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Insulation under flat roofs JFM 10-26-2006
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Posted by JFM on October 26, 2006, 8:13 pm


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

Posted by kevin on October 26, 2006, 8:21 pm



JFM wrote:
> 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 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


Posted by marson on October 26, 2006, 8:53 pm


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.


Posted by marson on October 26, 2006, 8:54 pm


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.


Posted by buffalobill on October 27, 2006, 5:53 am


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:
> 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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