Converting a vented crawlspace to non-vented

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  #1  
Old 03-06-06, 09:43 AM
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Converting a vented crawlspace to non-vented

I have a vented crawlspace that is currently insulated (a lot needs to be replaced) between the floor joists and no vapor barrier on the ground. Located in north central GA (suburbs of Atlanta). There are no drainage or water issues in the crawlspace; it's very dry.

So I have been thinking of sealing up the vents, removing the old floor insulation, putting down a vapor barrier on the ground and insulating the crawlspace walls (haven't decided between rigid foam board or batt yet).

Any ideas or thoughts?

BTW: How do you attached the vapor barrier poly to the block foundation walls?
 
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  #2  
Old 03-08-06, 06:07 AM
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Won't Work

Your idea won't work and in fact would be a code violation unless you also condition the air in the crawlspace, mechanically ventilate it, or ventilate it to the interior of the structure.

http://www.buildingscience.com/resources/foundations/conditioned_crawl.pdf
 

Last edited by manhattan42; 03-08-06 at 06:20 AM.
  #3  
Old 03-08-06, 10:08 AM
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That's precisely what I was suggesting Manhattan... I would be removing the insulation between the floor joists and relying on the conditioned air inside the house to condition the crawlspace as well. This would be done through the existing floor penetrations for plumbing and electrical (all air ducts are in the attic).
 
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Old 03-10-06, 03:09 PM
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But you didn't say you would condition the floor space......(shrugs)
 
  #5  
Old 03-13-06, 10:55 AM
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Sorry Manhattan, I wasn't clear in my first post. I should have said that when I remove the insulation from the floor joists that I do not intend to do any air sealing between the crawlspace and conditioned area above. Hopefully this will be enough to condition the air space as I have noticed that even with insulation between the joists now that the vented crawlspace feels cool in the summer and warm in the winter. This is with no duct work in the crawlspace as well so I'm assuming that there are quite a few penetrations allowing drafts from the conditioned area above.

What do you think about that? Of course I'm not looking at a permit since I'm a homeowner and doing DIY and not changing any structural, electrical and plumbing, so strict code compliance isn't exactly my biggest concern if it requires mechanical ventilation or ducting. ;-)
 
  #6  
Old 03-15-06, 04:37 PM
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You will need a dehumidifier and you will need to insulate the floor!
 
  #7  
Old 03-16-06, 02:34 PM
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I plan on insulating the sidewalls of the crawlspace to include 18inches of the floor if I use batts (haven't decide on rigid or batt yet). This is pretty much the correct method from everything I've read.

Why would I need a dehumidifier? I'm counting on it being conditioned space once I remove the insulation between the joists (as mentioned via the existing floor penetrations). I don't have a moisture problem now while it's vented, why would I then? Remember, I'm adding a vapor barrier that currently doesn't exist on the floor.

Am I missing something glaringly obvious?
 
  #8  
Old 03-16-06, 04:42 PM
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I have never seen a crawl space that does not have a moisture problem! Georgia is a green grass state where the RH gets very high! Your crawl might be dry! Try this to find out! The vapor barrier is the most important part of a crawl it helps SLOW the migration of vapor! Take a 5 * 5 piece of your poly and lay it on the ground in the crawl come back in 24 hr and see if it has moisture on the bottom of it! if it doesn't your plain might work!
 
  #9  
Old 03-17-06, 08:52 AM
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You've got that right Airman.. we sure do have a green grass state :-) with high humidity which is one thing that surprised me about how dry the crawlspace is. About the only grass that doesn't stay green during the winter is the hot-season grasses like bermuda that are becoming really popular with the new home builders. I definately prefer the fescue, but have to admit that well kept bermuda during the summer time is like a beautiful green soft carpet.

Great idea on the poly! I actually have a much larger piece that is down there already from the old vapor barrier (only about 1/4 of the original vapor barrier is still there, which is why I got started thinking about this project). I'll check it when I go over there tomorrow.
 
  #10  
Old 03-18-06, 07:36 AM
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Not a job for the DIYer

Pressure induced moisture flow is the logic behind sealing crawl spaces. Basically if you blow air into a confined crawl space an area of high pressure is created that forces the moisture away from the crawl space. This is the exception of the rule High to Low. Where normally an object of higher humidity will give humidity to an object of lower humidity. With pressure induced moisture flow an object of lower humidity can and in most cases, like your area, will have an object of lower humidity give humidity to an object of higher humidity.

First and foremost we have to understand that the volume of air within a confined space, including the house itself, remains constant. For example, if you take air from inside the home to pressurize the crawl space, then an area of depressurization occurs inside the house equal to the amount of pressurization in the crawl space. So as you move moisture away from the crawl space by pressurizing the area, you draw moisture into the house from depressurizing it.

This application comes from the air sealing industry. However, they would not air seal a crawl space without air sealing the house first. This is far more complicated than what I am explaining. In my opinion, your intended actions will probably cause a moisture problem than prevent one.
 
  #11  
Old 03-21-06, 09:11 AM
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I took airman's advice and checked under the portion of old vapor barrier that was still on the dirt floor. Sure enough, there was condensation on the underside.

I guess this means that if I decide to convert to the nonvented/conditioned crawlspace I will need to keep an eye on the humidity levels (no problem there, have a few hygrometers that keep highs and lows) and install a dehumidfier if the levels are too high.

What is the ideal level of humidity for a sealed crawlspace?

Resercon- I appreciate your post. One of my future projects is to house wrap the exterior (3 sides) when we replace the old siding. The front is brick, that will be staying.

Right now I am trying to decide what to do with the crawlspace as it already needs a new vapor barrier and new insulation (the old stuff is R-7 2 1/4in!!! the joists are 2x6).
 
  #12  
Old 03-21-06, 09:48 AM
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jganyard Id replace vapor barrier and insulation close vents and install a GOOD dehumidifier to keep RH under 50%
 
  #13  
Old 03-21-06, 01:50 PM
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Originally Posted by airman.1994
jganyard Id replace vapor barrier and insulation close vents and install a GOOD dehumidifier to keep RH under 50%
Are you suggesting that I insulate the joists or the crawlspace walls?

If you're suggesting I should insulate the floor, I would use unfaced batts right?
 
  #14  
Old 03-21-06, 06:44 PM
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Floor joist with kraft paper! Wall with 2in Polly styrene (spelling)! Walls? in GA you could skip the walls! 6mil Polly on ground overlapped by 2 feet glued at seems (spray glue) Polly up the wall and stop 2in under top plate! Use fearing strips and maze nails to pinch Polly to walls! Close vents at least 90 pint a day dehumidifier!
 
  #15  
Old 03-22-06, 09:06 AM
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Okay, I get you Airman... insulate the floor joists with faced batts (kraft paper down right? it would make it a heck of a lot easier to staple than use those insulation hangers).

Your saying using the rigid insulation on the walls is optional, I guess so since the floor would be insulated. Could I expect to gain anything by insulating the walls?

I get you on overlapping and sealing with spray adhesive. As far as attached to the cinder block walls, furring strips 1x1? What are "maze" nails?

90pint a day dehumidifier? That's a big one... I think I'll keep tabs on the RH after I seal it up to see how large of one I need. BTW: We're looking at about 1200sqft of crawlspace here, wouldn't 90pint be overkill?
 
  #16  
Old 03-22-06, 02:37 PM
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As far as insulating the walls Id think you would not gain a lot! Maze nails are a concrete nail. They hold better than a cut nail. 90 pint would be the minimum id put in! after a day or two of wet weather your 50 pint WILL NEVER KEEP UP! Plus a quality dehumidifier like thermo-star will use about have the electricity that the cheap $200.00 dollar ones use. They also are designed for low temp applications like a crawl.
 
  #17  
Old 03-23-06, 08:50 AM
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Originally Posted by airman.1994
As far as insulating the walls Id think you would not gain a lot! Maze nails are a concrete nail. They hold better than a cut nail. 90 pint would be the minimum id put in! after a day or two of wet weather your 50 pint WILL NEVER KEEP UP! Plus a quality dehumidifier like thermo-star will use about have the electricity that the cheap $200.00 dollar ones use. They also are designed for low temp applications like a crawl.
Where can I find a thermo-star dehumidifier? I can't find the brand using a Google search.
 
  #18  
Old 03-23-06, 10:00 AM
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I'm sorry it's Therma-Stor www.thermastor.com
 
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