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Insulation confusion

aztecrsf | Posted in General Questions on

We’re in the final stages of insulation. Some sections of the roof use trusses, and the height of the truss at the top plates is around 6″. Our required insulation is R30 (our Title 24) for ceilings, but that would compress the batts down to about half their height. No big deal, this is San Diego where temps are mild anyway. But I’m concerned that the last 12″ or so of the roof sheathing and the blocking between the trusses at the plates will be in contact with the batts and really poorly ventilated (e.g, the only air to get to that last 12″ will have to pass through the fiberglass). 

In other parts of the house where we had truly unventilated roof spaces (vaulted ceiling) we used rigid foam. That would be a very tough solution to implement at the last minute for the truss ends. 

Am I overthinking and overworrying here with the truss ends? Perhaps drop it to R19 for the last 12″ so it’s not packed tight? 

Thanks for your opinions, as always. 

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  1. Expert Member
    BILL WICHERS | | #1

    Put in vent baffles at the eaves to keep a minimum (per code) 1" gap between the underside of the sheating and the baffle interior for ventilation. Then you can stuff insulation under the baffle without blocking off the vent channel. If you're expecting to compress batts out at the eaves, I'd use site built baffles using 1/4" wafer board and furring strips as spacers / mounting flanges to support the wafer board baffle down from the sheathing. These will be more durable than the prefab plastic or foam baffles you can buy for the purpose.

    You'll find it's easier, cheaper, and actually better (from a thermal performance standpoint) to use loose fill (blown) insulation on the floor of a vented attic than to use batts. I'd use blown cellulose here, but you can get loose fill fiberglass too. Fit as much as you can out at the eaves under the vent baffles and don't worry about it -- in your climate zone ice dams aren't a problem, so you'll just have a little less insulation out at the eaves but not so little as to really be a problem.

    Note that it's better to use closed cell spray foam in unvented cathedral ceilings than it is to use "cut'n'cobble" rigid foam, since you can't keep rigid foam panels reliably air sealed over time, which can lead to potential moisture problems.


    1. aztecrsf | | #3

      I can always count on excellent answers here. Thanks!

      To be clear, the house is now insulated (too late for blown-in, and the attic is very tight, no floor anyway).

      I spotted this issue, among 100 other things, as something that I questioned whether correct. The insulation contractor is not helpful, and is out of the picture. It's up to me to get it right. Baffles are a good idea, I think I can get them locally, so I'll look into that.

      I would have loved to use spray foam. Here in San Diego it is very rare, very difficult to find a contractor for that, and the one bid we got was crazy expensive. Bummer.

  2. walta100 | | #2

    Seems to me you are putting the cart before the horse. Most homes lose more energy to air leakage than thru the insulation and if you install the insulation without air sealing first the insulation will make air sealing later almost impossible.

    Consider having blower door directed air sealing done before you insulate.


    1. aztecrsf | | #4

      Walta, we out the horse first. I spent a few days air sealing everything I could find, including gaps all around those truss blocks. Approx 50 tubes of caulk. My hands ached.

  3. aztecrsf | | #5

    Reporting back on this...our insulation contractor did what appeared to be just a slightly sloppy job at first glance. Lots of little things. But digging deeper, they skipped insulation in corners behind blind stud walls, over recessed lights, around the rim/band joist between floors all the way around the building, over interior name it. I spent 4 full days fixing all that and fitting the baffles as suggested.

    Thanks to GBA for the great reading all these years and the advice above.

    1. Expert Member
      BILL WICHERS | | #6

      Note that not insulating over recessed lights might have been on purpose: if those fixtures aren't
      "IC" rated (IC = Insulation Contact), then you aren't supposed to allow insulation to come in contact with the fixtures, and you'll need to "box over" them with rigid foam, leaving an air space inside between the walls of the box and the fixture itself. There are also purpose-made products to replace the "box". Another option would be to replace the recessed lights with IC-AT rated (insulation contact, air tight) fixtures. The "AT" rating is a little optimistic though -- I like to call those "leak less" fixtures. I wouldn't count on them to be all that air tight, despite the rating.


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