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-   -   vapor barrier question in dirt crawlspace w/ sump (https://www.diychatroom.com/f19/vapor-barrier-question-dirt-crawlspace-w-sump-97485/)

WillK 03-06-2011 09:38 PM

vapor barrier question in dirt crawlspace w/ sump
 
So as I'm adding beams in my crawlspace, my plan is to put down plastic sheet as vapor barrier over my new footings before posts are secured. I also plan to put in a layer of pea gravel at the bottom of the footings, then run a perforated PVC pipe next to the footings that will drain into a sump which I will pump out to a french drain in the yard.

So how do I install the vapor barrier where the sump pit will be? Do I just cut the hole in the plastic and tuck it into the ground where the sump pit is, then just try to seal the openning off with the sump pit lid? Kinda seems like I'm providing a mechanism to remove moisture but at the same time creating a big discontinuity in the vapor barrier...

jklingel 03-06-2011 10:53 PM

I don't believe holes in the vb matter on the ground, as there is no vapor pressure differential to deal with. You'll only be ruining a minor per cent. Verify my memory on buildingscience.com, but I recall one of the guru's there talking about "walk around w/ golf spikes if you want to".

WillK 03-07-2011 10:01 AM

Well when I did a search on buildinscience.com, it came up with this article:

http://www.buildingscience.com/docum...t-construction

In that article, it speaks of vapor barrier placed over your drainage pad under your slab and in that case holes in the poly sheet do not matter because the concrete slab acts as an air barrier, but if you aren't putting in a slab then the holes do matter because the concrete is not a continuous air barrier. And the only mention of the sump is when they talk about a drainage system to remove moisture.

It seems that I should just seal to the sump pit and the biggest thing to do is keep the sump pit working so it can remove moisture from there, whether it's groundwater drainage from under the vapor barrier or any potential plumbing leak coming from above the vapor barrier.

After thinking about it, really I'm trying to figure out how to adress the groundwater as my biggest priority so I can put an end to the foundation settling. That has more to do with the drainage system than the vapor barrier, so it seems I should worry more about the drainage system design than the vapor barrier.

Wildie 03-07-2011 02:54 PM

Quote:

Originally Posted by WillK (Post 604114)
So as I'm adding beams in my crawlspace, my plan is to put down plastic sheet as vapor barrier over my new footings before posts are secured. I also plan to put in a layer of pea gravel at the bottom of the footings, then run a perforated PVC pipe next to the footings that will drain into a sump which I will pump out to a french drain in the yard.

So how do I install the vapor barrier where the sump pit will be? Do I just cut the hole in the plastic and tuck it into the ground where the sump pit is, then just try to seal the openning off with the sump pit lid? Kinda seems like I'm providing a mechanism to remove moisture but at the same time creating a big discontinuity in the vapor barrier...

I'm under the impression that footings are to be placed on undisturbed soil. If you dig below the footing level and place peastone in the excavation, it would be disturbed.
I would think that a peastone collector around the perimeter of the footing would be a better idea. The collector would be drained into the sump.

stadry 03-07-2011 04:10 PM

IF holes in the vapor barrier didn't matter, we could use hardware cloth :yes:

most vapor barriers are the 6mil visqueen ( plastic sheeting ) & should be over 10mm minimum imo,,, supporting bases on compacted soil w/rebar,,, perimeter french drain leading to sump, pump,ck valve, & 1 1/2" discharge line is std op procedure.

WillK 03-07-2011 04:20 PM

I've done a lot of searching, and the tricky thing is that I've seen that but it is always in reference to footings placed below the frost line. I know that I'm getting into a question where I probably need to get an answer from a soil engineer, but I can't find out for certain that this applies to spread footings. My thinking is that I'm pretty sure that I do get water under my footings, and I want to ensure that it can escape.

My immediate problem is that while I'd be on board with putting pea gravel with drain pipe around the perimeter of the footing, I can't get to the exterior side where the water is coming from - not until I do the driveway later this year.

What I've gathered from my searches of geotech discussion boards is that pea gravel has load bearing capacity which may be comparable to undisturbed ground, the probelm with pea gravel is that it is easily disturbed by people walking on it but an evenly distributed load like a concrete pad won't disturb it. I have gathered that it can be used effectively under a slab.

And I asked the AHJ about whether crushed rock for drainage under my spread footings was a good or bad idea and she suggested pea rock would be good.

I'm still open to feedback on this one.

WillK 03-08-2011 06:33 AM

Quote:

Originally Posted by itsreallyconc (Post 604645)
IF holes in the vapor barrier didn't matter, we could use hardware cloth :yes:

most vapor barriers are the 6mil visqueen ( plastic sheeting ) & should be over 10mm minimum imo,,, supporting bases on compacted soil w/rebar,,, perimeter french drain leading to sump, pump,ck valve, & 1 1/2" discharge line is std op procedure.

I missed that since it was posted while I was typing my last message... That's what I needed, but I'm left with the question whether I need to figure out how to drain the complete perimeter or if I'm okay doing the 3 sides I can access from the interior? I supposed I'd have to excavate under the driveway from the hole for the footing before I set my concrete form in place to do this.

Wildie 03-08-2011 08:51 AM

Quote:

Originally Posted by WillK (Post 605050)
I missed that since it was posted while I was typing my last message... That's what I needed, but I'm left with the question whether I need to figure out how to drain the complete perimeter or if I'm okay doing the 3 sides I can access from the interior? I supposed I'd have to excavate under the driveway from the hole for the footing before I set my concrete form in place to do this.

A collector on 3 sides of the perimeter would be adequate. Water will always seek the lowest level anyway.
I doubt that excavating the drive way would improve the situation enough to make it worth while.

Derek1980 03-09-2011 12:57 PM

Willk,

I recommend placing your draintile with sock about 10 inches away from your footings, this will make sure you are not disturbing the foundation of your house. Then use your pea stone to hold it in place. As for your question about the seal of your vapor barrier to your sump basin I recommend that you dig a hole for your sump basin so that the top of your basin will be level with the the dirt in your crawlspace, take the top of your basin off when you cover the basin with the vapor barrier. Once you have the vapor barrier placed over the basin you want to cut the vapor barrier so it over laps the edges of your basin about 2 inches on all sides. ( so if the edges of your basin are 24 inches around you want the hole you cut to be 22 inches around) This will allow you to place the sump pump into your sump basin and attach your drainage pvc to it.(I recommend that you start with a 3 foot section on pvc so it will be easy to work with) Once your pump and starter drainage pipe are placed in there, place the lid of your basin back on and screw it in. This will create a sandwich effect making sure the vapor barrier is tightly secured to your sump basin. For more info on this subject check out . This should answer your questions but feel free to ask more if you have any!

stadry 03-09-2011 02:51 PM

ixnay on the sock ! :no: we'd use 4" ads corrugated slotted pvc pipe w/o the ' sock ' & line the excavation w/soil filter cloth instead to keep sediment further away from the pipe,,, don't forget 4" pvc sewer cleanout(s) so you can flush the system now & then :yes: & forget the apron store's new pipe w/foam peanuts, too !

WillK 03-09-2011 05:21 PM

Could you elaborate on that? I had been planning on using perforated PVC with a filter fabric sock. The way I've read it, the PVC will hold up to snaking where the corrugated pipe will be damaged by snaking, and I already know I have roots growing into the sewer pipes in the same area this will be.

Derek1980 03-11-2011 10:48 AM

willk,

Well whichever drainage pipe that you use i recommend that you use one with a sock, this will help prevent any sludge and mud from getting into it and clogging it! I also recommend covering it with pea stone to!

WillK 03-12-2011 10:58 AM

Thanks, I think that's going to be about what I'll do. The end of the branches forming the 3 sided perimeter, I think I'll go up and leave them sticking up so they can be used as cleanouts.

stadry 03-13-2011 07:05 AM

don't know any pro using sock's pipe as it allows sediment to come too close to the pipe perforations & clog the system thereby limiting access to the water you're trying to manage,,, typically we open a trench 12" wide x 12" deep then line it w/soil filter cloth,,, 2" of bedding stone - pipe - # 57 stone cover - wrap the filter cloth above the stone,,, cleanouts are too cheap to leave them out :yes: yes, a little more labor but less maintenance & failure

WillK 03-14-2011 08:10 AM

I had actually been planning to line the trench with landscape fabric regardless, whether I used a sock or not. With pea stone I'd expect the sock to be necessary to keep the stone from washing through, but I can see where that wouldn't be an issue with the larger stone and the sock would have the potential for blockage.

I'm no soil engineer, but my understanding is that by ground is fine sand. I'm seeing water pooling around my plumbing stack when it gets warm and snow is melting off, once it gets back below freezing the water goes back down within less than half a day (haven't watched any closer than that.) The sand provides good natural drainage, but I'm after 1) making sure that it's not providing drainage through the area that's supporting the house and 2) keeping moisture out of the crawlspace.

This might file under fun things to do if I had any time, but it would be interesting to build a plexiglass box with a small section of each option and pour water into it to see if either system allowed the sand to get through and clog or not.


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