blown insulation

EHH week 11: roof venting

The roof of this house is one large plane tipped up to the western view.  This shape allows us to stay under the 25 foot height limit (not easy to do with a two story house) and simplifies rainwater collection because everything flows towards one side of the house.


Because we are superinsulating, we are using the full depth of the roof joists for blown-in insulation.  Well, all except the upper 1-1/2 inches which must remain void to ventilate the roof.  This 1-1/2 inches allows air to carry away any vapor that condenses into water on the underside of the sheathing.  Without the ventilation space, the roof structure would rot.


The ventilation space must run continuously from the eave at the low end to the eave at the high end in every single joist bay.  I like to think of an ant crawling on the underside of the plywood from one edge of the roof to the other. 


Trouble is, seismic design wants to have the roof sheathing fully nailed at its perimeter to the tops of the exterior walls.  On this house, we solved the issue with semicircular holes in the perimeter blocking that allow nails between the holes.  At the rakes (the sloping sides of the roof) we had to lower the required cross blocking to avoid blocking the air passage.  And at the one part of the roof that is overframed, we drilled over 100 holes through the lower sheathing to let air pass up and out.