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

[Reuben Deumling]

On Sat, Nov 26, 2016 at 9:42 AM, Michael O'Brien <obrien at hevanet.com> wrote:

> 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,
>
When you say 'bottom of rafters' can you explain? Are you saying staple the
foil sheet to the inside faces of the purlins - which would then touch the
back side of the drywall?


> not sure of your configuration.
>
standing seam painted metal roof -> tar paper -> 5/8 plywood -> 2x6 rafters
+insulation batt -> 2x4 purlins (horizontal)+insulation batt -> sheet rock
-> prime+ paint

>
> 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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>
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