Replies

1. 
   Jon_R | Sep 30, 2019 10:33am | #1

   Your plan is for an unvented/conditioned attic or a vented/unconditioned attic? Unless you have ducts up there, I suggest the latter - all insulation on the floor.

2. 
   BrianPontolilo | Sep 30, 2019 10:47am | #2

   Hi John.

   As Jon R mentions above, the approaches--insulating the roofline vs insulating the attic floor--are very different. Insulating the roof line (vented or unvented) brings the attic into the conditioned space of the house. Insulating the attic floor keeps the attic outside of the conditioned space (cold in the winter, hot in the summer).

   In your location, R-49 is what the International Residential Code would like to see in your attic or roof. Most high-performance builders shoot higher than that. Since an unvented, insulated roof is a riskier assembly, and closed cell spray foam more costly in many ways, I think insulating the attic floor with blow in fibrous insulation is a better path for you. Of course, after doing some really diligent air sealing. I assume that you have seen these articles, but in case you haven't, check out:

   How to Build an Insulated Cathedral Ceiling
   Creating a Conditioned Attic
   Air Sealing an Attic
   How to Insulate an attic floor

3. 
   johnchard | Sep 30, 2019 11:24am | #3

   Thanks Jon and Brian.

   I forgot to mention. Our second floor mini split will be ducted and the air handler and duct work will be up there. Its an old attic with a flimsy timber door and narrow stair case. The attic floor does have cellulose blown in. The efficiency contractors from Eversource have tried some simple weather stripping. This will be our first winter here and I worry it will be like a fridge with a plywood door!

   So given this and the ductwork, I thought foaming it would make sense to at least make it airtight?!

4. Expert Member
   Dana Dorsett | Sep 30, 2019 01:11pm | #4

   It can sometimes even be cheaper to build an air sealed utility-chase/platform over the mini-duct cassette & ducted region and install R49 above the platform than to put R49 at the roof deck. Installing short ducts to ceiling diffusers at the interior side of the rooms with ceiling diffusers designed to throw air toward the exterior walls/windows can keep the size of that platformed area small. An insulated weatherstripped hatch or door to the utility chase under the hatch can provide service access. That approach would require air dams in every joist bay that crosses under the mini-wall of the chamber.

   Since it's not structural (it's only holding up insulation, not the ceiling, not the roof) it can all be done with 2x4 framing and sealed half-inch OSB on the exterior side of the framing.

   Some thought has to be given to the return path ducting. A common return grille in a section of ceiling (perhaps set up for swapping the filter media without going into the attic) using partition wall cavities as jump ducts to the common areas can usually work. With a grille into the partition stud bay near the floor in the doored-off room and near the ceiling on the common space side of the walls provides better sound & light isolation than door cuts at the threshold, and better mixing than transom grilles or jump ducts penetrating the ceiling.

1. 
   johnchard | Oct 01, 2019 10:22pm | #5

   Thanks Dana. We have a somewhat awkward little space up there and the flimsiest of attic doors on super old stairs.

   So what I am hearing is-
   1. No ductwork, then R49 the floor
   2. Ductwork, then I could create an air sealed utility chase with R49 outside (ie a room within the room) or I foam and seal the roof deck at 4" at <R49

   Is this right? Is there an option where its roofline PLUS attic floor?

   thanks,

   John