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Old 11-06-2009, 06:22 PM   #1
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Knee wall in basement of split level.....


I have put a sub floor on the basement (concrete floor). There are walls on top of the concrete foundation. I want to insluate and build walls on the concrete part below. They are about 40 inches high.

So far I have glued polybead (styrofoam sheets) to the concrete. Next I need to build a wall and insulate. When I get up to the concrete ledge how do I tie these together. DoI run a 2X4 across the ledge.


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Old 11-06-2009, 06:35 PM   #2
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Knee wall in basement of split level.....


Here's a couple of pics
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Knee wall in basement of split level.....-knee-wall.jpg   Knee wall in basement of split level.....-knee-wall2.jpg  

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Old 11-07-2009, 10:17 AM   #3
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Knee wall in basement of split level.....


Is this forum really slow or doesn't anybody know what the answer to my question is?
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Old 11-07-2009, 03:13 PM   #4
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Knee wall in basement of split level.....


You may not want to hear my answer..... The white foam board is not the correct type to use as it lets moisture through readily. (also stores moisture) http://www.energysavers.gov/your_hom.../mytopic=11620 You need to seal the seams of the extruded board an follow these recommendations for your locale: http://www.buildingscience.com/docum...gs?full_view=1
Be safe, Gary
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Old 11-07-2009, 05:40 PM   #5
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Knee wall in basement of split level.....


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Originally Posted by GBR in WA View Post
You may not want to hear my answer..... The white foam board is not the correct type to use as it lets moisture through readily. (also stores moisture) http://www.energysavers.gov/your_hom.../mytopic=11620 You need to seal the seams of the extruded board an follow these recommendations for your locale: http://www.buildingscience.com/docum...gs?full_view=1
Be safe, Gary
Thanks for the response but that's the proper way to do the wall. Polybead, then frame it out, insulate and a vapor barrier. I asked a couple contractors working in the neighbourhood and thay were cool enough to tell me how to do it.
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Old 11-07-2009, 06:21 PM   #6
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Knee wall in basement of split level.....


The "proper" way may be the way they do it, but the extruded foam insulates 30-40% better and does not transmit moisture or vapor, like the same stuff they make foam coffee cups out of.

Not all contractors know the best way because they have to bid low to get work.

Dick
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Old 11-07-2009, 07:38 PM   #7
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Originally Posted by concretemasonry View Post
The "proper" way may be the way they do it, but the extruded foam insulates 30-40% better and does not transmit moisture or vapor, like the same stuff they make foam coffee cups out of.

Not all contractors know the best way because they have to bid low to get work.

Dick
I'm not going to argue this. 2 contractors showed me how to do it and I just went down the cul de sac when I seen the city inspector. Polybead on the concrete wall, frame it, insulate with R20, moisture barrier and tie it in to the wall above knee wall. Acoustical chalk or tuck tape, then sheet rock.

Inspector said 90% of the houses here have the knee wall regardless if they're a split or 2 story. In his 30 years he personally has never seen or heard of a moisture problem when houses are done to this code.

In eastern Canada where I live, contractors are bringing in carpenters from other parts of Canada and the US because the housing market here is booming. Contractors don't have to bid low, they're setting their prices because of demand. A 1000 sqf split level with unfinished basement will cost you $300K on a 50X100 lot. It takes about 15 months to take possession. Listed houses are gone in a week.
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Old 11-07-2009, 07:50 PM   #8
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Knee wall in basement of split level.....


In answer to your question of how to tie in your wall, I usually use a wider pressure treated lumber to make my top plate. I use a lumber wide enough to go from the edge of the wall and have it extend over top of the concrete wall. This way I can anchor the lumber into the top of the wall and tie everything together.
As far as the insulation is concerned, you probably wont have too much problem with that type of styrofoam as long as you don't have too much moistrue. That type has a tendancy to decintigrate over time when exposed to maisture. As long as you are dry, you probably wont have too much problems, but the other type is better for that situation.
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Old 11-07-2009, 09:36 PM   #9
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Originally Posted by RobQuillin View Post
In answer to your question of how to tie in your wall, I usually use a wider pressure treated lumber to make my top plate. I use a lumber wide enough to go from the edge of the wall and have it extend over top of the concrete wall. This way I can anchor the lumber into the top of the wall and tie everything together.
As far as the insulation is concerned, you probably wont have too much problem with that type of styrofoam as long as you don't have too much moistrue. That type has a tendancy to decintigrate over time when exposed to maisture. As long as you are dry, you probably wont have too much problems, but the other type is better for that situation.

Thanks, that's another good option. A 2X4 fits perfect on the ledge and I could use that to tie both walls together. Did you ever see a sill gasket? They use them here on the concrete so that you don't need to put pressure treated on top.

I lived in TN for 12 years and I noticed they used pressure treated as a bottom plate on bare concrete slab, but I'm almost sure there wasn't a sill gasket. I'm still learning and I know they do different things in different parts of the country(ies) but I'd have to think the local guys know best.
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Old 11-07-2009, 10:06 PM   #10
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Knee wall in basement of split level.....


We use something called sill seal here in Michigan. It is a foam membrane that is sandwiched between the sill and the foundation. It is to create a berrier between the sill and the foundation but also to be an insulator also. I usually use pressure treated anyway because I have seen moistrue get trapped between the sill seal and the sill. Seeing that it is in a basement you could probably get away with a non treated material if you have a berrier between the concrete and the wood. So if you have a 2x4 wall studded up in front of the wall and 1 in foam I would probably use a 2x8 board as a top plat to connect everything. If it is a little wide you can rip it down. The nice thin about this is that you now have nailing for any type of finish you want for the top of the knee wall. I hope it works well for you, it always has for me. Good luck
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Old 11-08-2009, 03:36 PM   #11
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Knee wall in basement of split level.....


I think that's great you found out the way to do it locally.

The reasons I said not white foam board, partly from my generic files: Did you do a water test on the concrete wall? Perhaps by taping a piece of plastic down to the wall and checking it 2 days later for condensation against the concrete. This test is good for a slab, only to see if it has a vapor barrier under it, or not. Even then, the test does not show if the slab has a continuous, sealed, no punctures in the vapor barrier plastic, only the small section where it was tested for that particular time of year. After a rain, the test results may be far different, especially after the ground water has had time to soak in and follow the strata by capillary action. The water may not drain off as readily and back-up following a different path to the slab while underground. This is true for a concrete wall, also. Because a wall is exposed to the basement air movement before the finishing construction, if there are any wet spots where water has saturated the wall and shown itself, the air dries it leaving little sign of leakage. (Unless it has pooled below and left obvious water/mineral marks behind). This may be due to poor application of the exterior waterproofing material, or lack of sealed foam boards that help divert water down to the perimeter drainage system. The ideal way would be to plastic the whole wall, for a whole year, but who would do that. It is easier to figure you will have water coming through the slab or wall, and plan for it. As water is pushed through concrete, if there is an air space big enough or any air movement, you wouldn’t know it was there. Think of a sponge soaked with water in the sun after awhile, just by looking at the outside, it is hard to tell how much water is inside.

In your cold climate it is suggested to use foam boards inside to slow the water that gets past the latex coating to the concrete wall from the exterior. The wall should dry to the outside because of the vapor barrier installed inside behind the surface finish drywall to stop moisture from within.
White, expanded foam board could be used to slow any water from getting to the warm framing or added insulation, but not as effectively as the blue or pink foams (extruded). I see three problems:

  • white foam has a permeability rating of 3.5 per inch (blue is 1 per inch), or it lets water in 3-1/2 times easier than blue foam.
  • white foam stores water inside it, between the beads, when blue foam repels water only letting through a small amount, which doesn’t store inside.
  • white foam needs 20-30% more thickness to attain comparative R-value vs blue foam – not a huge deal as it’s way cheaper.

Think about #2 above, the water is now on the warm side of the foam, able to access the wood framing and insulation, unable to dry inside (v.b.) or outside (wall of wet foam that is slowly trying to dry out). The moisture is not heading back into the cold concrete, but towards the warmer spaces in the wood wall and warm studs.

Not taping the foam seams allow paths for any water present to follow to the interior, and heated air to infiltrate the space between the concrete/foam.



I hope you never have problems with your installation, here is the poly foam to use under the sill plate: http://www.lowes.com/lowes/lkn?actio...433&lpage=none
Be safe, Gary
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Old 11-08-2009, 03:53 PM   #12
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Knee wall in basement of split level.....


I think it's wonderful that you did it that way. How are you going to attach the drywall to the foam?
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Old 11-08-2009, 09:48 PM   #13
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Knee wall in basement of split level.....


Check his picture, I think his bottom plate is down already with the mod sub-floor butting tightly. Maybe he went to get some capillary/thermal break material sill sealer from box store, to keep the slab moisture from entering the wall space under the bottom plate.

Notice figure 2 under photo #11: http://www.buildingscience.com/docum...ms?full_view=1
Be safe, Gary
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Old 11-09-2009, 01:51 PM   #14
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Knee wall in basement of split level.....


Looks like you have the rest under control. Good luck!
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Old 01-05-2010, 07:33 AM   #15
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Knee wall in basement of split level.....


Yes it's common to use a treated bottom plate without a sill gasket. Others use just the gasket. Code generally requires either the treated plate or a separator like sill gasket, at least in my area.

Regarding your foam choice selection--and I just myself used extruded pink foam--you'll probably be ok. Although extruded gets a slight leg up on recommendations, some others do use expanded foam like you have done. Its lower R is fine for you if you're putting batts in the walls. It does tolerate moisture generally less well but on the other hand it breaths better than extruded.

Where in eastern canada does a 1000 sqft house cost $300k? Maybe in the heart of Halifax...my parents just sold one more than twice that size with a finished bedroom for that price.

FWIW I'm building around a window very similarly to yours and just tying it into the wall as a whole; the wall will be adhered to the concrete slab with tapcon screws and then to the joists above. I've not yet fully decided between a PT bottom plate or just a sill gasket plus normal wood. I'll use a sill in any case, though as it's cheap and a total capillary break.

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