[Greenbuilding] thermal implications of a vault vs flat ceiling at R-38?

[RT]

On Fri, 25 Nov 2016 12:17:20 -0500, Reuben Deumling <9watts at gmail.com>  
wrote:

> restoring farmhouse ...roof .. 12-in-12 pitch,

> new rafters are 2x6 and I'm going to run 2x4s perpendicular to them

> What I haven't decided is whether to leave the ceiling vaulted or frame  
> in a normal flat ceiling at 8 or 9 feet

Reuben;

It's unclear to me whether you're leaving the existing roof as-is and just  
adding a "cap" over the flat roof at the peak or installing new rafters  
 from the top plate of the second storey walls.

If the latter, I'd be inclined to configure the new 2x lumber as 24" deep  
spaced rafters (aka site-built parallel chord trusses using 12" wide x 24"  
deep plywood gussets spaced at 1/3 or 1/5 points along the span of the  
rafter) rather than as the proposed 2x6 rafters with 2x4 purlins.

The 24 inch rafter depth would make for very strong, stiff rafters  that  
would also allow for 18"-plus  of insulation and a ventilation air space  
over the insulation .

As for vaulted  vs flat ceiling :   I'd put a collar tie across the  
rafters and utilise that collar tie as the top of ceiling with sloped  
ceiling below that point.
"Soaring" (or as one realtor's mis-print appropriated called it  
"sorrowing") vaulted ceilings that extend all the way to a pointy peak get  
old pretty quick.  Pointy-peaked vaults may be well suited for structures  
intended for the Divine but for mortals,  modest vaults truncated by  
ceilings at collar ties are IMO, more appropriate to the human scale.

                                 ---- * ----

If the proposal is just to extend the slope of the existing roof by adding  
a "cap" over the flat roof at the peak of the existing roof, I think that  
I'd look at making the "cap" portion as a glass over-roof that would  
function as an enclosure for a solar batch heater (if feasible in that  
locale) or a small wintergarten or rooftop greenhouse or clothes dryer or  
[other].

PS: This message was CC'd to Reuben just in case the Listserver software  
inexplicably removes the body of the message and sends out a null message  
again.

-- 
=== * ===
Rob Tom T6015O
Kanata, Ontario, Canada

< A r c h i L o g i c at Y a h o o dot C a >

(manually winnow the chaff from my edress if you hit "Reply")
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