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Dens-Pac the roof?

Gene_Davis | Posted in Energy, Heating & Insulation on December 21, 2008 07:44am

We’re in a very cold climate here, and big snow loads, too.

I’m dreaming up something for someone that may have a roof framed with parallel chord trusses, 16 to 18 deep.  The spans are just a little much for 2x12s, and the lengths needed are beyond what is stocked in the yards.

Would one mesh this and dens-pac the roof, no venting?  Or is that a big no-no.

 

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“A stripe is just as real as a dadgummed flower.”

Gene Davis        1920-1985

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  1. mike_maines | Dec 21, 2008 08:47pm | #1

    Gene, you could use a parallel chord truss, I-joists are used all the time as rafters so PC trusses would work too.  The only problem I can see is it would be hard to get a dense-pack without sealed sides to the rafter bays.  The cellulose would be inclined to blow through the open chords.  If you used I-joists, the thin 1/2" web wouldn't allow for too much cold bridging and you can easily get a proper dense-pack.

    It's almost always better to vent it you can, but with enough R-value and good vapor retarding details on the inside you can get away with a "hot" roof.  With the R-60+ that 16-18" would give you I don't think your roof would be too hot though!

    1. User avater
      Gene_Davis | Dec 21, 2008 08:58pm | #2

      Thanks, Mike.

      I found this with a web search.  Something somewhere around Freeport.  Maybe I'll email the architect and see what he has to say, as regards the dens-pac details.

      I saw a guy do fiberglass batts in a 2x10 cathedral roof here, then he did one layer of 2" foilfaced Thermax isocyanurate foamboard, taping joints with the specifiede foil tape, then strapping with 1x3 screwed through to the rafters, then the finish.  No venting, and reportedly, no problems.

      Your suggestion re I-joists as rafters probably makes better sense than the parallel chord trusses.  There is a lot of side-cleating work needed for web pack-out, but what the heck.  I'm going to go to my I-Level spec guides and see what I need. 

      View Image

      "A stripe is just as real as a dadgummed flower."

      Gene Davis        1920-1985

      1. mike_maines | Dec 21, 2008 09:34pm | #3

        Curt Jensch, the architect who designed the house you linked to, is a good friend of mine.  He started a new company, Spring Island Builders:  http://www.springisland.biz/index.htm.  His own house is nearly done, and it's really cool.  Hopefully it makes it into a future FHB.  He designed the first LEED-rated house in the country. 

        Taggart, who built the house you linked to, is our closest competitor but they wear their "green" on their sleeve more than we do. 

        We did several jobs with unvented R-38 cellulose, but now have gone to "flash and batt" on most jobs, a combination of closed-cell foam and fiberglass.  The project I'm working on today will be vented R-38 dense-pack though.  This is the company we use for dense-pack:  http://www.warmtechsolutions.com/homepage/. 

      2. Piffin | Dec 21, 2008 09:45pm | #4

        "I saw a guy do fiberglass batts in a 2x10 cathedral roof here, then he did one layer of 2" foilfaced Thermax isocyanurate foamboard, taping joints with the specifiede foil tape, then strapping with 1x3 screwed through to the rafters, then the finish. No venting, and reportedly, no problems."I do similar with no troubles.I also used PC tyrusses for a flat roof 15" deep and filled it full of chopped FG wityh vapor retarder and IC can lights - no vents and no problems. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      3. User avater
        BillHartmann | Dec 21, 2008 09:52pm | #5

        I think that Mike used to put in the chutes and denspack around them.But IIRC one of this later pictorials did not show them and I ask him and said that they wheren't needed.But he is not is as cold a climate as yours..
        William the Geezer, the sequel to Billy the Kid - Shoe

  2. MikeSmith | Dec 21, 2008 10:28pm | #6

    this wound up being about 24" for the cathedral ceiling

    http://forums.taunton.com/tp-breaktime/messages?msg=112323.56

    one thing about  dens-paking a ceiling  ( or not even  DENS )... we like to strap below the insulmesh  to provide reinforcement

    it also means we can get a 3/4 " bulge and still not bother the  board guys

     

    Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
  3. Marson | Dec 22, 2008 12:27am | #7

    We have done that successfully with I joists. However, our local building department now requires that ALL roofs must be vented. IIRC, it was a change in the IRC.

    1. User avater
      Gene_Davis | Dec 22, 2008 02:36am | #8

      Thank goodness we've no inspection, here. 

      View Image

      "A stripe is just as real as a dadgummed flower."

      Gene Davis        1920-1985

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