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My house has raised tie trusses:

Raised tie trusses

This means that on the second floor the central part of the ceiling is flat, but near the long walls it is slanted. The trusses are 2x6 timber construction, meaning that there is minimal space for insulation in these areas (red squares in the image).

In the space outlined in blue there is more space for insulation, but this area needs to be vented to prevent the roof from overheating and damaging the asphalt shingles.

We are planning to replace the roof with metal in the next year or two. While making this change, I would like to increase the roof insulation. Short of replacing the trusses, what are my options for improving insulation with this roof construction type?

These are the ideas we have considered:

  1. Open the ceiling from the inside and replace the blown-in cellulose insulation with closed cell spray foam.
  2. Add a layer of rigid foam above the sheathing and below the new roof. This may cause problems with roof venting.
  3. Remove the exterior sheathing and sister in 2x6s or even 2x8s to create additional space for insulation.

Below is a picture of the roof from the outside showing the small vent.

Picture of roof

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  • Query: You mention blown in cellulose insulation is already in place. I'm assuming it's loose. Is this type of insulation just above the horizontal blue zone or is it also in the red zones? If it is in the red zones has the entire triangular section formed by the angled ceiling & wall been filled in with loose cellulose fiber, or does the thickness of the insulation vary from very thick at the bottom to thin at the top, near the blue zones?
    – Fred
    Apr 30 at 3:27
  • @Fred that's a good question. There is no venting at the rafter tail, only the peak, so I assume the whole space in red is filled with cellulose (or at least that an attempt was made to fill it). But the attic access is only large enough for my head, so I can't actually investigate without opening the ceiling. I added a picture of the exterior.
    – LShaver
    Apr 30 at 14:28
  • From the photo of the external of the house, I would be surprised if there is any blown in insulation in the red zones. The gap between the ceiling and the roof looks very narrow. The type of insulation for such a situation would need to be solid & sheet like, something similar to what @blacksmith37 mentions. I think it would be easier to replace the insulation at the same time as the roof is being replaced, particularly in the red zones. The people at SE Home Improvement might be able go give some ideas.
    – Fred
    May 1 at 11:56
  • Regarding the blown in insulation in the roof. Prior to doing anything, ensure it's something benign like cellulose fiber & not asbestos.
    – Fred
    May 5 at 20:32

1 Answer 1

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In a similar situation; I cut one inch thick foil faced foam board to fit snuggly between between the joists. I left one inch space between the foam board and roof sheathing for air circulation up from the continuous eve vents. I then put 3.5 in. paper covered fiberglas batts in the remaining space between the joists. Drywall went over the batts in sloped sections. A contractor was building the house but I put in insulation except the fiberglas blown into the attic

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  • Was this a renovation? Did you notice much improvement?
    – LShaver
    Apr 30 at 23:05
  • New construction, nothing to compare it to. And this design was was a small fraction of the total roof ; less than 10%. May 1 at 0:00

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