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Basement rigid board installation


qtoask's Avatar
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03-14-12, 01:15 PM   #1  
Basement rigid board installation

I am trying to install 2'' extruded polystyrene in my basement. I have been reading about it but I am having some challenges because of my basement wall not being smooth at all.

1) Does the board needs to be very tight up against the wall or can I have a gap between concrete and the board to avoid making the wall smooth all around? I will be using PL300 to glue, but it is nearly impossible with the way concrete wall currently is.

2) If it comes to making the walls flat then what is the best way to do it, I am guessing it will be such a time consuming and labor intensive job by a hammer and chisel?

 
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03-14-12, 01:32 PM   #2  
If flat is possible that would be best. The objective is to eliminate any air movement into and out of that space. Also, the wall above grade will be colder than the wall below grade. Add in the moisture from below and you get a transfer of moisture that will freeze and collect at the top. Concrete always has moisture passing through it. The moisture vapor will actually pass through that 2" extruded as well, so no vapor barrier, let it dry to the inside.

To smooth out the wall, if it is just those darn seams, it is actually very easy. A right angle grinder and a diamond cup will walk right through that job. I also have a small blade for my Dremel, a bit slower, but the diamond disk works great on concrete.

Bud

 
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03-14-12, 01:48 PM   #3  
Thank you for your reply. I will see what I can do to make it flat.

Not sure if I understand this part though:
"Concrete always has moisture passing through it. The moisture vapor will actually pass through that 2" extruded as well, so no vapor barrier, let it dry to the inside."

So would putting extruded up against the wall, sealing top, bottom and joints would be enough? or what did you mean let it dry to the inside?

 
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03-14-12, 04:51 PM   #4  
He means you're going to get moisture moving through the concrete to the inside but the foam will let it pass through, which you want. Don't try to stop it with a plastic or other vapor barrier.

 
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03-14-12, 04:59 PM   #5  
The pink or blue extruded foam board is considered permeable, although 2" is very limited. What that means is that it allows moisture vapor to pass through slowly. The same thing is happening to your concrete walls. One of the laws of physics is moisture will move from wet to dry, and if it can't evaporate it will accumulate until the inside is as wet as the outside. Many basement walls look dry, but your dehumidifier is telling us a different story. The black tar they put on the outside of a foundation is never 100% and thus, between the rain, splashing, and moisture that penetrates that barrier, yes, moisture is always passing through. The trick is to not block it to where it will accumulate.

Building Science corp has several articles on basements, vapor barriers, and moisture, plus I ran into some good ones recently (would have to backtrack). But there is lots of reading on this subject.

Bud

Note: at the end of this document there are more links.

http://www.buildingscience.com/docum...s/?full_view=1

 
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03-21-12, 08:48 PM   #6  
Thank you for all the information.

"To smooth out the wall, if it is just those darn seams, it is actually very easy. A right angle grinder and a diamond cup will walk right through that job."

I got my angle grinder (dewalt 4 1/2) but I am having difficulties finding the diamond cup you mentioned. Which kind is the best for this kind of job since there are few different types of diamond cups. Would you mind sending a link so that I know what exactly I am looking for?

 
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