Replies

1. Expert Member
   Dana Dorsett | Oct 03, 2018 02:32pm | #1

   With an inch of air gap.channel vent you're looking at about R23-ish compressed batt in the remaining 6.25" for that sloped 4' section of roof, which isn't a lot, but way better than nothing. If you can add rigid polyiso insulation on the interior side of that sloped section the thermal performance can be improved a lot, at the expense of a few inches of headroom.

   What is the R-value of the rest of the attic?

   What is your climate zone/location?

1. 
   CwTs68Z | Oct 03, 2018 02:52pm | #3

   I’m in climate zone 5, Chicago north suburbs. Rest of attic is empty right now, getting ready to air seal attic floor. Was planning on lots of cellulose insulation.

1. Expert Member
   Dana Dorsett | Oct 03, 2018 03:18pm | #4

   There's no definition for "...lots of cellulose insulation..." but at a code min R49 every square foot of the sloped section with R22-R23 has more than twice the heat loss of an R49 roof. With 3" of continuous polyiso (~R19) on the interior side of the slope it'll be a lot closer, and might even hit code-min on a U-factor basis.

   If installing 20"+ cellulose everywhere else the ceiling losses will be dominated by the sloped section, even if it were filled with 6" of 2lb polyurethane unless there is some rigid insulation on the slope.

1. 
   CwTs68Z | Oct 03, 2018 03:59pm | #5

   Dana, for the rigid polyiso, do you mean inside the rafter bay against the backside of the plaster ceiling?

1. Expert Member
   Dana Dorsett | Oct 03, 2018 04:28pm | #6

   Behind the ceiling gypsum, but NOT between the rafters.

   If slipped between the rafters the thermal bridging of the R1/inch rafters would subtantially rob R6/inch polyiso of it's potential performance. Put the cheap stuff between the rafters. The 6.25" of available rafter-bay fill can be the same cellulose you're blowing elsewhere, and would perform at about R23 at that depth.

2. GBA Editor
   Martin Holladay | Oct 03, 2018 02:42pm | #2

   John,
   This article describes all your options: "How to Build an Insulated Cathedral Ceiling."

3. 
   CwTs68Z | Oct 03, 2018 04:48pm | #7

   Dana,
   All 3 2nd floor rooms have plaster walls and ceilings. I’m not willing to take that down.
   This house is 68 years old. What other options do I have?
   The smart baffles that I was going to use are rigid plastic and provide a 2 inch airspace.

4. Expert Member
   Dana Dorsett | Oct 03, 2018 05:33pm | #8

   With a 2" baffle you only have 5.25" of space below the baffle. If you're not going to re-do the ceiling install R23 rock wool batts in that space. It'll only perform a R22 when compressed to 5.25", but that's still way better than nothing. If you ever re-do the interior you can improve on it at that point with interior side foam.

   For a huge amount of money and only marginal benefit you COULD install 5" of HFO blown closed cell foam bringing it up to R35 at center cavity, but barely moving the needle over what you'd get with R23 rock wool, due the severe R5-R6-ish thermal bridge through 5.25" of rafter.

5. 
   CwTs68Z | Oct 04, 2018 03:27pm | #9

   Thanks Dana,
   I think the R23 rock wool batts is what I’ll use in the sloped ceiling area and 20 inches of cellulose in the remaining attic area. I really appreciate your help with this.
   Thanks much!
   John McLoughlin