a month to operate but that is normally offset by a reduction in the lowered HVAC operating cost. These fan are designed to run contiguously and last upper wards of 10 to 15 years. To remove the air from under the plastic take some black corrugated pipe like you get in the garden section of Home Depot and lay it under the plastic. You can normally just run a 15' section under the plastic be fine on averaged sized house (2000 sq ft give or take) and be fine. Then you connect that to 3" PVC piping and pipe that to the outside of the home. Attach the fan and vent it away from the house. Normally we vent it above the homes eves or either we chase the piping to the attic and mount the fan in the attic and vent it through the roof. The fan actually connects inline with the piping. The plastic membrane that we use in a dual layer reenforced with nylon but the heavy black stuff from HD will work it's just not as strong. Then it's attached to the wall with Liquid Nails and furring strips (1"x2") are mounted over the plastic with concrete anchors to attache it to the wall until the Liquid Nails dry.

Here is a pic of how we seal the basement.







If you decide to go this route let me know and I can walk you through the steps I helped a few people on here who are ambitious DIYers  do it them selves. Also for the average consumer Amazon.com has the best price on the fans.

One final thing this same process minis the plastic and the  corrugated pipe is used to reduce or eliminate humidity in basements as well. You just have to get the piping under the slab or cut the slab to get it under there.


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## jonkayak (Jun 21, 2010)

One other thing. The reason why this works so well after the vent are closed and the plastic seals it up is due to the fact that most of the moisture in a crawl space comes from the ground and not the air. So in years like this one where it rains non stop the ground is saturated.


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