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Scissor truss insulation advice

John_F | Posted in Energy Efficiency and Durability on

I am building a house in 4a, Southern Indiana, near Louisville KY.  It’s a generally mild climate but gets cold (had a few days around 0,°F in January) and the summers are hot and humid.   The roof is set up to be a vented roof.  Part of the house is a trussed flat ceiling, which I plan to insulate with blown cellulose.  
The part of the house in question is a 12:12 over 7:12 scissor truss.  I know, I have been reading up on this forum (too late) and I should have had an insulation plan together before I got to this point.  I’m not a builder, I’m just a guy building a house on a farm property.  The finished ceiling will be T&G in this part of the house.  My initial plan was to drywall the ceiling and blow cellulose above it and count on some settling and maybe even have to come back next year and add insulation to the peak.  I was going to spray foam the walls and the spray foam contractors say that the cellulose in this ceiling won’t work, will slide down too much, and it will be difficult to heat and cool this space.  The foam contractors all say I should spray the roof deck of this part of the house with 5-6″ O.C. foam.  This comes to like R-20 which doesn’t seem like enough.  They say the R value doesn’t matter because it’s airtight.  Then they say I could and should skip the drywall and put the T&G on.  My HVAC is roughed in already and it would be difficult to add a supply and return to the attic at this point but I could add a fan either to the outside or to the room below.  
I really liked the initial plan of a vented roof for this house with the humid weather and I’m worried that I will regret foaming the roof deck.  It seems like the quickest and cheapest option but not the best.  
Any advice or creative options to consider out there?  Thanks in advance for any comments, and take it easy on my for getting myself here!

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  1. Expert Member


    Stuff happens, you are where you are. The main thing is to end up with a solution you don't have to worry about.

    You can always use batts, rather than worry about the cellulose settling - or the problems you can get with spray foam. If you want to reduce wind-washing, choose high density ones.

    If they are suggesting open cell spray foam, that is risky as it can accumulate moisture and cause the sheathing to experience damage. Their comment about the R-value being unimportant is just dumb and wrong.

    I know you didn't ask about it, but flat roofs with permeable insulation like cellulose are tricky. It might be worth running what you are thinking of there by us for comment.

    1. John_F | | #2

      Thanks for the response. I understand the risk of the OC on the roof deck, that is what is keeping me up at night!
      Another idea I have is to put 1"-2" rigid foam on the bottom of the trusses, tape the seams, attach 1x3 strapping through the foam board to the bottom of the trusses, T&G to the strapping and be able to put R-38 batts on top of the foam board. Which batts are considered high density?

      I'm still thinking about OCSF in the 2x6 wall cavities and the contractor said they can also spray the top of the flat ceiling drywall to achieve an airtight seal.

      Any further thoughts would be greatly appreciated.


  2. John_F | | #3

    Side note is that we like to keep windows open and not run the HVAC for long stretches of time throughout the year as the climate is pretty mild (windows would be open now with a low of 56°f last night). Seems like the conditioned attics perform best when you are always running the HVAC.

  3. John_F | | #4

    Another side note the trusses do all have 15" heel heights

  4. gstan | | #5

    Many years ago I built a similar house with a scissor truss roof over
    the living room - not quite as steep as yours. I installed fiberglass
    batts (9" I think) first then 10" cellulose over the batts. Six years later
    there was no sign of sliding or settling when we sold the house. If
    I remember right it was a 9/12 upper chord and a 5/12 lower.

  5. sommerbros | | #6

    My house has a similar scissor truss design 10/12 exterior 8/12 interior. My insulation contractor specializes in the BIBS (Blown In Blanket System) and his approach was to install cardboard baffles parallel to the ceiling creating a 12" (R50) cavity between the ceiling and baffle. Once the baffle was complete, netting was attached to the ceiling and the cavity was dense packed with fibreglass, essentially a thick wall laying on the slope. This left loads of vented space above the insulation and no risk of settling or wind washing. The work was done in 2008 and I have recently looked at my ceiling through infrared and there has been no sign of settling.

    I have another project coming up with the same scissor truss design. This time we are targeting closer to R80 so we are taking a slightly different approach. Since the space between the bottom (ceiling) chord and top (roof) chord is only about 30" at the deepest we have opted to essentially fill the entire cavity aside from our air path, this may leave us with a higher R value then required but the simplicity of our baffle system out weighs the cost of a few additional bags of cellulose.

    I have attached a couple of drawings illustrating our approach. one is of the actual truss we are using and the other is an example from another post that shows the baffle placement.

  6. Danan_S | | #7

    > The roof is set up to be a vented roof.

    > The foam contractors all say I should spray the roof deck of this part of the house with 5-6″ O.C. foam.

    I may have missed something, but if the roof/attic are vented, spraying insulation on the underside of the roof deck doesn't provide any insulation, since the attic is basically outdoors.

    Perhaps they meant they would spray foam the attic floor? But as others have mentioned that could cause moisture issues since most foam is vapor impermeable.

    Also, if you are worried about the cellulose sliding, you could hold it in place with the same fabric they staple up to hold it in wall cavities, or just roll out batts over the peak.

  7. John_F | | #8

    Thanks for the responses. I had considered blown cellulose over batts but wasn't sure if the 7/12 was too steep.
    I just now looked into the BIBS, looks like it could possibly work in my situation I'll look into it more. Doesn't appear to be a BIBS contractor in my area and might not be diy friendly
    I meant that the roof is currently set up to be a vented roof but if I foamed the roof deck of the scissor truss part of the house, they would foam over the eav and ridge vents making that part of the house an unvented roof.
    3 different contractors have recommended 5"-6" open cell on the roof deck, seems to be an inadequate amount of insulation in a climate where most people are either heating or cooling most of the year, not to mention the problems associated creating an unvented roof with vapor permeable insulation.

  8. John_F | | #9

    I think my plan is to put 1" polyiso under the trusses, tape and seal the edges the best I can and put 1x3 furring strips below the polyiso and t&g to the furring strips. R38 mineral wool batts (if I can find them?) Above the polyiso between the trusses.

    1. Expert Member
      MALCOLM TAYLOR | | #10


      Of the proposed solutions, that's probably the one I would pick. It sounds like a good compromise between efficiency and creating a safe assembly.

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