Framing, Avoiding Wood Against Concrete

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Old 01-20-15, 04:22 AM
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Framing, Avoiding Wood Against Concrete

When framing a basement, what is best to use between the bottom plate and the concrete floor? I would like to avoid direct contact between the wood and the concrete. I'm using rigid foam as a vapor barrier against the walls. Thanks
 
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Old 01-20-15, 04:32 AM
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I usually just use pressure treated wood for the bottom plate and then the correct fasteners for everything that attaches to it.
The rigid foam against the basement walls is not intended to be a vapor barrier. It is a layer of insulation that holds the wall framing away from the concrete without leaving an air gap. All water intrusion needs to have been eliminated before the rigid is installed but moisture vapor will still need to be allowed to pass through to avoid accumulation behind the wall. This issue is still being debated, but I still prefer to use the pink or blue rigid as they do have a low level of permeability and can dry to the inside. The amount of moisture is extremely small so does not create a problem for the basement.

Bud
 
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Old 01-20-15, 05:15 AM
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Just wrap the bottom plate with flashing tape. It'll be in the roofing section at the big box store.
 
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Old 01-20-15, 06:55 AM
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Old 01-20-15, 07:09 AM
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Thanks, I didn't know there was such a thing.

To Bud's point, I just cannot seem to get a consistent answer on the vapor barrier question (a much debated topic, I know) when using rigid foam. Mike Holmes and others claim the rigid foam IS the vapor barrier, other say leave a space between the foam and framing with a traditional plastic vapor barrier. Perhaps I'll post another thread on this debate.
 
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Old 01-20-15, 07:14 AM
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The foam is a vapor retarder, not a vapor barrier. Mike Holmes is typically spraying foam on when I watch and that does a good job air sealing as well, which is important.

I know Bud is of the mindset that you don't use a vapor barrier on a below grade wall and you also don't have any air gaps to allow condensation to occur and this makes sense to me. Hence, if I was using rigid foam, I would build the stud wall right against the foam and no plastic vapor barrier anywhere.
 
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Old 01-20-15, 09:44 AM
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The educated expert who warned against having a vapor barrier against a foundation wall just changed his mind, Nah couldn't happen! But there is some hedging going on so I'm not sure the debate is over. Yes, concrete does not mind being wet, but anything that can serve as food for mold will certainly result in things growing back there. So, I'm still going to advise the rigid foam without a foil facing because as Mitch stated it serves as a vapor retarder, allowing a small degree of drying to the inside.

I have long followed the "leave a gap for air flow" approach between the studs and the concrete, but having read an article with pictures that explained how air circulation in very cold climates will transport the moisture that passes through the soil near the bottom of the wall, up to where the concrete is exposed to the elements and very cold, thus depositing its moisture and building up significant ice. That ice later thaws and looks like a leaky basement wall.

It is the rigid foam that best fills that gap isolating the wood from the foundation and providing a nice uniform insulation layer.

PS I love Mike, but when you know a lot about a particular field it is easy to spot the mistakes he makes and there are many. But I do give him credit, he tries. I even saw him playing with an infrared camera, but I'll bet he never went for certification, makes a difference.

I have to run and I'm running long so will stop and wait for more questions.

Bud
 
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Old 01-20-15, 11:39 AM
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Thanks for the help. I will likely go with 2" rigid foam and the framing up against the foam, when I get to that point.
 
  #9  
Old 01-20-15, 02:03 PM
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If you seal that 2" rigid to the foundation walls and then find the walls are not perfectly straight or plumb then, leaving a gap to make the wall straight is not a problem, as the insulation will keep the inside surface warm enough to avoid any condensation.

Be sure to detail the rigid over the top of the foundation and then air seal the joist cavities and insulate them.

Bud
 
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