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Blown-in fiberglass in 24 inch parallel chord truss

joenorm | Posted in General Questions on

I had the fourth insulation contractor out to bid my job. And they are the fourth to be reluctant to blow in the cavity of my 24 inch deep parallel chord truss. It is territory nobody around here seems to be familiar with. The last company was concerned with the weight and whether the netting can hold the product. He also told me it’d be around R-80 or 90 which does seem a bit overkill. Not to mention it will cost about $3K more than putting R-60 worth of batts up there.

At this point I am being reminded how I felt when I set out to do an unvented crawlspace and was only met with opposition. I eventually just got tired of it and caved, and went the easy route of sticking with what everyone does around the area.

My feeling is the weight of fiberglass is not that much, but I do not know for sure. Another option is to install 5/8 sheetrock first, then blow in the cavity.

Part of me just prefers not to challenge these installers too much, they’re good at what they know but I don’t want to end up with a crappy install because it’s the first time they have done something.

Any advice or thoughts appreciated.

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  1. user-723121 | | #1


    What is the pitch of this roof? I have done as you are describing with a dedicated air space and placing the ceiling drywall on except for about 24" near the peak. This allows the insulation installer to blow the rafter bays full from the top and then net the top 24" to complete the blow. Should be cost competitive with batts and fill around the trusses much better. Any number of quality insulators in MSP would do this job.

  2. joenorm | | #2

    Roof is 2/12 pitch

    Agreed, it would fill around all the truss intersections much better. As far as being cost competitive, it will be a $2,200 adder to do the roof blown-in rather than batts. Worth it?

    I get a little nervous when the contractor is out of their comfort zone, but how hard could it be?

    How did you detail your dedicated airspace?


    1. user-723121 | | #4

      Pine strips (1 1/2") fastened below roof decking, then 1/2" fiberboard sheathing fastened to the pine strips with 1/2" crown staples. I would look around for another contractor, blown insulation should be cost competitive with batts. A mess can be made using batt insulation in between roof trusses. There will likely be gaps as most insulation is 15" or 23" wide.

      1. joenorm | | #6

        There would certainly be gaps in the truss webs. but not on the chords. They are two foot on center, insulation is designed for that.

        Every contractor so far has charged a premium for blown-in. It's just not very common in this area.

  3. Expert Member


    If you go with batts you don't end up with as efficient an installation as you would with blown insulation. But you save 2G, and even with the loss in efficiency end up with a much higher R value roof that is usually used in the PNW.

    Use batts.

    1. joenorm | | #7


      This is very likely good advice. I am not sure if I would even notice the difference between the two and $2K is not insignificant. As you know, the PNW is generally a pretty mild climate.

      1. Expert Member
        Akos | | #8

        If you go with batts, make sure to order batts for metal studs for the ones between the truss webbs. These are wider and will fit better.

        1. joenorm | | #9

          Are these widely available? The idea being that they'd be forced into the truss voids a little better?

          1. Expert Member
            Akos | | #10

            My local box store stocks certain sizes, but any commercial drywall supplier carry them.

            The 24 oc metal stud batts are actually 24" wide, so it will be a tighter install.

  4. StephenW81 | | #5


    I am probably going to be facing almost exactly the same issue. Have you considered Johns Mansville Spider Plus blow-in fiberglass insulation? My understanding is that it will stick to the underside of the roof/ventilation channels and the truss members without requiring netting. However, I have no personal experience with it.


    1. joenorm | | #11


      I have read about but not considered Spider insulation. I live in a rural area with limited options as far as who services the area. It's a vacation destination so building is a big industry but people tend to do things one or two ways. The guys doing things like cellulose, or anything different for that matter, tend to charge a major premium.

      If blown-in fiberglass was the same cost as batts for me, It would be the better choice with the trusses and trying to get insulation around all the nooks and crannies.

      At this point I am leaning toward the suggestion from others here to use two layers of tighter batts. I can save over $2,000 and put that money toward the cost of my PV system. In my(what I consider mild) climate I may never notice the difference of the two insulation installs so the cost savings is significant. If I lived somewhere colder, I might be thinking differently.

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