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Putting batt insulation between 2×4 top chord truss for unvented low slope roof

WildBunchFarm | Posted in General Questions on


It’s been awhile since I’ve asked a question here but I need some of this community’s expertise yet again.  

I am connecting an a two story addition to an existing home.  The project qualifies for a historic rehabilitation tax credit so I have to follow the guidance of the historic rehab people at the State and Federal level.  They don’t want me to have a “flat” shed roof, but rather a low slope gable roof.  And this roof has to fit under the eaves of the existing home.  It’s a tricky problem for me, but I think I’ve figured it out.

My build is in Climate Zone 4A.

I will have low slope trusses made, but the height from the bottom chord to peak of the truss can’t be higher than 18″.  I will make this an unvented roof/attic assembly.  I only have enough room to put 3″ of foam board above the roof deck.  2″ of polyiso foam board (R-13) and 1″ of EPS (R-5) for a total of R-18.  

Under the roof decking in my “attic” I can fulfill the rest of the R-20 to meet R-38 minimum by putting in R-21 batt insulation.  However, how do I attach batt insulation to 2×4 top chord trusses?  Should I just dense pack my entire attic since there’s barely any room in there anyways?  Maybe I should specify the truss maker have a 2×6 top chord in order to use tiger teeth, but that would raise the price of trusses by a few hundred bucks.  Not a big deal if this is the easiest method. 

Thanks for any advice one can give.   

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  1. Jon_R | | #1

    Not what you are thinking, but with the right amount of adhesive, cellulose can be wet sprayed overhead. With less air movement, performance should exceed batts.

  2. WildBunchFarm | | #2

    Yeah that's an idea. I have done wet spray cellulose on walls before. I don't know how it will hold up over 5" in a nearly horizontal application.

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