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Use plastic to cover dirt floor of crawl space or not?

Use plastic to cover dirt floor of crawl space or not?

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  #1  
Old 04-25-11, 08:24 PM
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Use plastic to cover dirt floor of crawl space or not?

I am reading posts about using plastic to cover the ground of the crawl space. Some say to do so, some say that it would be better to use a dehumidifier.

Also, some suggest closing the vents instead of leaving them open during the summer.

This is confusing.

Can someone shed some light on whether I should put the plastic down or not?
Also, open the vents or close the vents for summer?

Thanks in advance.
 
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  #2  
Old 04-26-11, 03:15 AM
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There are several schools of thought on this, none is more correct than the other, except location. Here in the south we are required to provide ventilation to all craws spaces and cover the earth with 6 mil plastic. We carry that plastic up on the walls and I like to use HVAC duct adhesive. Seems to work pretty good. You can staple it to the bottom rim plate, but you have to make sealed concessions regarding the vents if you go that high. I use automatic vents and don't have to worry about opening or closing seasonally.
NOW, the other side will be presented by Airman, so give him a few minutes to wake up. I don't know of anyone who would not advocate the 6 mil plastic, but the others will recommend sealing up, conditioning, dehumidifying, etc. the space mechanically, so hang in there for the other point of view.
 
  #3  
Old 04-26-11, 04:11 AM
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Plastic. Open. That's how our previous residence in NC was configured and it seemed logical to me.
 
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Old 04-26-11, 06:12 AM
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I strongly also suggest the automatic opeing vents and 6 mil. black plastic.
Every home I've ever worked on with no vents has wood fiber eatting fungus growing like crazy. And if there a dehummidifier it had not been checked for years to even see if it had been working.
 
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Old 04-26-11, 06:46 AM
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There is new thoughts on venting crawls. I sealed mine up. Plus I insulated around the sill plates.

Here is some info, with links.

I suggest you read and educate yourself. You will get too many conflicting answers here. Its old school vs new school. Sometimes towns require vents by code. But the engineers are old school. They do not want to change the building codes. They make too much money to care.

http://www.advancedenergy.org/buildi...0To%20Vent.pdf

Mike NJ
 
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Old 04-26-11, 08:45 AM
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There seems to be some misunderstanding as to how water vapor works.

Water vapor is created when the sun shines of the sea and land round the equator.
It rises and moves north with the rotation of the earth.
It lands in your yard as dew, frost, rain, hail and snow.

About two hundred times a year (if you get up early enough) you see dew or frost on your greenhouse, car or lawn.
This is due to warm humid air meeting a cold surface and condensing.

You also see condensation when you open your freezer and its full of white frost, or take a bottle of beer from the fridge and within moments its covered in condensation.

Water vapor is carried in warm air, when it meets your home it dives through the holes trying to reach a cold spot in your basement or crawl space to condense.

If the joists in your crawl space are cold enough it lands as condensation and makes the wood wet, if there is a mold spore there it can grow and rot your wood.

Some people seem to believe that the cold water in the ground under their home will suddenly heat up and turn into water vapor/steam it does not.

Water vapor only escapes from the surface of water when heat is applied.

When you heat a kettle and boil the water it mimics the effect of the sun and steam is formed, that condenses on your cold windows. Don't boil the kettle and the water stays inside.

As you can see from the above, the water vapor in the air enters the crawl space through the holes in your home, it is helped to do this by the heat rising inside your home escaping through open windows and doors (and cracks and holes) and pulling in fresh air from the crawl space/outside.

What you should do is block all the holes in your floor and the rest of your home to stop this.

There is also the question of radioactive Radon Gas, this rises from the decaying Uranium in the ground and is pulled into your home by the escaping warm air.

Radon Gas kills over 20,000 people a year with lung cancer.
 
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Old 04-26-11, 12:21 PM
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Originally Posted by lawrosa View Post
I suggest you read and educate yourself. You will get too many conflicting answers here. Its old school vs new school. Sometimes towns require vents by code.
I really think these studies try to couple the entire world into a particular class and dictate what is best for them based on laboratory studies. It is wrong. Different parts of the country almost scream for sealed crawl spaces, conditioned and dehumidified, while others are pleasantly happy with the flow of fresh air given through the vented systems. Here in the south we have dry crawlspaces with open vents. Close them up during moist summer time and you start getting white mold on the joisting. Although we are required to do it, it just makes sense where we are located. It is good information, but you have to dissect the data to get the full impact.
Oh, Perry, read all the information regarding radon deaths. It almost always is aggravated by other cancer causing problems such as smoking. To say radon itself causes that many deaths is just an EPA statistic. Face it, the EPA is the one shutting down oil rigs in the ocean and on land, so I don't put too much credence in their "studies", either.
 
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Old 04-26-11, 07:46 PM
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I'm awake!!! Everbody is saying that you need a VB. Like said seal seams and run up wall. Now to the other side! you are in a green grass state so having vents will add more rh to the crawlspace close the vents. Monitor the rh levels and if they are above 60% you will need to add a dehumidifier. Once sealed I have only seen the need for one about 2% of the time.
 
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Old 04-26-11, 09:27 PM
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And where I grew up and now where I live it is just the part of your yard that is under the house. Often it isn't even closed in. Never seen a vapor barrier used there but like I said it is just the part of your yard that's under the house. Who puts a vapor barrier on their yard?
 
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Old 04-27-11, 03:10 AM
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If we look at the Radon map, we see that an awful large part of the country is emitting Radon gas, you may live on a hot spot. You can buy test equipment to indicate how bad it is. You may know that Radon is one of the heavy gases, it doesn't blow away as quickly as light gases. It tends to linger in corners and cupboards where it turns the dust radioactive.

Radon gas builds up in the winter when the windows and doors are closed and we are trying to keep warm, it is less of a problem in summer with lots of ventilation.

Of course a lot of people who have/will get Radon induced cancer, have worked in mines or in other underground jobs where Radon gas is impossible to avoid or as children they played on the floor.
 
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Old 04-27-11, 04:56 AM
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Perry, I'm having a hard time with your assertion that water will only evaporate with direct sun light or a hot flame applied to it. I don't see how the water in the soil in a crawl space does not evaporate and add to the humidity.
 
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Old 04-27-11, 08:59 AM
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After an April shower, it is frequently possible to see steam rising off a fence or off the ground when the sun is warm enough to do this. This does not happen in a crawl space......there's no sun and its too cool.

The Laws of Nature, Physics, whatever, say that hot always moves to cold and water vapor always moves to cold.

To illustrate, think of a steel poker held in a fire, gradually the heat moves up the poker until its too hot to hold.

Or a room you are trying to heat in winter, when the heat continues to disappear though the windows and the water vapor condenses on the windows. It also disappears through the walls and ceilings, but that's not so obvious.

Air that is warm holds water vapor and air that is cold does not.

Ground that is hot from the sun gives up its water as steam, ground that is cool does not.

When you take the air temperature of a crawl space it is usually around the 40f mark. Cool.

Under the home the air is usually cool, its in contact with the ground and parts of the home that are cool. Even in summer, it holds little water vapor.

The ground and walls are colder than the air, so any warm air that arrives deposits its water vapor onto the nearest cold surface usually the ground, if this is below the "Dew Point"

Rather than the water vapor leaving the ground and moving into the air and then condensing on the joists, it moves from the air into the ground if its below the "Dew Point."

Dew Point Calculation Chart (Fahrenheit)

% AMBIENT AIR TEMPERATURE IN FAHRENHEIT
20 30 40 50 60(70)80 90 100 110 120
90 18 28 37 47 57 67 77 87 97 107 117
85 17 26 36 45 55 65 75 84 95 104 113
80 16 25 34 44 54 63 73 82 93 102 110
75 15 24 33 42 52 62 71 80 91 100 108
70 13 22 31 40 50 60 68 78 88 96 105
65 12 20 29 38 47(57)66 76 85 93 103
60 11 19 27 36 45 55 64 73 83 92 101
55 9 17 25 34 43 53 61 70 80 89 98
50 6 15 23 31 40 50 59 67 77 86 94
45 4 13 21 29 37 47 56 64 73 82 91
40 1 11 18 26 35 43 52 61 69 78 87

An example at 70f and 65%RH the "Dew Point"
is 57f that's when condensation starts to form.
 
  #13  
Old 04-27-11, 04:25 PM
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I've said it many times you don't need condensation to grow mold. All you need is rh above 60%.
 
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Old 04-30-11, 08:33 AM
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Ok, it appears the best bet would be to lay down a vapor barrier and close the vents in Summer since I'm in Indiana.

Thanks for the info and input. I'll keep my fingers crossed.
 
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Old 04-30-11, 10:13 AM
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May I suggest that you buy a weather station with two remote sensors.
Being able to monitor whats happening and keeping a record on your computer, will enable you to make an informed decision?
 
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Old 04-30-11, 12:13 PM
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Correct. VB and closing the vents is the way to go. About the only thing I agree with perry is to go get a remote rh sensor from radio shack for about twenty dollars.
 
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Old 04-30-11, 04:23 PM
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I was just gonna ask where and how much.

THank you both so much for the help!
 
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Old 04-30-11, 04:56 PM
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From a practical standpoint, 10 mil is not that much more for a complete floor. It is far more durable, especially if you go once in a while. - It sure beats taping and sealing tears and rips that require another trip in and out after getting the sealing materials. You never find the deterioration of the VB until you have occasion to go in and the soil settles unevenly, creating low spots and tears even as you leaving. This is especially true in a shallow (low height) crawl space.

Why skimp on materials when the labor is free and the satisfaction can be high?

Dick
 
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Old 05-10-11, 04:48 PM
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Thank you for the advice. I will go that route as long as my store carries it.
 
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Old 05-16-11, 07:19 PM
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In the south definitely keep the vents closed, and 100% vapor barrier coverage. A previous poster mentioned bringing the VB up the wall and attaching the rim joist; It is local code for encapsulated crawlspaces to leave a 4" termite inspection strip open at the top of the foundation wall. I think it is a good idea everywhere. We attach the VB with 1/4" plastic pins and adhesive caulk.
If anyone wants some great info on how to deal with a crawlspace, these guys are building scientists with no vested interest. They have performed a dozen year test on controlling moisture in crawlspaces and there findings are considered the end-all by most experts.
www.crawlspaces.org and Advanced Energy, an Energy Efficiency Services Firm
 
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Old 05-17-11, 03:50 AM
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Although I respectfully disagree with CrawlspaceTech, both schools of thought should be examined in each situation. Advanced Energy can't operate without vested interest in products. Sort of like making a perfect vacuum. It won't work. Here in the mountains of North Georgia, I have encapsulated crawlspaces at the request of general contractors. I go back in a couple of months to a small pool of water all over the area. Where we abide by the Southern Building Code, install adequate ventilation around the entire perimeter, no moisture on the VB, no mold on the joists. I believe it has to do with constant wind velocity and its ability to move air in and out of the crawlspace, keeping the RH the same inside and out. But thanks for your comments. Keep 'em coming.
 
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Old 05-17-11, 07:14 AM
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Let me repeat.

Condensation does not form on warm wood, only on wood that is cold, below the "Dew POINT" of the surrounding air.

You cannot see the water vapor that lands on cut timber, the molecules are so tiny they easily slide between the fibers of the wood.

When wood leaves the kiln it has had its water content reduced, as soon as its in the open it starts to pick up water and its water content increases until it matches the humidity of the area.

What you don't want is for its water content to reach the level where mold grows.

Keeping it dry by keeping it warm works.

There is no point in laying plastic on the ground to stop water vapor rising, when the humidity/water vapor is in the air and it is pulled into the crawl space/home by the warm air that rises in the home and disappears out the windows and doors.

It is a simple matter to box in the joists and floor to keep the joists and floor warm and to stop the water vapor from getting to the wood. This has a beneficial side effect of helping to keep the home warm/cool and lowering the power bill.
 
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Old 05-18-11, 08:53 AM
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I am the poster formerly known as Crawlspace Tech. Apparently I cannot use my business name as a forum handle, sorry mods!

Chandler:
Advanced Energy are research scientists at N.C. State University, so the money comes from somewhere, but not directly related to any products. The issue with using ventilation in the southeast is this; the vents are to dispel moist crawlspace air to the exterior. You cannot remove air without replacing replacing it. The source it is replaced from is the outside air, which in the southeast typically has 80-90% humidity levels from May-September. Removing moist air only to replace it with more moist air does not change the conditions. With that said, a sealed/encapsulated crawlspace requires dehumidification or conditioned air. You cannot just seal it up and leave it alone. If you do not have either, you can certainly get the conditions you describe. If a crawlspace has ductwork, you will have a negative air pressure in relation to the exterior anyway due to ducts all leak to some degree. When the HVAC is turned on in the house, that negative pressure pulls more moist air into the crawlspace, and the crawlspace air is pulled into the ducts.
There is no set solution for every house though. My advice is only for those experiencing condensation/mold. If your crawlspace is dry, keep doing whatever it is you are doing.
 
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Old 05-21-11, 09:42 AM
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Originally Posted by Perry525 View Post
It is a simple matter to box in the joists and floor to keep the joists and floor warm and to stop the water vapor from getting to the wood. This has a beneficial side effect of helping to keep the home warm/cool and lowering the power bill.
When you write, "to box in", do you mean I should enclose the crawlspace ceiling? Should I use insulation between the joists and then seal this over? What should I use to enclose this? A vapor barrier?
 
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Old 05-21-11, 10:17 AM
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When you write, "to box in", do you mean I should enclose the crawlspace ceiling? Should I use insulation between the joists and then seal this over? What should I use to enclose this? A vapor barrier?
==============================

The best way/cheapest way to do this rather depends on where you live.
In a cold climate, boxing in with polystyrene/Styrofoam or similar product that is at least three inches thick will provide the insulation you need. You are doing two things. Keeping the wood warm and dry and keeping the heat inside your home/floor/joists. To make this work, you need to closely but the sheets of insulation, avoiding any holes that the cold air can enter. Any gaps/cracks/holes are best filled with spray foam.
Obviously filling the spaces between the joists is a good idea, any normal insulation, such as fiberglass will work, as the polystyrene will keep the damp air out of the fiberglass and the fiberglass will stop any convection between the joists.
 
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Old 10-23-11, 12:39 AM
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Originally Posted by Perry525 View Post
When you write, "to box in", do you mean I should enclose the crawlspace ceiling? Should I use insulation between the joists and then seal this over? What should I use to enclose this? A vapor barrier?
==============================

The best way/cheapest way to do this rather depends on where you live.
In a cold climate, boxing in with polystyrene/Styrofoam or similar product that is at least three inches thick will provide the insulation you need. You are doing two things. Keeping the wood warm and dry and keeping the heat inside your home/floor/joists. To make this work, you need to closely but the sheets of insulation, avoiding any holes that the cold air can enter. Any gaps/cracks/holes are best filled with spray foam.
Obviously filling the spaces between the joists is a good idea, any normal insulation, such as fiberglass will work, as the polystyrene will keep the damp air out of the fiberglass and the fiberglass will stop any convection between the joists.
For the end of spring I had a vapor barrier down but the large amount of rain lifted it up and bunched it into my sump pump. Boy was I ticked off at all the wasted effort.
So, I removed that mess and put in a large dehumidifier and closed the vents.
That worked very well.

Well, winter is coming and I was recently informed that the wood floor has some curled edges that may possibly due to cold weather underneath the floors.
I am going to insulate the crawlspace as you have suggested but I am confused about the two paragraphs where you mention the polystyrene.
Did you mean I should use the styrafoam sheets first and then fiberglass underneath the sheets? Or just one or the other?
If I use fiberglass, how do I keep it from sagging and creating a pocket above them?
I ask because I have this situation in the garage right now where the insulation is sagging.

http://foo.gearsector.com/foo/images/insulation.JPG
 
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Old 10-23-11, 09:50 AM
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There are two ways you can deal with this, depending on ceiling height, number of doors, skirting boards, heating, cost, skill, time etc.

One way is to cover the existing floor with at least three inches of polystyrene/blueboard/Styrofoam etc. Having a wood/OSB t&g glued floating floor that has no physical connection with the existing floor.

The other is to encase the whole of the floor fixing polystyrene sheets to the bottom of the joists and filling the spaces with fibreglass, this keeps the heat from the above rooms inside the floor and this means that no condensation will form on the joists.
Condensation only forms on cold surfaces that are below the dew point of the surrounding air.
 
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Old 10-23-11, 07:28 PM
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Thanks for replying. I just wanna make sure I'm getting this right. I definitely don't want trapped moisture.
Here is a pic of what I think you mean. Is this right?

http://foo.gearsector.com/foo/images/styrene.png
 
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Old 10-24-11, 09:31 AM
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Perfect!
May I suggest that you fill the spaces at the ends of the joists with pieces of polystyrene cut to a push tight fit and make this at least three inches thick.
Fibreglass is not very good as an insulator, except when it is in a box or a plastic bag where the air does not blow over it, or the air cannot circulate inside it.
 
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Old 10-24-11, 08:59 PM
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When you say ends; Do you mean at the end lengthwise or width-wise? I am assuming you mean lengthwise ends since it would mean cramming the insulation between joists if I stuck the polystyrene between each joist with 16" of insulation crammed between them(width-wise). From what I've read condensing/compressing insulation reduces it's effectiveness. So I am thinking at the ends of the joists where they will meet the end walls/cross beams. I drew this picture up as well to be sure I'm getting this right. This should be near the cold outside walls is what you mean to insulate against right?

http://foo.gearsector.com/foo/images/styrene_ends.png
 
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Old 10-25-11, 10:55 AM
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Yes, that is correct.
Best of luck!
 
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Old 10-25-11, 11:56 AM
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Thank you for all the help.

I just have one more question concerning the insulation for anyone that will answer:

http://foo.gearsector.com/foo/images/insulation.JPG

This is in the garage.

The facing in this picture is on the bottom and that's where the extra paper is on the sides to staple the fiberglass in place and supposedly hold it up between the joists. Why is the facing on the bottom? Should I flip the insulation so that it's against the ceiling?

I wonder why it doesn't have facing on both sides if one side needs to be used to hold it up...
 
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Old 10-27-11, 07:07 PM
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I'm going to do that again before spring hits. I gotta get that insulation in first before winter.
I did have it in but didn't have it held down very well. Lesson learned...
 
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Old 11-03-11, 07:27 AM
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My heat pump (ducted) is installed in my crawlspace, which might serve some ventilation to my crawlspace (coming from duct leakage). Do you guys suggest installing vapor barrier to my unfinished crawlspace?

Is there any possibilities that the radon gas (if any) will be sucked by my heat pump and blow it inside the house?

Not sure from previous house owner, why they cover the window vent in the crawlspace.

Btw, my house location is at Albany, NY. We got lots if snow.

Thanks in advance.
 
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Old 11-03-11, 01:04 PM
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Yes cover the ground and run up the walls. Make sure it is sealed
 
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