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Old 04-03-2009, 01:56 AM   #1
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insulation basement


I would like to know how to insulate my basement walls the correct way. I have 8ft.poured walls in my basement i left about a 1/4 to 1/2 inch air gap then framed up the 2x4 walls i put R-15 kraft faced on all poured walls with a 6mm vapor barrier ,all the other walls that are on the interior of the room are R-13 kraft faced i thought to put the 6mm vapor barrier on the backside of the stud wall too is this overkill.

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Old 04-03-2009, 05:53 AM   #2
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A vapor barrier on both sides of the insulation will trap moisture in the insulation, not good. But there should be a 6MM barrier on the concrete walls. I prefer to use rigid EPS foamboard, insulation and vapor barrier in one.

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Old 04-03-2009, 06:41 AM   #3
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Not overkill but wrong. As posted a double barrier. You need to stop moisture from the concrete wall into the basement. This can be done with rigid insulation against the slab. never use batt insulation for this. No vapor barrier should be used in the wall. The foam board provides a vapor retarder at the concrete wall side and the kraft face insulation (to the inside surface) provides a vapor retarder for moisture moving from the warmer inside air to the colder outside wall. Read the articles on www.buildingscience.com
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Old 04-03-2009, 09:52 AM   #4
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It's typically done with XPS rigid foam. I wouldn't use fiberglass in a basement.
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Old 04-03-2009, 02:46 PM   #5
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At this point i can't fit any rigid foam behind the stud walls against concrete would it be a good idea to take the 6mm off the front of the wall and take off the kraft faced paper then put the new 6mm back on the front side.

Last edited by tjsplaine; 04-03-2009 at 03:28 PM. Reason: spelling
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Old 04-03-2009, 03:40 PM   #6
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Remove the wall and do it right now, or spend more money and time and remove it latter. But latter you will have to toss it to git rid of the mold. There are not short cuts. Well maybe some.. like using a wood stud to start with. Very poor idea. A basement wall will get wet at some time. Anything that fails when it gets wet and is not allowed to dry will fail. Batt insulation is the biggest no-no but not sealing the concrete with rigid foam leaves you with no way to stop incoming moisture and no way to stop warm inside air condensing on the cold concrete wall.
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Old 04-06-2009, 08:37 AM   #7
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Hey all, not trying to hijack, but based on the above comments, would either of these methods be acceptable for framing and insulating basements? The diagram has a daylight wall, but it could represent a full size basement wall as well.

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Old 04-06-2009, 10:14 AM   #8
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Quote:
Originally Posted by Bob Mariani View Post
Not overkill but wrong. As posted a double barrier. You need to stop moisture from the concrete wall into the basement. This can be done with rigid insulation against the slab. never use batt insulation for this. No vapor barrier should be used in the wall. The foam board provides a vapor retarder at the concrete wall side and the kraft face insulation (to the inside surface) provides a vapor retarder for moisture moving from the warmer inside air to the colder outside wall. Read the articles on www.buildingscience.com

As Bob said, you need to read the articles at Building Science before insulating. They've done the research and will show you what happens if it is not done correctly.
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Old 04-06-2009, 01:55 PM   #9
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Did not read my references did you? Anyway, no these are both wrong. With image one you can have moisture moving through the batt insulation and condensing on the cold plastic during the winter. In the summer you will have moist air moving through the concrete that will be trapped inside this poly. Now you have summer and winter mold....

Scenario # 2 Is better... but moisture will still get in from the warm basement and destroy the batt insulation. If and when you get a wet wall (it will happen) the wall has not air flow to allow it to dry. Failed again.
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Old 04-06-2009, 02:15 PM   #10
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Bob After reading the building science article it seems the second senario in the above is exactly what they are showing in fig14. Am I missing something when you say it's better but it will fail. How do you improve scene 2 . I am just following this because I am in the thinking process of framing and insulating also.
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Old 04-06-2009, 02:38 PM   #11
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Quote:
Originally Posted by jeeper View Post
Bob After reading the building science article it seems the second senario in the above is exactly what they are showing in fig14. Am I missing something when you say it's better but it will fail. How do you improve scene 2 . I am just following this because I am in the thinking process of framing and insulating also.
Use kraft faced insulation of the batt insulation if you insist on using this. Maybe due to costs.

Leave a 1/2" space or 1" if you can (better) between the wall and the foam against the concrete wall. this is the important part. It is why I explained in my first post you will have a shelf at the above/below transition.

Better ways:

Use foundation sill insulation under the sole plate. This will allow air to flow into and out of the wall space you need to provide.

Use metal studs... these will last and not be effected by water

Use spray insulation or foam in place of the batt. Batt is fine for the upper section of the wall.
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Old 04-06-2009, 02:52 PM   #12
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Gotcha, i missed the spacing of between the wall and the EPS. Thanks
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Old 04-07-2009, 09:45 AM   #13
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Final additional dumb question. On the EPS I have heard of people removing the film on both sides. Why would they do this? seems this would kill the vapor barrior. Is it so the is breaths?
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Old 04-07-2009, 10:10 AM   #14
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not sure. I see no reason to do this. Maybe some others here will have some idea.
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Old 04-07-2009, 10:36 AM   #15
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Bob, you may want to clarify this: "Use foundation sill insulation under the sole plate. This will allow air to flow into and out of the wall space you need to provide."

Sill sealers are to seal the air and moisture out from the living space. They don't allow air through them. Be safe, GBR

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