Insulation under flat roofs

I posted this once before using FreeAgent, but it never showed up. I even went to Google groups and searched there without finding it, so it apparently just evaporated. I'm therefore trying again and hope it does not result in double posting:
Post ---------- A flat roof covering an addition to my house had to be completely re-done from the walls up--new rafters, decking, roof. I had it framed, decked, and re-roofed by a roofer, but I must finish the inside.
Maybe if I can describe this properly, no photo will be necessary: The rafters run from the inside wall and rest upon (and extend beyond) the outside wall. There is plywood decking with rubber roof on top. No soffit yet. So it is completely open between the rafters from the soffit area, above the walls, to the inside. I will insulate the inside room area between the rafters. I will have vinyl soffit and facia applied later, and the soffit, will, natrually, be vented.
The question is how should the insulation be separated from and protected from the soffit area which will be vented to the outside?
Seems to me that I should nail up some planking or staple up some plastic (at the walls between the rafters) to protect the insulation from the outside air.
Or, should this be left open for ventilation purposes?
JFM
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JFM wrote:

The whole point of ventilating the soffits is to ventilate the attic, and in particular, to keep the *underside* of the roof at the same temperature as the outside air.
If you have an attic of any sort up there (i.e., a large-ish gap between the inside finished ceiling and the roof sheathing), you want all that air to be directly vented to the outside as much as possible. The more the better. If there is no space (like my house, which has cathedral ceilings and just a 6 inch gap between the drywall and the roof sheathing), then you want to create a small, well ventilated air gap between the insulation and the underside of the roof seathing. There are styrofoam baffles available for this purpose, that create a thin channel from the soffits right up each rafter cavity. There are other similar baffles that just keep the insulation from closing off the tight space where the roof meets the wall, so that air can pass from the soffits into the attic space.
Look up attic ventilation for some nice images.
-Kevin
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here in minnesota, energy code requires you to apply "windwash blocking". that is because the cold air strips the heat out of the insulation where it is exposed to the outside. We used to use a scrap of OSB between the truss heels, leaving about a 1 inch gap between it and the roof. then, if you don't have enough room for insulation, you will need a foam baffle fastened to the roof sheathing to allow an airspace for ventilation. Recently, i have discovered cardboard insulation chutes, which are stapled down to the top plate, then go up to within an inch of the roof sheathing and fold over. provides a wind wash block and a ventilation space in one.
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here in minnesota, energy code requires you to apply "windwash blocking". that is because the cold air strips the heat out of the insulation where it is exposed to the outside. We used to use a scrap of OSB between the truss heels, leaving about a 1 inch gap between it and the roof. then, if you don't have enough room for insulation, you will need a foam baffle fastened to the roof sheathing to allow an airspace for ventilation. Recently, i have discovered cardboard insulation chutes, which are stapled down to the top plate, then go up to within an inch of the roof sheathing and fold over. provides a wind wash block and a ventilation space in one.
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see climatized answer from manufacturer at: http://www.owenscorning.com/around/roofing/zip.asp?ckie=complete see comprehensive info at: http://www.buildingscience.com/resources/default.htm
JFM wrote:

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Sounds OK, with a vapor barrier below the insulation on the inside.
Nick
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This is called "birdblocking" where I've been. Its the same size lumber as the rafters. A hole make be cut and blocked with heavy galvanized screening for air flow. If you go this route you don't need a soffit.

Up to you.

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google "windwash barrier"
Jonny wrote:

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