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  1. #1
    Mark B Guest

    Default Insulating a Basement

    I'm finishing a 1950's basement in Michigan. The basement is dry, but before we started we ground out all cracks in the walls and filled them with hydrostatic cement and then used DriLock paint on the walls. Now I'm putting up steel stud walls, including around the perimeter and I'm trying to get advice on how to insulate these walls. Everything I've read says don't put anything directly against the block wall, because if warm moist air touches the cold block it will condense and you don't want insulation for eaxmple getting wet. Therefore, I'm planning to build the perimeter walls 1" away from the block.

    How do I insulate? I don't want to glue rigid foam insulation board to the block wall because I'm afraid that if moisture gets between the wall and the insulation board mold will grow. I don't want to use fiberglass because I believe it will sag over time and end up resting against the block wall anyways. So, I'm thinking about using rigid foam board in the metal track between the studs, 1" away from the block wall, no vapor barrier. Does this make sense? Am I getting any R value by doing this or should I just not bother with insulation?

    Thanks for the help.
    ...Mark

  2. #2
    Join Date
    Jun 2008
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    Default Re: Insulating a Basement

    I like your 1" gap idea. Rather than put the foam board insulation between the studs, I think it would be easier to sheath the metal stud wall with it (less cut & fit). It will also provide a thermal break to keep the metal stud from being a thermal bridge and eventually showing the ghost shadows of the studs on the drywall.

    Also, how about putting some poly on the block wall before the stud wall goes up?

  3. #3
    Join Date
    Nov 2007
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    Default Re: Insulating a Basement

    most of the rigid insulation boards come in 4x8 sheets and are already groved at the 16 and 26 where you can just snap them and place them right in the c channel of the stud and ofcourse 1 and 5/8 inch studs work best for that because it fills the c part of the stud. only thing is if you use one and 5/8 studs you need to put a shoe (or a piece of stud cut to make an L shapee) on the side of the stud about half way up and shoot it to the wall to make the stud stiffer

  4. #4
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    Default Re: Insulating a Basement

    Quote Originally Posted by Mark B View Post
    Everything I've read says don't put anything directly against the block wall, because if warm moist air touches the cold block it will condense and you don't want insulation for eaxmple getting wet. Therefore, I'm planning to build the perimeter walls 1" away from the block.
    Mark ... you are correct in that if warm moisture laden air was to meet the cold surface of the foundation wall .... condensation could occur.
    However .... the method you propose of leaving a 1 inch air gap between the inner wall and the foundation wall would create this condition .... even though you have insulation between the studs..... because the wall is not air tight.
    In the cold winter months you might even find frost forming.

    Also because the wall is not air tight ..... having this air gap would create a "chimney" effect of air movement inside the wall cavity which will reduce the insulation performance ... it's the law of nature.


    How do I insulate? I don't want to glue rigid foam insulation board to the block wall because I'm afraid that if moisture gets between the wall and the insulation board mold will grow.
    Yes mold is always a concern ... but ... mold feeds on organic materials and wouldn't feed on rigid foam insulation and the metal studs.


    I don't want to use fiberglass because I believe it will sag over time and end up resting against the block wall anyways. So, I'm thinking about using rigid foam board in the metal track between the studs, 1" away from the block wall, no vapor barrier. Does this make sense? Am I getting any R value by doing this or should I just not bother with insulation?
    Ultimately the choice is yours..... as a recommendation ....

    I'm sure many of folks are tired of this method that I preach.... here goes...

    Attach rigid foam insulation to the foundation walls first completetly covering the walls..... if you use 2 inch you are getting approx. R9.
    Use a house wrap tape to cover the seams where they meet at the corners and at the butt joints.
    The closed cell rigid foam insulation has a low air and miosture perm. value which means it doesn't pass air and miosture very easily and is an acceptable vapor barrier.


    Then put up the wall framing in front of the foam and apply your wall finish to the studs.

    What you end up with is a continious thermo break behind the wall framing which is far superior to the interupted method of insulating each stud cavity.
    This will be especially advantageous since metal studs have about zero R value .... actually will transfer the cold all too well.

    There is no need to apply any additional vapor barrier with this method.

    If you decide not to do it this way and leave a 1 inch air gap then don't apply plastic to the wall ... you'll end up with moisture and possibly frost build up in the winter.

    Hope this helps.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
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    Default Re: Insulating a Basement

    Canuk:

    I am ready to start my basement renovation and I like the method you are proposing. I will be using wood studs with PT plate on the bottom. I live in Southern Wisconsin so we have cold winters and fairly hot summers.

    I have three questions regarding this method:
    1. Do the wood studs have to be placed directly against the rigid foam or can I leave a gap between the foam and studs in order to easily run electrical behind the studs?

    2. What is the preferred way to attach the foam to the foundation wall? Construction adhesive?

    3. Is there a risk of condensation building up between the foam and foundation wall running down to the floor and feeding on the bottom plate boards?

    Thanks a lot.

  6. #6
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    Default Re: Insulating a Basement

    Quote Originally Posted by simon View Post
    Canuk:

    I am ready to start my basement renovation and I like the method you are proposing. I will be using wood studs with PT plate on the bottom. I live in Southern Wisconsin so we have cold winters and fairly hot summers.
    Depending on your local code you may not need to use a PT bottom plate.
    Here we can use some sort of isolation membrane under regular lumber. Something that I like to use is "sill gasket seal" ... which is a thin foam sheet that comes in widths for 2x4 or 2x6 lumber in rolls of 50 or 100 feet ... it's commonly used for home construction.

    You would have to check with the inspectors in your area.





    1. Do the wood studs have to be placed directly against the rigid foam or can I leave a gap between the foam and studs in order to easily run electrical behind the studs?
    No ... the studs don't have to tight against the foam.
    The thing that comes to mind with your idea for running electrical .... how will you fasten the lines?

    2. What is the preferred way to attach the foam to the foundation wall? Construction adhesive?
    My preference is this line of construction adhesive :
    http://www.stickwithpl.com/ProductsL...TION-ADHESIVES

    Whichever brand you choose use a construction adhesive that's compatible with foam ....... otherwise with the wrong type you may end up with a undesirable chemical reaction that will melt the foam.

    3. Is there a risk of condensation building up between the foam and foundation wall running down to the floor and feeding on the bottom plate boards?
    There shouldn't be an issue with condensation if the basement wall is isolated from the warm air within the room.

    This will be a reason to ensure the insulation is properly installed and well sealed to prevent warm moist air getting in behind the insulation.

    When gluing the foam in place use a continuous bead along the 4 perimeter edges.
    In the corners you can use spray foam in a can to seal ... all butt seams use a house wrap tape to seal those joints.

    Another area that is important to insulate will be the rim perimeter joist.

    By the way .... you can use foam on the floor with 5/8 plywood over top. if you go this route use the spray foam to seal where the foam on the floor and the bottom of the wall insulation meet.

    Hope this helps and good luck with your project.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  7. #7
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    Default Re: Insulating a Basement

    Quote Originally Posted by canuk View Post
    No ... the studs don't have to tight against the foam.
    The thing that comes to mind with your idea for running electrical .... how will you fasten the lines?
    This is a good point. Might as well just put the studs up agains the foam, drill through the studs for the electrical and gain the extra space in the room.

    Quote Originally Posted by canuk View Post
    Another area that is important to insulate will be the rim perimeter joist.
    For this I was planning on using unfaced fiberglass insulation to fill the cavities and placing the foam on the outside of that.

    Quote Originally Posted by canuk View Post
    By the way .... you can use foam on the floor with 5/8 plywood over top. if you go this route use the spray foam to seal where the foam on the floor and the bottom of the wall insulation meet.
    We were planning on putting down berber carpet, should I go the foam/plywood route or can I just put the carpet on the top of a pad with an integrated vapor barrier?

    Quote Originally Posted by canuk View Post
    Hope this helps and good luck with your project.
    Thanks for the help, I will post pictures once it is complete.

  8. #8
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    Default Re: Insulating a Basement

    Quote Originally Posted by simon View Post


    For this I was planning on using unfaced fiberglass insulation to fill the cavities and placing the foam on the outside of that.
    For the perimeter rim joist / sill area it might be difficult for you to secure the foam board over top of the batt insulation.
    As a suggestion ... you might consider just using the foam board it will likely be easier for you ... depends. Whichever way you go make sure the area is well sealed with caulk or spray foam to stop any air infiltration from outside.

    We were planning on putting down berber carpet, should I go the foam/plywood route or can I just put the carpet on the top of a pad with an integrated vapor barrier?
    The choice is up to you ... you don't have to use the foam and plywood sub floor. If the added expense and the 1 5/8 loss of head room isn't an issue ... this would be a deluxe way of having a comfortable basement floor..... in my opinion.

    Thanks for the help, I will post pictures once it is complete.
    Glad to ...... Looking forward to the pictures.
    Last edited by canuk; 08-21-2008 at 03:37 PM.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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