[Greenbuilding] innovative solution sought

[conservation architect]

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.  

Eli 
www.lightfootdesignbuild.wordpress.com

From: Sacie Lambertson 
Sent: Friday, September 9, 2016 10:59 AM
To: Greenbuilding 
Subject: [Greenbuilding] innovative solution sought

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. 

(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).  

Will appreciate all your useful suggestions.

Sacie


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