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  1. #1
    Join Date
    Jan 2009
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    2

    Default How to insulate an overhang

    Good evening all,

    Question about properly insulating an under-insulated overhang that is resulting in very cold floors. I live in northern Baltimore County Maryland, the rear of the house faces north, the house is 14 years old, there is no house wrap, the basement is unfinished. The overhang is 24 inches deep and 9 inches high and the width between the floor joists varies. Spray foam is not an option at this time.

    Could I:

    A: Add more fiberglass insulation to the existing insulation (for some reason I do not think this will really work well)
    B: Use rigid foam insulation to create a "box" in each void and insulate the remaining void with fiberglass insulation - would I use adhesive (like liquid nails) to fasten the foam board to the inside of the overhand and well as the ceiling of the overhang?
    C: Use a house-wrap material (i.e. Tyvek) on the outside of the overhang and do A or B as well

    Would/could using rigid foam boards create an unfavourable moisture situation?

    Winter has started and the floors are getting cold. Any advice is appreciated.

  2. #2
    Join Date
    Jan 2008
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    731

    Default Re: How to insulate an overhang

    Mud sil? Eave space? Cantilliever? Bay Window? etc... Not sure what your "overhang" is, its elevation, exposure, what's above it....?

    I could guess you have a 2' cantillever of your first floor beyond the north foundation wall - but x9" from 14 years ago? which is why I'm asking about what precisely this "overhang" is, suspecting a garden window, bay, box bay, etc. extension. Is this 9" measurement the inside of a mudsil above the foundation sill plate?

    Need more construction details (beam/pier, joists, i-joists,? 9" band/rim joist?) please, and/or a picture to get the answer.

  3. #3
    Join Date
    Jan 2009
    Posts
    2

    Default Re: How to insulate an overhang

    Thanks for looking Blue RidgeParkway. Here is some more info and pictures:

    The overhang (we can call it a cantilever) runs the entire length of the house ~45 feet. The 9" is the total height of the area I need to insulate (also the height of the floor joist), each "hole" is 24 inches deep.

    Attached are four pictures - 2 of the cantilever and 2 of the basement looking up into the under-insulated area. In the second picture from the basement, currently there is a sleeping bag stuffed in the hole. It was here when we moved in earlier this month - free sleeping bag.
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  4. #4
    Join Date
    Jan 2009
    Posts
    1

    Default Re: How to insulate an overhang

    I have nearly the same problem, except my "cantilever" is actually just 2 feet of house above a 2-3 foot thick stone foundation. Our "mud sill" is an 8x8 inch beam! Stout construction in 1901, unfortunately, it's as drafty as you'd expect from a 107 year old house.

    It's a similar design challenge as above - how do you insulate such a space?

    To compound my particular problem, I have an area where the joists run parallel to the foundation and the access available to this space is about 1" wide between a joist and the top of the foundation - It's nearly impossible to get in there. The kitchen cabinets are above, so I was thinking of removing the cabinets and going through the floor to gain access. The cold pours out any time we open a drawer in the kitchen!!

    Personally, I like the idea of the foam box, sealed along the edges with adhesive or expanding foam and fiberglassed inside. The foam will stop the air infiltration and the fiberglass will bring up the R value.
    Last edited by millenniumtree; 01-16-2009 at 11:48 PM.

  5. #5
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: How to insulate an overhang

    Geez .... looking at the date stamp on your photos and seeing green grass .... it's hard for me to fathom you have a problem with cold.

    Considering we are knee deep in white stuff and it was 53 below zero with a howling wind on that date.

    All kidding aside ......

    The first thing that comes to mind at looking at the pictures ..... is there actually insulation in those joist bays of the cantilever?
    If so .... is it completely filling the cavity from the bottom of the joists to contacting the underside of the subfloor ?

    I have a suspicion the insulation was done after the construction instead of during.
    In which case the insulation contractor likely did this from the inside ( from the basement ) and probably somewhat haphazardly done since it can be awkward to get into that space.

    Insulating is only one part to achieving the desired performance. The other important step is preventing air movement within the insulated space.....especially when it comes to using batts.

    While the batts can be an adequate insulating material ....... moving air will hamper it's ability to function properly and will reduce the insulating performance .... as much as a 40% reduction of it's R value.

    Other factors like compressing the batts and moisture absorption will also negatively impact batt insulation's performance.

    Fiberglass insulation's performance also reduces as temperatures drop ..... though I doubt this would affect you much.


    There are a number of ways this can be tackled.

    You could try adding the batts from the inside ( basement ) however it will be tricky to ensure you isulate this area properly.

    Otherwise you can do this from the outside..... depending.

    You may be able to get away with simply applying a 1 or 2 inch layer of rigid foam to the underside of the cantilever. Using a house wrap tape to seal the joints of the sheets of rigid foam along with finish covering such as plywood or the like.
    This will provide a continious thermo break and enhance the insulating performance.

    Or ...

    Remove the sheathing from the underside of the cantilever to expose the area from underneath. Apply cut sheets of foam attached to the rim ( band ) joist fill the entire cavities with batt insulation ..... seal all points of air infiltration.
    Apply a 1/2 sheet of rigid foam to the underside of the joists and attach the plywood over that.

    Some obstacles you will run into will be where the joists are closely spaced ( for structural reasons ) which will make it difficult to properly fill with batt insulation.

    Fortunately or unfortunately ..... this is one of those applications where spray foam would be ideal.


    BTW .... the picture showing the down spout .... it would be a good idea to add at least a 3 foot extension to better direct the drainage away from the foundation instead of relying on that plastic pad.


    Just a thought.
    Last edited by canuk; 01-17-2009 at 09:25 AM.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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