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Thread: Insulating a cantalevered bump out

  1. #1
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    Dec 2006
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    Insulating a cantalevered bump out

    Ok you know how the small projects turn into the project from he!!....

    I have a 2' by 8' cantalevered bump out in my breakfast area. The underside has rotted out. The only wat to get to it was to tear down the deck, which was due for demolition anyway. The deck is/was only 2' off the ground and support beams 6" off the ground which was why I could not get access.

    Now that I have removed half of the deck I can get access to the underside of the bump out, I can do the necessary repairs. I recall great debate when Marty was insulating his shop floor as to how best install the insulation. My plan is to put in bats of insulation and cover it with exterior grade plywood to keep mice and bugs out.

    Question: Should I use faced or unfaced insulation? I was also thinking of putting soffit vents on the underside to help vent any moisture. Any recommendations would be very much appreceated.

  2. #2
    I'm thinking this would be a great place for rigid foam insulation. Why?

    You can cut to exact size and hold up under the floor where fiberglass wants to sag. 2" 4'X8' ssheet is about $10-12 at HD and so the project you discribe would allow two layers from one sheet or a cheap fix. As you have severl more inches in your floor joists you could add additional layers, Use a foam insulation "Great Stuff" and seal any cracks or seams.

    How do you cut to exact size? With a battery charger and a wire. Stretch a wire between two leads from a battery charger, the short will heat the wire and make a hot knife that will slice through with ease. I did this with my S-I-L's garage and have all the details, if you are interested.

  3. #3
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    Hey Mike!

    The great debate we had over the insulation under the shop was influenced greatly by the fact that we have to cool more than heat. That's not the case where you are...I know, we spent many a freezing cold winter up there...

    What we did under the shop (or more correctly...what DENISE did ) was install the paper-faced batts with the paper down. That was largely to keep the fibers out of her face while she was stuffing the batts. I think we finally decided that the small benefit we 'might' have gained from the paper barrier wasn't important enough to worry about.

    Again, that's not the case in your area. Technically, you should install the batts barrier up (towards the living area). I'm not sure I'd install plywood over (under) the batts. Moisture getting trapped in there might cause problems down the line. We only put galvanized hardware cloth over (under) the batts...as a critter barrier.

    With all that said...

    I think Bill's approach is better, neater, and far less messy. Although, I think I'd stuff multiple layers...or get thicker than just the 2" stuff the borg sells.

    Good luck...
    - Marty -
    Fivebraids, Inc.
    When you earnestly believe you can compensate for a lack of skill by doubling your efforts, there’s no end to what you can’t do…

  4. #4
    Join Date
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    Salt Spring Island, BC Canada
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    Hey Mike I agree with Bill and use spray foam insulation to fill in any gaps that might be had. Finish it off with vented soffit to allow max air circulation. Just make sure that you seal it up good to keep mice and bugs out. If you use pressure treated lumber or paint on a wood preservitive it will help repel the bugs.

  5. #5
    Hey Mike, I thought I tossed the picture of the foam cutter I made to slice the 2" foam, but I found it. Not a pretty device but it got the job done...

    I made a table and attached a frame similar to a Jig saw frame. Instead of a blade I strung a piece of Ni-chrome wire (you can use guitar string) to each end, a lead to a battery charger, Tension was applied to keep the wire straight with an old spring from the junk box. a Fence was clamped to the flat plywood base so I could "rip" to a given width. Adjust where on the wire you attach the BVattery leads to control the heat, You want just beginning to turn red. You will be surprised how easy it cuts and to w/in tiny fraction of an inch . With NO mess.



    (another long story, but...) We got ahold of a stack of 2" and 1 1/2" 4'X8' sheets of the Foam and sliced it to the width of the space between the studs on my S-I-Ls garage, we had enough to put two layers thick (3 1/2")
    Then covered with OSB. On another note, We took the foam and insulated a box to put around the Cistern Pump using the same 2 layers and cut as smooth as it did there is no leaks and the pump has stayed "Freeze Free" for the past 3 years. A friend of mine used the same rig to slice some of the foam to fit the insides of his thin walled metal Garage door.. Snug fit and friction fit to keep it in place.

    Foam and OSB were almost free as we were dumpster diving... Another friend has a son who is a crane operator, He called one day and said to bring a truck and hurry. Not knowing what he wanted, but I did just that. Seems he was erecting a Resturant that was preconstructed in sections (Outside walls finished all the way through to the finished inside walls, just bolt them together. But the sections were shipped with 2" and 1 1/2" thick slabs of foam to protect them in shipment. Contractor was having to pay to have it hauled off to the landfill. We stepped in and each evening (3 nights) we took what was stacked up to be tossed. 90 sheets in all. And the OSB, Same fellow called again and said bring truck, so we did.... Seems they were removing an old roof and replacing with new on a factory. They stripped the tar paper and removed the OSB, I brought a Case or two and the fellows were careful to remove the OSB intact. (They were replacing with some high energy metal faced foam roof material.) two truck loads of OSB for a couple cases. not a bad deal. So when my friend's son calls and says bring a truck, I come running.

  6. #6
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    Mike did you determine what caused the rot in the first place?

  7. #7
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    Hey guys thanks for the detailed responses.

    Personally, I'm leaning towards a treating it like a roof soffit and having a hard cover that is vented. That will keep bugs and mice out. There was a mouse hole or two in one of the joist bays. Plus one of the joist bays has an HVAC duct that goes to the breakfast area. The opening to the basement is pretty hard to seal there. I was going to cut a piece of tin to cover it from the out side.

    The rigid foam insulation is a lot easier to work with and does not sag over time so I might use that instead of fiberglass.

    I did fill the gaps with Great Stuff.

    Drew - to answer your question, the rot on the rim joists was caused by the deck not allowing proper drainage between the two rim joists. The plywood face of the bump out arond the windows and doors was cut to just above the height of the deck by about 1/4". This let water in to the rim joists and then down to the plywood under the cantalevered section. I just love adopting other people's construction problems!

  8. #8
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    mike, be very carefull about installing double or tripple vapor barriers, it`s possible to do more harm than good....do some reading at owens corning or certainteed`s websites about the propper use of insulation, vapor barriers and ventilation.
    [SIZE="1"] associated with several importers and manufacturers.[/SIZE]

  9. #9
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    Mike the ridgid insulation is a vapor barrier in itself. Though a vapour barrier isn't really nessisary in a floor and can cause problems if moisture gets trapped in there. I am personally not a big fan of the sealed home theory as our parents told us when we were kids don't put a plastic bag on our heads and that was for a good reason. A house needs to breath to allow all the moisture and chemically filled products that we fill our homes with to escape. The ideal location of a vapour barrier is 2/3 inside the wall as that is where the dew point is. Are you re-building the deck again Mike? wasn't there any building paper covering that gap?
    Daily Thought: SOME PEOPLE ARE LIKE SLINKIES..... NOT REALLY GOOD FOR ANYTHING BUT THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS...............

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