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Discussion Starter #1
My front and left side walls are exterior cement.

I originally planned to put 2x4 framing directly against the cement walls, but I noticed that a lot of people leave a 1" gap between the cement and the framing. What is the purpose of this?


My room width is already restrictive so every inch counts. If I left a 1 inch gap could I frame that wall with 2x2s to make up for the lost width?
 

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Mine are spaced from the foundation so that I could decouple the walls from the rest of the structure using the RSIC DC-04 clips. Also, if you don't leave a space, you'd either have to use treated lumber or have a barrier of some sort between the concrete and the framing.


No, I wouldn't frame a "free standing" wall with 2x2's. It would have too much flex.


Dan
 

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I did it since the building inspector recomended it. Would have had to used pressure treated wood if not. It was something about mold prevention or something.
 

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I've read about some others doing this and plan on doing it myself.... although I'm still a bit unsure. The nice thing is that I will run all of my wiring on the back of the studs (thanks to Larry Fine for clearing this up for me). It will save a lot of drilling. A couple questions I have though....


1. Do I have to fill that "gap" with insulation or anything else?

2. Should the concrete or new wall be covered with plastic or something like that to eliminate the chance of moisture?

3. Is there a potential issue with fire code? I've heard of issues with a space between 2 walls, but can't remember the specifics.


Jerrod
 

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Generally, the framing is not in contact with the concrete foundation; however, do check your local codes. Also, understand that this 1" gap is a firechase and, at the top of the wall, the gap must be sealed to prevent flame from travelling up the wall into other parts of the structure.
 

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From a thermal perspective, many basement people recommend at least a 1" gap. My basement is like yours, 2 exterior walls so I read around a bit & based on what I found, I went with a gap for a couple of reasons.


Insulating a basement HT from the exterior wall is a very good thing from a thermal perspective. However if you insulate an exterior wall (whether for thermal purposes or not), you need to be careful not to create a bigger problem by doing so. Assuming that you use 2x4 studs and you use batted fiberglass insulation, the fiberglass should not touch the exterior wall – there should be a gap between the wall and the insulation. If the exterior wall touches the fiberglass, this can lead to condensation and the fiberglass remaining wet & potentially moldy. Most basement people recommend an inch between exterior walls and the fiberglass. This space provides some degree of air flow – that’s a good thing.


R-13 fiberglass insulation is 3 ½†thick, meaning if the studs are tight against the wall, the insulation will just touch. This means you either 1) use a lower (thinner) value or 2) you stud the walls out an inch or more from the wall & use R-13 or greater. This is what I did – while losing another inch was painful, it was worth it (or I hope so – if I ever get done!).


Another issue is a vapor barrier for the wall. Most code requires some sort of vapor barrier between the insulation and the exterior wall in a basement. You can use unfaced fiberglass insulation and use a poly sheet to provide a vapor barrier or you can get Kraft-faced insulation – this provides a vapor barrier. I find the Kraft faced easier to use & install, but YMMV. The vapor barrier is not to keep moisture from coming into the room – it is to prevent warmer (& more humid) air from the HT from reaching the cold exterior wall & then condensing. Condensing water on the wall is a problem, you don’t want to tear out walls in the future to address a mold problem.


For the sole plate (bottom, on the concrete floor), you should use pressure treated lumber to reduce the risk of rot.


Bob
 

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As an aside ... it is far, far better to install thermal insulation on the outside of a concrete foundation wall; however, since almost no builder does this, you're stuck with your thermal mass outside the envelop of the home. Drats.
 

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In my case, the foundation walls were not plumb or straight, so moving the framing out allows me to ensure that the interior walls are as perfectly positioned. I have gaps that range from 1" to 3" or more in some spots.
 

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Discussion Starter #9
Thanks for all the great information!

So it sounds like I could gain that space back if I really wanted to, by using a combination of pressure treated lumber, thinner insulation, and a vapor barrier although it is probably better to leave a 1" gap there.


What about metal studs? I haven't heard of anyone using these in their construction. Any reason for that?
 

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I have heard a variety of reasons, including:


1. Wood is less prone to rattles

2. Wood can't rust if exposed to moisture

3. Can't hang anything on metal studs - or you have to plan ahead of time where you will hang things and back it with wood

4. Wood is warmer

5. The materials available at home reno centers is generally too light a gauge


Of course, metal stud fans will say:


1. Its faster (once you get the hang of it)

2. Its always straight (no argument there)

3. Its faster to run wiring (no drilling)

4. Its cleaner - no sawdust


That said, what it boiled down to for me was familiarity - I'm just more comfortable working with wood. There may also be acoustical differences, but I'll leave that one to the experts.
 

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Dennis:
Quote:
As an aside ... it is far, far better to install thermal insulation on the outside of a concrete foundation wall; however, since almost no builder does this, you're stuck with your thermal mass outside the envelop of the home. Drats.
From the perspective of thermally and vapor barrior/mold, yep I agree.

However the problem, at least locally, with having insulation oustide is that there are a bunch of bugs that like to stay hidden. So we have a foot or so of exposed concrete higher than the soil to keep them out of the house. Yes spiders and fly's will walk up that no problem, but for some reason the termites and others do it much less. If we ran insulation on the outside to cover that, then the bugs would come inside a lot more than they do today, possibly nibbling on the wooden structure.

I don't know if this is building code, or an urban legend.
 

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strange_brew:

Here's a couple more.


Steel studs conduct heat/cold, leaving cold spots on your wall, and promoting condensation.


Wood studs also absorb and release humidity slowly, acting as a kind of buffer. There are days when it rains for example that the humidity goes way up, with steel studs there's less place for that humidity to go so it condenses, whereas with wood studs it gets absorbed into the wood for a day or two, until it dries up outside.


You can buy engineered/recycled studs that are straight. They're just as strong, and only a little more expensive (although Jonnio found them cheaper). There's a thread about it here .
 

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I am finishing my entire basement. About 1500 ft2. I am using 3/4" extruded polystyrene against the concrete foundation walls. Then the studs are snug up against that. Then I will add unfaced insulation in the cavity mostly for sound attentuation as all but one side of my basement is below grade.


The foam acts like a beer "cozy" to prevent condensation. Also, by using unfaced fiberglass, air can move between the wall cavities and the finished space and there will be less chance of mold growing since it will "breathe" better.


Now, up at the rim joist area, I cut and fit polyisosianurate (1") to the rim joists between the floor joists all the way around the perimeter. I live in a very humid climate (outside DC in VA). Also used unfaced FG on top of that foam (r-19). The point here is to keep out the moisture that will easily migrate through the wood rim joists. But allow all air to move between finished space and cavities so there is less chance for mold, since certainly some moist air will get through.


Note all studs are non-PT since they do not contact the concrete walls.


The HT room has concrete walls on two sides. It is framed as a room wihtin a room. I have 2X6 staggered stud construction on one wall, and a double stud wall on the fourth side. The two concrete sides are single 2x4. The ceiling is new 2X6 ceiling joists between the existing floor joists hung about an inch or 2 below the bottom of the ceiling joists (the bottoms of the new joists are 2 inches below the bottoms of the existing joists). This way, the HT room ceiling is decoupled from the floor above.


Probably more than you wanted to know.
 

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Sleepy hollow:
Quote:
The foam {3/4" extruded polystyrene} acts like a beer "cozy" to prevent condensation. Also, by using unfaced fiberglass, air can move between the wall cavities and the finished space and there will be less chance of mold growing since it will "breathe" better.
In Ontario the building code says that anything that resembles a vapor barrior has to be on the warm side of the insulation, just behind the drywall.


I'm not a builder, I'm an internet reader.


Two vapor barriors are always an incredibly bad thing.

As you move through the insulation, the temperature drops. At the top of the concrete, where it's exposed to air outside, and even several feet down from there once the insulation is up and the house furnace is no longer keeping it warm, that part of the concrete can be cold enough to freeze the water out of air on its surface.

If there's insulation between the foam and the drywall, then the temperature between those two points is linearly different.

If the condensation point ends up being somewhere between the two (fluffy fiberglass pink and the rigid foam), and there's no vapor barrior on the drywall side, well that would be bad too, because water will drip down the drywall side of your polystyrene.

Concrete lets moisture through it, both ways.

It's good to have water tight stuff on the outside of the concrete where it's underground, but above ground I think you want it air permiable in the winter to dry out (dramatically lower the humidity) the space where the insulation is.


Can you explain how your's works in winter and summer? Where does the humidity go? Where does the humidity not go?


(Also, the polystyrine won't help sound TL from either an air-spring point of view, nor a decoupling point of view if they're against the studs.)
 

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Bob, thats an interesting comment. The BSC recommends foam on the foundation wall, then stud, then fiberglass batt and no vapor barrier.


This is an american corporation but I have heard that this method is gaining popularity in Ontario. However, it seems to violate your 'vapor barrier to the inside' code requirement.


To my way of thinking the foam (which is actually a vapor controller and not a barrier, perm rating 10X that of a 6mil vapor barrier) allows the wall to dry to the inside at a controlled rate. I firmly believe this is the correct way to go for below-grade.


Andy K.
 

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kromkamp and Sleepy hollow:
Quote:
Ontario Building Code 1997:

9.25.4.3 Installation of Vapour Barriers

Vapour barriers must be installed on the warm side of insulation in thermally insulated walls, ceilings and floors.
In the 'famous mold PDF' they've got two types of foam. One is semi-permiable and I think the other is not. Also if they are not taped and sealed it might be ok too. I don't know.

I think that building code requires vapor barrior on the inside, which means there has to be some way for the moisture to get outside. As long as both are true, then that's as good as it gets.


I can't find the old "famous mold PDF" as it seems to be gone from the building sciences website.

This one says
Quote:
Sheet polyethylene is an almost “perfect†vapor barrier because it does not allow

any moisture in the vapor form to pass through it.
and then a little bit later on pg 25 has a diagram with polystyrene that seems to let vapor through it, and polystyrene is what Sleepy hollow is using. I had thought from his post that Sleepy hollow thought it was a vapor barrier, whereas now I'm wondering if Sleepy hollow has any vapor barrier.
 

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Well, it comes down to the same debate we've seen before - below grade, moisture simply will not dry to the exterior. Which means if you have a vapor barrier on the interior, moisture from the outside gets trapped in your walls.


From what I can tell XPS and EPS (and urethane spray foam) have a perm rating around 1 at 1 inch (ie. .5 at 2 inches, etc) and are classified as vapor retarders . A true vapor barrier has a perm rating of around 0.1. Icynene (open cell spray foam) has a perm rating around 14 and its debatable if its even a vapor retarder.


Andy K.
 

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Quote:
Originally posted by BasementBob
...In the 'famous mold PDF' they've got two types of foam. One is semi-permiable and I think the other is not....

... I had thought from his post that Sleepy hollow thought it was a vapor barrier, whereas now I'm wondering if Sleepy hollow has any vapor barrier...
Bob: I was a little confused by your earlier post when you mentioned two vapor barriers... because, I don't see one listed either.


XPS (eXtruded PolyStyrene) is the pink or blue stuff, which is more of a vapor retarder than EPS (Expanded PolyStyrene). But the building science website cautioned against using too thick of a layer (of either) or it becomes a vapor barrier. I don't remember the cutoff (and I'm too lazy too look it up) but I'm pretty sure it was more than the 3/4" of XPS mentioned.


My oversimplified interpretation of what the building sciences website says is that vapor barriers are bad in basements.


Like Andy K. said. :)
 

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mbishop
Quote:
My oversimplified interpretation of what the building sciences website says is that vapor barriers are bad in basements.
My read is that it depends how far north you are. For example, if you're in Fargo North Dakota, vapor barriors on the warm side of insulation are a good thing. If you're in the middle of the USA then the rules change.


Right now, here in Oakville Ontario Canada (south of Fargo BTW), it's below freezing outside. This afternoon, May 2nd 2005, it was snowing, fortunately also outside. Admitidly it melted pretty quick.


Although
Quote:
Basements should be designed to dry to the interior (see Figure 20). These principles are often in conflict with some common misapplied energy conservation and moisture control practices – for example the use of sheet polyethylene as an interior vapor barrier.Sheet polyethylene (or vapor barriers) should never be installed on the interior of interior basement insulation assemblies or on the interior of interior insulation in below grade wall assemblies
seems to agree with you. (from pg 24 of the above PDF link )


The latest episode of "Holmes on Holmes", shot in Burlington Ontario Canada, they took apart a house in the wintertime, and there was an improperly sealed {concrete, fiberglass, lousy bit of vapor barrior with holes} vapor barrior, and as they took away the fiberglass there was ice on the inside of the concrete wall.


Anyone know how permiable 3 layers of 5/8" gypsum/drywall are?
 

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1/2" drywall has a perm of 38. I believe its purely additive ie. 3x5/8" = ~2" = 38/4 = 9.5, still pretty high. Green Glue (if you are using it) is water based and probably not a complete seal so I suspect its effect is negligible.


Andy K.
 
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