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<DIV>I suggest you define the thermal envelope above the roof sheathing.
Install eps foam above the sheathing across the rafters and install purlins
above fastened to rafters with a long screw. Unlimited space allows you to
choose the thickness of foam that is optimal for thermal and cost benefit.
I much prefer a vapor permeable foam to a non permeable foam because humidity
can pass through. Therefore, providing ventilation above the foam allows
vapor to passively exhaust from the envelope. The space between purlins
can provide ventilation horizontally. Using corrugated metal roof that can
span between purlins, the rib provides an opening for air and vapor to rise
vertically. A Ridge vent is needed. The complications of this
approach are in how the thermal envelope transitions into the vertical plane
(wall). However, this approach achieves complete continuity. </DIV>
<DIV> </DIV>
<DIV>Eli </DIV>
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href="http://www.lightfootdesignbuild.wordpress.com">www.lightfootdesignbuild.wordpress.com</A></DIV>
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<DIV style="font-color: black"><B>From:</B> <A title=sacie.lambertson@gmail.com
href="mailto:sacie.lambertson@gmail.com">Sacie Lambertson</A> </DIV>
<DIV><B>Sent:</B> Friday, September 9, 2016 10:59 AM</DIV>
<DIV><B>To:</B> <A title=Greenbuilding@lists.bioenergylists.org
href="mailto:Greenbuilding@lists.bioenergylists.org">Greenbuilding</A> </DIV>
<DIV><B>Subject:</B> [Greenbuilding] innovative solution
sought</DIV></DIV></DIV>
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<DIV class=gmail_default
style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif">All, We have an old
house in which we want a well insulated unvented attic. We are replacing
the existing roof (with metal) and installing a minimal layer of XPS on top of
the sheathing. We plan to use closed cell spray underneath followed by dense
pack cellulose between the rafters. The problem is the rafters, which are
only 2x4s in this old house. I am looking for a creative least expensive
way to provide more rafter depth for the insulation. </DIV>
<DIV class=gmail_default
style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif"> </DIV>
<DIV class=gmail_default
style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif">(We do not want to
build up the insulation on the top of the roof sheathing because doing so
requires the added cost of completely rebuilding the existing soffits, which are
in good condition). </DIV>
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style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif"> </DIV>
<DIV class=gmail_default
style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif">Will appreciate all
your useful suggestions.</DIV>
<DIV class=gmail_default
style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif"> </DIV>
<DIV class=gmail_default
style="FONT-SIZE: small; FONT-FAMILY: verdana,sans-serif">Sacie</DIV></DIV>
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