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Community and Q&A

Insulating Decking in Unvented Roof

storunner13 | Posted in Energy Efficiency and Durability on

We just bought a 1928 house in Minneapolis (zone 6).  It has an open gable roof with a cross gable (addition) addition at the front.  The roof needs to be replaced, so I’m planning to redo the insulation and venting.

One of the previous owners tried to insulate the between the rafters in the kneewall space with what looks like R15 fiberglass batts.  Nearly 75% have fallen out (no wonder we get ice dams).  Additionally, there are currently no soffit vents, nor is there a channel from the eave to the ridge, which has resulted in some mildew on the backsides of the decking in the attic.

I’m looking for some feedback on a plan to insulate below the roof on the back side.  I’m going to pull out all the old fiberglass batts, and stuff them into bags to fill the wall cavities in the kneewall space.  I’m trying to source some reclaimed 4″ polyiso boards to cut and fit between the rafters (5.5″, leaving a 1.5″ space below the decking for an venting channel to the ridge.  I then plan to attach 2x4s horizontally to furr out the rafters — I’ll place an additional 4″ of polyiso to this space, and finally attach an outer layer of .5″ of foil backed board to the outside and redo the drywall in the sloped closet.  The attic has some blown in insulation (<4″) already.  I plan to foil board over the joists to air seal and then blow in more insulation on top (17″ or so).

With the new roof being added, I’m planning to have the roofers install good attic ventilation near the ridge (with a ridge vent or similar) and then cut in continuous soffit vents at the back eaves.

Please poke holes in my plan.  I will likely not be able to add full R49+ to the cathedral ceiling that is above the bathroom.  Additionally, we don’t have the time or money at the moment to try to overhaul the insulation on the front of the house (kneewall spaces are sealed), so I will get to that at a later date.

Should I consider adding insulation (3-4″ of polyiso) to the current roof decking and redecking the whole roof?  What is the latest information on sealing up roof decking with insulation and having an unvented roof?  If this is worthwhile, I may consider the added expense.

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  1. Jon_R | | #1

    I'd start by reading IRC 806.x carefully. Then present your plans and questions in terms of the clauses there.

    1. storunner13 | | #2

      Hi Jon. My plans for venting the roof complies with 806.1 - 4. I'm mostly asking about plans for insulating with 8+" of polyiso between the furred out rafters in the kneewall space. What am I not considering in this plan?

      My secondary question was regarding adding insulation to the top of the roof sheathing--this would comply with 806.5.1.4 as it would be >R-5. However, adding insulation to the top of the sheathing means that I would need to air seal all the attic spaces (unvented), which likely is not an ideal option considering the 1.5 story construction, and poor insulation under other parts of the roof.

  2. Jon_R | | #3

    A properly vented design, good interior side air sealing and as much insulation as practical sounds good.

    Where did ">R-5" come from? Z6 unvented designs need much more foam.

    1. storunner13 | | #4

      Upon further review, R-5 is in reference to the minimum amount of air-impermeable insulation that needs to be BELOW the decking.

      We've gotten off topic. I'm still looking for feedback on my original questions, which are process related, not code related.

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