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

[Reuben Deumling]

On Sat, Nov 26, 2016 at 9:44 AM, RT <ArchiLogic at yahoo.ca> wrote:

> 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.
>
I've scabbed 2x6 rafters onto what was still usable of the original 2x4
rafters and run them to the ridge. Where this was not feasible I started
afresh with 2x6s from fascia to ridge/hip. 26 years ago I did a version of
your cap idea to this house, but in no small part thanks to what I've
learned here on the greenbuilding list over the past 16 years it was time
to redo this particular fix. The flat part of the original (up to 1990)
roof left less than five feet of height upstairs, which was not exactly
ideal as living space, inspiring the first modification.

>
> 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 .
>
That would indeed be a lot of insulation. Given that there are a number of
dormers poking through this hip roof, the logistics of making this work
would be daunting. The plywood gussets were definitely a close second to my
chosen criss-cross pattern of rafters and purlins. I settled on the purlins
mostly because it seemed to me to offer the simplest/most effective thermal
break option I could think of.

>
> 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.
>
Yes to collar ties. But the length of these collar ties still remains
undecided.
In the last one of these I did I sized the collar tie to be invisible by
keeping its profile low enough to not poke through where the 11" thick
insulated panels on both sides of the ridge met. I'm leaning toward a flat
ceiling thinking through the ventilation issue more, but am open to any and
all suggestions.

>
>                                 ---- * ----
>
> 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")
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20161126/8193a761/attachment.html>