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Thread: Insulating open rafters in my shop

  1. #1
    Join Date
    Jan 2007
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    Snohomish, Washington
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    Insulating open rafters in my shop

    I should know better than this, but it sure would be easier if I could get away with it.

    I have a 24 X 28 double car detached garage that Ive turned into a woodshop. I started out by completely gutting the interior walls. I then stubbed out the wall studs with 2X4s ripped down the center. This gave me a total of 5 1/4 depth to my walls. I then insulated the walls with 6 insulation. I than took off the old cedar shake roof and put a monier lifetile heavy shake roof on it, which matched the house. As an underlayment I used Grace Ice & water shield. This product makes my roof completely water proof. The trusses are open (I prefer it that way) and I would like to insulate the rafters, but I am not sure how to go about this. I live in the Pacific NW and it can be humid and wet most of the year. Because my roof is water proof, could I get away with putting 6 unfaced batts up against the underside of the roof without creating a moisture problem. My shop is a hobby shop and I heat it only when I want to. This can be two to three times a week. Would there be enough air flow through the batts to dry any condensation that would appear? Im temped to try a section to see what happens. Does anybody have expertise in this category?

  2. #2
    Join Date
    Feb 2010
    Location
    Collegeville PA (30 min west of Philly)
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    440
    I'm not an insulation expert, but I plan to use roofing baffles when I insulate my barn's roof this year. Google "roof baffle" and you'll find lots of information.
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  3. #3
    Join Date
    Nov 2010
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    Amsterdam, NY
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    Not sure about your area, but I believe you need roof baffles or you can use spray foam insulation which goes right to the underside of sheathing.
    No airspace required.

  4. #4
    Join Date
    Feb 2009
    Location
    League City, Texas
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    1,629
    The thought of open rafters, Washington state, and insulation all scream MOLD to me...
    Trying to follow the example of the master...

  5. #5
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    Jan 2007
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    Snohomish, Washington
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    James, I was thinking of using a spray insulation, but this would seal the sheathing between two water proof layers. If by chance I had a slight leak in the waterproof underlayment, this could eventually rot the sheathing. The insulation should buffer a rapid temperature change that could cause condensation to form. The unfaced fiberglass insulation should allow some breathablility to expel any moisture. I'm trying to rationalize here, I have no expertise.

  6. #6
    Join Date
    Nov 2010
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    Amsterdam, NY
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    Quote Originally Posted by "Patrick G Rainsberger" View Post
    James, I was thinking of using a spray insulation, but this would seal the sheathing between two water proof layers. If by chance I had a slight leak in the waterproof underlayment, this could eventually rot the sheathing. The insulation should buffer a rapid temperature change that could cause condensation to form. The unfaced fiberglass insulation should allow some breathablility to expel any moisture. I'm trying to rationalize here, I have no expertise.
    Patrick, I am not sure of the climate there so that makes it a little hard. But here in upstate NY, the temps don't get above 32F too often in the winter, so insulation directly against the sheathing will cause it to condensate. Because the insulation does allow some heat to pass at least over time, and the sheathing is always cold thus causing the condensation and the decay of the sheathing.

    I get what you are saying about the potential for leak, but generally if you have a slight leak, it will rot the sheathing before you can find it even with fiberglass insul.
    We have used the spray foam in the past and so far so good.

    Just another idea for you

  7. #7
    Join Date
    Jul 2004
    Location
    Vernon, Connecticut
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    I'm in CT and it gets cold here. I sealed the rafter bays with spray-in foam. I'm not too worried about moisture rotting the roof decking because 1. the foam seals the rafter bays tight preventing room moisture from reaching the decking and 2. my workshop has fewer sources of moisture than a house (no shower, appliances, etc). There are several good building science articles about vented vs unvented roofs, and the debate still rages on.


    I'm not worried about roof leaks, and given your use of water & ice shield, I wouldn't either if I were you.

  8. #8
    I had Icynene foam sprayed directly on my roof sheathing, 10", and in the 2x6 stud cavities. The theory is that the surface of the foam will never get cold enough to condense moisture, and the foam prevents moisture from getting to the rafters and sheathing. That was two years ago and the addition is close to being finished, but the HW baseboards are not in yet. Residual heat from the main house and sunlight keep the addition about 55 on the coldest 0 degree winter days, and the addition is bigger than the house. Icynene is 3 times the price of fiberglass, but the payback can be as little as 5 years.

  9. #9
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    Mar 2010
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    Grottoes, VA.
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    Is there a reason to keep the trusses open? Other than "I prefer it that way"? With them open, it will cost more to heat your shop, when you do heat it. It will also extend the amount of time it takes to heat your shop to your comfort level.

  10. #10
    Join Date
    Dec 2008
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    South Central Pennsylvania, USA
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    Quote Originally Posted by "Patrick G Rainsberger" View Post
    Would there be enough air flow through the batts to dry any condensation that would appear?
    Actually, this is exactly the problem. Air with water vapor moves up through the batts and then condenses against the colder sheathing, thereby soaking the insulation, creating mold, and reducing the effectiveness of the insulation - not to mention all the other problems...

    A closed cell spray foam insulation will prevent the infiltration and you won't have the condensation/moisture problem.

  11. #11
    Join Date
    Apr 2007
    Location
    Seattle suburbs
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    69

    spray foam

    consult a reputable spray foam company ... ask for local builder references and call them.

    I believe the foam will do fine.

    your other option would be to make a second cieling at a different pitch than the current roof to allow for space to accomodate insulation and air movement - think of a capital letter "A" as the shape. This is how the grand room in our home was done, except the truss' were designed to have no horizontal chord -

    All in all, just put in a ceiling with batts above it - most cost effective and best result. Anything else will cost a ton. use the $ saved to buy some LN planes (or ONE Brese plane!)

  12. #12
    Join Date
    Jan 2007
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    Snohomish, Washington
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    Sorry about not following through with my own post. I probably should not have started it just before my work week started. I work three 12hr. shifts 1 hrs away from my home. By the time I finish each day I’m pretty much spent. I think your combined rationalization was much better that mine. I’ll be looking for a spray foam contractor in the near future. My only issue with spray foams is what I see when I google Icynene dangers problems. The negative side doesn’t look good.
    Last edited by "Patrick G Rainsberger"; 02-17-2011 at 11:38 PM.

  13. #13
    Join Date
    Dec 2008
    Location
    South Central Pennsylvania, USA
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    I did a quick search and the main "problem" link that turned up dealt with a water leak and mold issues with Icynene. A quick review of the Icynene website failed to turn up whether it is a closed cell or open cell foam - although it sounds like it is an open cell foam. There are distinct differences between the two and the use should be considered before selecting one or the other. Particular attention should be made to whether the wall (or ceiling or whatever) system needs to "breathe" or not.

    However, leaks are an entirely different matter. Whether it is plumbing, roofing/sealing, or groundwater you're going to get into trouble if the source of the problem isn't addressed. From the "danger" website about Icynene that I saw I really don't see that the Icynene reacted that much differently to any other insulation system. But perhaps you are talking about other issues than the one that I found?

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