Question for HVAC gurus.

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I was hoping this one would be different, at least for this one question, my first post to the group.
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Hey dummy, your the dummy considering bad advise from the wifeys ex, just remember the ex might make you shrink in size till your wife needs the ex.
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Someone with your spelling and grammar skills calling anyone else dummy is funny.
If it is bad advice, explain why. Do you know anything about the subject matter or do you just like to spew your ignorance as a part of daily life?
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Alim Nassor wrote:

You need good ventilation under the roof deck to cool the roof. Otherwise, the shingles will die early.
Insulating the roof will require more insulation for the same job.
Why foam? You can probably use fiberglass for way less $, and it can be removed if you need to work on the wireing. In an unused attic, you can have as much insulation as you want.
Will you really have an attic space, or just a bunch of gaps between trusses?
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The roof will be galvanized R-panels. The roof framing will be trusses. Foam is a good air barrier as well as insulation.
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Alim Nassor wrote:

A layer of plastic will be just as good of an air barrier.
How does the foam survive over time when subjected to the extreme heat a metal roof will transfer? Especially if there is not a ventilated airspace between them.
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I don't know. Another reason to insulate the floor and not the roof?
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Alim Nassor wrote:

That's my thought.
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Thanks Bob
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Your builder and wife are both correct but I don't think you understand that they meant doing both the ceiling and the roof. Your logic is also correct given your apparent misunderstanding of what they meant. The roof insulation must be installed properly with an airspace between the roof and insulation providing an air duct from the eve to the ridge. If you don't do this the life of your shingles will be greatly reduced. I suggest you don't let this attic space go to waste.
Jimmie
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Either method will work well if done properly. Heat loss (in the winter) or gain (in the summer) depends on the surface area of the thermal envelope and insulation values. The actual volume of the conditioned space does not matter directly. Since the roof is a larger area than the ceiling, if you make the roof part of the thermal envelope you will have somewhat more heat loss/gain, unless you insulate it to a correspondingly higher R-value.
But it is not a large effect, and there can be good reasons to condition the attic space. For example, if your HVAC equipment will be in the attic space, then there are advantages to having all the ductwork in conditioned spaces.
As others have mentioned, if you do insulate the roof, then the roof deck can get quite hot in the summer unless you take measures otherwise. Depending on the roofing material, this can be a problem (e.g. with asphalt shingles). But I believe you mentioned using a galvanized metal roof; I don't think roof deck temperature would be an issue for that material.
Cheers, Wayne
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Thanks Wayne.
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Alim Nassor wrote:

Are you sure you want to build a house with a woman who's still that involved with her ex?
I'd recommend seeking advice from a clinical psychologist rather than an HVAC guy ;-)
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LOL. I'm glad they can be civil. With me and my ex it is more like civil war.
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On Fri, 5 Mar 2010 00:20:25 -0800 (PST), Alim Nassor

Not a Psychologist.
We live in the Mojave desert. A friend's new construction a few years ago had the attic ceiling sprayed with ICYNENCE tm foam.
See this page: _Insulating an attic ceiling_
http://www.oldhouseweb.com/how-to-advice/insulating-an-attic-ceiling.shtml
Product web site:
http://www.icynene.com /
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Thanks.
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