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

[Michael O'Brien]

Hi, Reuben--

I think some metal roofing has high reflectance and emissivity, would reduce the heat in the attic. I wonder if a foil sheet stapled to the bottom of the rafters would add more reflectance, not sure of your configuration.

Best, Mike
Sent from my iPhone

> On Nov 26, 2016, at 7:20 AM, Reuben Deumling <9watts at gmail.com> wrote:
> 
> Thanks for your reply, Topher, and good to hear from you. This list is not quite what it once was... Have folks drifted elsewhere, and if so I wonder where those places might be?
> 
> 
>> On Sat, Nov 26, 2016 at 7:03 AM, Topher Belknap <topher at greenfret.com> wrote:
>> 
>> Greetings,
>> 
>> Thermal losses should be considered on the basis of surface area, not
>> volume. 
> Agreed.
>  
>> Heat stratification (for good or ill) is pretty insignificant in a modern, well insulated and sealed house. 
> I've noticed this myself and tried to discuss this with the heating man who came to advise/make a bid for the additional duct work to the new upstairs rooms), but I don't think I was able to persuade him. 
>  
>> Venting ought to be the same for either option (if I am envisioning correctly).  Since it should
>> run up the outer surface, not the interior. 
> I am aware that standard practice has me installing vent channels between the insulation-in-the-rafter-bays and the sheathing, that connect vents at the bottom to ridge vents--we've discussed this here over the years--but I also think I remember folks arguing that in certain situations one could skip those channels. Am I misremembering this or just experiencing wishful thinking? I confess that getting the venting right seems to me the biggest challenge at the moment. 
>  
>> My concern would be ensuring that the vapor barrier (if your climate wants one) is put in
>> the right place. 
> Ah, vapor barriers. I'd love advice on this. There is currently a layer of roofing felt on the outside of the sheathing. The batts of insulation I'm planning on using for the walls and sloped roof sections of the envelope are paper-faced fiberglass. I was going to rip the paper off the batts that run vertical and keep it on the inner layer that runs horizontal. Is that a viable approach?
>  
>> If you decide on the flat ceiling, fill that space with insulation.
> I was certainly planning on insulating above the horizontal sheet rock, but are you suggesting filling the entire triangle?  
>> 
>> Thank you kindly.
>> 
>> Topher
>> 
>> --
>> Topher Belknap
>> Green Fret Consulting
>> Kermit didn't know the half of it...
>> http://www.GreenFret.com/
>> 
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