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Crawl Space Encapsulation- Can you use the existing vapor barrier?

Crawl Space Encapsulation- Can you use the existing vapor barrier?


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Old 12-13-16, 02:19 AM
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Crawl Space Encapsulation- Can you use the existing vapor barrier?

Having very high humidity readings in my crawl space I decided to start down the path of encapsulation. I got a pretty high bid having the job done, so I decided to attempt this myself. I sealed the crawl space vents and out of curiosity I began taking temperature and humidity readings. Upon the sealing of the vents the readings became very stable. Very encouraging considering in the summer I got a reading of 91% RH.

Now to my question- I already have a 6 mil vapor barrier that was installed when the home was built. Can I simply tape all the seams of the existing vapor barrier instead of replacing it with something else?

I realize I need to seal the piers and insulate the wall, but if I can use the existing vapor barrier it will knock out a chunk of work.
 
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Old 12-13-16, 07:47 AM
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If the current vapor barrier is in good shape, I would reuse it. If it was pockmarked with holes and rips, I would replace it.
 
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Old 12-16-16, 08:20 PM
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Yep, reuse it only if it's in good shape. I did self encapsulation and was in the same boat - guage showed 99% at times in the summer. A single $200 humidifier is keeping it under 50 easily now. I used Viper vapor tape initially for all of the seems (plastic on plastic). I was down there last night to do a better sealing of my sump pumps (2) lids and used 3m all weather flashing tape since I ran out of the viper. Both tapes work great but the 3m is some crazy good tape to have around. For the walls and pillars (plastic on concrete) I never found a tape that would stick to my liking so I used a few tubs of Duct Mastic applied liberally. Worked great although you have to make sure you leave some slack at the walls so stepping there doesn't pull it away.
 
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Old 12-16-16, 08:23 PM
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I'll be glad to detail my whole setup if you are still planning yours out.
 
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Old 12-17-16, 02:11 AM
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This really sounds encouraging. Thank you for your responses.

I would be interested in hearing/seeing more details of how you encapsulated your crawlspace. Did you take temperature and humidity readings throughout the encapsulation process?

I have been astonished at the effects of just sealing off the foundation vents.
 
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Old 12-17-16, 04:58 AM
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In the South, sealing off the vents can cause problems IF you don't in, some manner, "condition" the air in the crawl. You have to make it a part of the whole house air conditioning/heating system so it will equalize and have a place for the humid air (which you now have enclosed in the crawl) to go. Dehumidifying may work, but I have been under a couple of encapsulated crawls where the demumidifiers couldn't keep up and they had literal rivers running under the house.
 
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Old 12-17-16, 05:50 AM
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Hi Chandler,

That is a great point. So far all my measurements have been during the Fall/Winter. I was wondering what the humidity would be in the Summer if I did nothing else other than sealing the foundation vents. Sounds like even if sealed off from the humid outside air the humidity would increase to mold producing levels. I am going to start in the early spring taping off the vapor barrier joints and work on the piers and exterior walls.

I have seen two ways to address the walls.

1- run the plastic up the walls terminating under a 2x4. Another where the plastic was terminated under foam board attached to the wall. Can someone tell me the pro's and cons of both?
 
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Old 12-17-16, 06:45 AM
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Everyone has the mistaken impression that the air from the outside is laden with humidity. Nothing is further from the truth. The air in the crawl is humid and the cross breezes afforded by the vents allows fresh air to enter and move the humid air out. I have set myself up for repudiation by others on the forum, so bear with it for a while, please. You will gain more information as we go along.

So, if you close the vents, you are committed to a total encapsulation. I would run the plastic up to the sill plate, using the mastic to seal it against the wall or foam.
 
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Old 12-17-16, 10:23 AM
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Up here, we tend to encapsulate and open the space to the house air. In the south, it's much more common to seal the space from the house air and open the vents instead, as Chandler discussed.
 
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Old 12-17-16, 07:45 PM
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As has been mentioned, there's a combination of things that can be done and not all of them may work. Fortunately closing the vents isn't a permanent thing (don't EVER get rid of the vents permanently ) so they can always be opened if you can't get the humidity down. And regardless of the vents, a good job on the vapor barrier is necessary. So you're not committed to anything. You will definitely want a remote sensor to keep a close eye on it. I've got a remote sensor, a regular mechanical sensor, and my dehumidifier sensor down there. I am pretty anal about checking it and you will want to be as well. I've got a camera down there so I can read the dehumidifier led as well as to see if it's running (a receipt taped over the exhaust so I can tell if it's flapping).

In my case the vapor barrier got it down from the 90s to the high 60s/low 70s. So definitely helped but still in the mold range. That's when I sealed the vents and put the 70pint down there. Your vapor barrier may be enough without encapsulating.

What you have down there and your floor material also comes into play. Anything but carpet and encapsulating it will help because it will keep it somewhat warmer in the winter. Mine is at 63 and it's 9 outside right now. Ductwork and pipes will also benefit from this.

For my encapsulation I used a lot of the 1 inch blue Dow foam board. It was time consuming but wasn't really all that expensive. I measured each floor joist cavity and cut 2 squares for each with one having slightly beveled edges. Table saw worked great for this. Place both squares into the cavity - the tighter the better. I then used spray foam to seal them all with the beveled edge helping to give the spray foam some room. I now have a foam gun and would recommend purchasing one vs the disposable can but either works.

You'll also want to use spray foam between the sill plate and foundation. A lot of air gaps here.

I ran my vapor barrier about halfway up the wall and attached with mastic. You won't want to run it all the way up or you run the risk of trapping moisture against the sill plate.

I ripped more of the foam board and placed two layers (staggered if possible) longways along the foundation wall. This will help hold the plastic in place while the mastic dries also. I didn't want to 'permanently' fix mine in place so I could do inspections easily so on the walls perpendicular to the joicts I cut little blocks of foam and wedged them between the foam board and the floor joist to keep it all in place. On the other two walls I cut longer pieces and wedged them between the closest joist and the foam. It's all held everything together well. I also had some of those long-ish scalloped landscaping pieces that I propped up around the wall to keep pressure on the foam. Scattered bricks around the floor as well.

So this may all help or none may help in your situation but hopefully gives you some ideas.

Also, before you encapsulate, I strongly recommend a radon test on both the floor level and in the crawlspace for reference points. You'll want to plan for radon mitigation during the process if your tests call for it. That's what I am battling now.

If you go the dehumidifier route you'll also want to think about a continuous drain plan. Don't even consider dumping it by hand periodically I have mine draining into one of my sumps.

You should also check with your electric company to see if they will reimburse you for some of the cost.
 
 

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