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Old 08-19-2008, 06:01 PM   #1
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i am starting to finish my basement. Basement has exterior concrete walls, finished w/ insulation bolted to concrete. should i take it out or install new wall in front. if yes how much clearance should i leave between face of insulation and back of studs? (by doing which i will loose some area of the basement). To start laying out walls should i install bottom plate first considering all the plumbing on outside walls and then build wall on floor and set it up between floor plate and bottom of floor joist? Is it the right way. Please advise.

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Old 08-19-2008, 10:45 PM   #2
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It is typical to lay out walls with a chalkline on the floor. Build the wall laying down on the floor, then tilt it up and fasten in place. I wouldn't recommend fastening the sole plate down before the wall is completely framed and plumb.

I'm assuming you have foamboard insulation on the exterior basement walls. You should leave a little wiggle room between the framed walls and the insulation. That way, subtle foundation movements over time don't telegraph through the finished walls. A small gap will also allow you to use a level to plumb the wall perfectly.

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Old 08-20-2008, 02:17 AM   #3
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You should be aware that your basement floor is a "floating slab". That is, the basement walls and the footings under the jack posts holding up any beams will be reinforced with rebar and will extend deeper than the slab. The foundation and footings will have been poured first, and then the floor slab separately so that it can move up and down with changes in the clay moisture content under your basement floor.

So, I agree with the previous advice to build your wall on the floor and then stand it into position, but I would build it a full 1/2 inch shorter than the floor to joistbottom height, and I'd build it with a pressure treated bottom plate. When you stand the wall into position, drill a 5/16 inch hole through the top plate of the wall into the joist bottoms above the wall. Then drive a 5/16 inch lag screw through the top plate into the joist bottoms so the head of the lag screw is a full 1/2 inch below the bottom of the top plate. NOW plumb your wall and ramset the PRESSURE TREATED bottom plate to the concrete floor.

Doing it this way, your floating slab concrete floor can rise and fall up to a full 1/2 inch before there's any tension or compression on that basement wall. Where I live, that type of construction is called a "floating wall". And, people tell me it's not necessary to build that way because there will never be any movement in the concrete slab floor. My feeling is that I hope there isn't. But if there is, and you haven't allowed for any movement in that floor, then you're gonna have a major problem with your walls crushing.

And, call me a neurotic, a perfectionist or just plain insane, but I would install a ceiling fan in the wall you build in front of your insulated basement walls to suck air out of the air space between your walls and your insulated concrete foundation walls and blow that air outdoors. Maybe allow a basement window to feed cool dry air into that space from the other side of the basement. That way, you can ventilate that space between the insulated foundation walls and the wood walls by turning on that ceiling fan mounted in the wall. Dry wood and dry concrete are happy wood and happy concrete. Please, do it for the wood and concrete.

Undoubtedly, someone's going to bark at me for the above post. They're going to say that the concrete basement floor isn't gonna move a hair, and in most cases they'd be right. But, if you have a lot of clay in your soil, and you have exceptionally dry conditions so the clay dries out and shrinks and lowers the slab, or exceptionally wet conditions so that the clay swells and lifts the slab, then you're dealing with one of he cases that isn't part of the "most" cases.

Maybe check with your local building contractors to see if floating basement walls are needed in your area.
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Old 08-20-2008, 08:45 AM   #4
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Good advice about the movement of the basement slab. They do move, and leaving a small gap is good practice.
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Old 08-20-2008, 09:06 AM   #5
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Quote:
Originally Posted by Nestor_Kelebay View Post
your floating slab concrete floor can rise and fall up to a full 1/2 inch before there's any tension or compression on that basement wall.
While I understand the concept of the floating wall...wouldn't it be fruitless once the wall was rocked? I wonder what the drywall would look like after moving up to an 1" (1/2" up and 1/2" down) Call me crazy, but if the house or basement floor has that much movement, I'm guessing there's other structural deficiencies.

OP: What type and how thick of insulation is fastened to the concrete? As others have said, PT bottom plate, plumb and fasten (I'm a huge fan of the ramset or Remington fasteners) to the floor. Before you get too excited about chalking the wall layout...you'd be better off spending a few hours with a paper an pencil with a scale drawing....it'll save lots of head scratching down the road. Look at the cieling as well, are there limitations i.e. pipes, wires, HVAC stuff, that if you were to re-configure the layout it might be easier to work with. It's a big project...but certianly worth the rewards when it's done.
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Old 08-20-2008, 09:36 AM   #6
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Quote:
Originally Posted by Nestor_Kelebay View Post
You should be aware that your basement floor is a "floating slab"....
This is an incorrect statement. Not all basements contain floating slab floors. This is relative to region, and regional construction requirements. Warmer vs. colder freezing climates. Example: You will not find floating slab floors in colder regions with freezing temperatures.

If one has a floating slab floor, there is a different process to the framing of the walls, in order to allow the wall to move vertically with the floating floor.

Link on framing information: http://www.betterbasementideas.com/i..._in_basements/
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Last edited by AtlanticWBConst.; 08-20-2008 at 10:05 AM.
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Old 08-20-2008, 11:15 AM   #7
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thank you very much guys for your advise. Insulation on foundation wall is not rigid insulation. It is regular 4" (i think) insulation with alum. foil on inside face attachched to wall with fastners at every 3-4' at top, bottom and middle. House is in new jersey and is 18 months old.
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Old 08-20-2008, 11:35 AM   #8
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Before this gets too convoluted, perhaps we should define "floating" floor slabs for the OP. We might be crossing our terminology

Correct me if I'm wrong, but my pea brain would consider most basement slabs as floating, or potentially floating. That's not to be confused with suspended structural slabs, which free-float without subgrade underneath them. Basement slabs are normally poured after the footing and foundation walls are placed, and are in no way tied or integrated with the footing or foundation. Although not particularly intended to move or self-support, upward heaving or expansion of the subgrade can cause the slab to shift upward. For this reason, structural columns are often isolated from the slab, so the structure doesn't shift with the slab.

The reason for the gap at the walls is to prevent a heaving slab from negatively affecting the structure of the home. There are ways of doing it that allow the base to slip up and down without boogering the sheetrock, but in my opinion the sheetrock is somewhat sacrificial. As rippy states, it will crack if and when the floor heaves. This type of basement wall construction is required in several cities in this area, and could potentially be of benefit in any region (regardless of climate) that has expansive soils or sites with any hydraulic pressure issues.
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Old 08-20-2008, 12:26 PM   #9
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Quote:
Originally Posted by thekctermite View Post
... This type of basement wall construction is required in several cities in this area, and could potentially be of benefit in any region (regardless of climate) that has expansive soils or sites with any hydraulic pressure issues.
I would guess your right...about the moving or potential of a moving slab for the OP. I'd also guess that if the floor is moving significantly it would have already been noted.

I'd like to dig in a bit more on the 2 scenarios you provided, the expansive soils I can understand, and assume were talking about clay type soils and their shrink/swell capacity. IF there's hydraulic pressure issues, I automatically assume that would mean water, no? If water issues are enough to drive the floor up, then one should maybe reconsider their options of creating an additional living space. Clearly if a floor moves that much, yes the drywall is sacrificial, but I'd think there would be many other structural issues going on and wouldn't even consider moving the project forward.

For the OP, since you have fiberglass I think it's fair to assume the foil will serve as a vapor barrier, if you can find a brand, do some research for your self to verify. I'd keep the walls an inch or less away from existing insulation.
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Old 08-20-2008, 05:51 PM   #10
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Quote:
Originally Posted by RippySkippy View Post
IF there's hydraulic pressure issues, I automatically assume that would mean water, no? If water issues are enough to drive the floor up, then one should maybe reconsider their options of creating an additional living space. Clearly if a floor moves that much, yes the drywall is sacrificial, but I'd think there would be many other structural issues going on and wouldn't even consider moving the project forward.
Very true. You might be shocked, however, at the amount of basement finish projects I see in basements that have no business being finished. People assume that sheetrock and carpet will miraculously transform their wet swampy humid basement into nice fresh living space. Not so.

Simply installing a sump pit and pump will usually rememdy water issues under the slab, but won't always keep expansive clays from swelling (and subsequent slab/wall movement).
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Old 08-21-2008, 08:46 AM   #11
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I'd believe it. Sometimes "wants" over power rational thinking. A little drywall, paint and carpet is amazing...but only in the RIGHT environments.

Here in central Iowa, my neighbor said he looked at a house when they were look to buy or build, and the basement floor was peaked in the center of the house...to the tune of 8+ inches. Apparently caused by hydraulic pressures, he walked. So what you're saying is real, and dependent on the local environment

For those that may be interested...there's a decent tool on the web (best viewed in IE) and a boat load of information there about soil. Another site isn't as fancy...but it might be a bit easier to to navigate.
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Old 08-21-2008, 10:13 AM   #12
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Good advice on wall placement - I would also suggest you give a listen to The Handyguys Podcast series on basement finishing.
It was reviewed here http://www.buildingonline.com/news/v...ubcategory=346 and the series can be found here
http://www.handyguyspodcast.com/tag/...nishing-series

Good luck on your project

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