Replies

1. Expert Member
   Dana Dorsett | Oct 19, 2018 09:10am | #1

   By "...being forced to double / sister ALL rafters with 2×8’s..." is that for structural reasons? Even with some polyiso edge strips for mitigation doubling the framing fraction is going to take a thermal hit. (BTW: That reduces the cavity width to 13" , not 14.5".)

   In stead of sistered on 2x8s, can you make a truss out of it? Use a 2x2 chord and half-inch 6 " x 6" plywood gussets 24" on center glued and nailed to the chord. Install 3/4" edge strips on the 2x6s (or chord) to fill the space between the 2x2 chord and the 2x6, then glue & side-nail the gussets to the 2x6. That will be the same 7.25"total depth as a 2x8, and almost as strong. If you were going to take it to 8.25% with 1" edge strips, use 2" polyiso instead of 3/4".

2. 
   bradesp | Oct 19, 2018 09:41am | #2

   Unfortunately we're converting a stick built truss to a true cathedral and we're using steel tie-rods to hold the two walls together once we remove the bottom chord. So, yes, the 2x8 sistered Rafters are unfortunately a requirement from a structural POV.

3. Expert Member
   Dana Dorsett | Oct 19, 2018 03:22pm | #3

   With R14 of closed cell foam you're good for about R28 of fiber or open cell foam insulation from a dew point perspective. If you're at 8.25" total depth (2x8 + 1" polyiso) that leaves 6.25" of space to fill.

   R28/6.25= R4.5/inch.

   Half pound foam is only R3.7/inch, so at 6.25" would deliver ~R23 for R37 total. Open cell 0.7 lb foam is about R4/inch, or about R25. for R39 total, which is better.

   But with polyiso on the rafter edges you won't be able to trim the open cell foam flush using the normal tools without scarring up the polyiso.

   In this instance sculpting an R30 rock wool batt designed for 2x8s but compressed to 6.25" from it's 7.25"manufactured loft will deliver about R27, which is still fine from a dew point control perspective, an you'd be at R41 total.

   Damp sprayed 1.8lb JM Spider would deliver about R26, for R40 total. This is probably your best bet, since it can be rolled flat without crushing the polyiso, and would deliver a complete fill. Other 1.8lb fiberglass (Optima, L77 et al) would have to be blown in netting adding an extra step to the process.

   To stick with the open cell concept, using 1x furring over the edge strip polyiso (Bonfiglioli strip style) would add about R3 to the open cell numbers, and would stand up to the abuse of foam trimming. At 7.0" total depth 0.7lb foam would deliver R28, which is still OK from a moisture accumulation risk point of view- it won't get into the humidity ping pong situation that sometimes occurs with all open-cell solutions.

4. 
   bradesp | Oct 20, 2018 08:15am | #4

   Dana, this is amazing and gives me several options to explore... Most importantly, it sounds like my plan for 2" inches of HFO closed cell foam plus the right compliment to fill the cavities keeps me out of harms way from a moisture pov.

5. 
   Deleted | Oct 20, 2018 08:19am | #5

   Deleted

6. 
   bradesp | Oct 20, 2018 08:26am | #6

   I found your furring design reference! For others, this style of furring for insulation foam can be found here:

   https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

1. Expert Member
   Dana Dorsett | Oct 20, 2018 10:38am | #7

   I apologize for not linking to that useful detail document- I usually do when mentioning Bonfiglioli by name. Glad you managed to find it without me!