Does anyone know where I can get info on how much air flow is resticted when horizontal strapping is used on top of roof joists? I have 14 inch TJI’s with R40 insulation which leaves only 2 inches of unrestricted space. This would normally be fine with sufficienr soffit and ridge venting but unfortunately substantial areas of these chases will be blocked by chimneys, skylights and dormers. Years back I read somewhere that cross strapping on top of roof joists in this situation actually decreases air flow because the air moving up the cavity along the underside of the roof sheathing is obstructed by the strapping. It supposedly causes a turbulence which restricts the upward air movement in the rest of chase cavity. Thanks!
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I'm not sure about all that. But I do know that we've worked for one GC that had several hand framed roofs, he had us strap the roofs with 2x4 before sheathing. I'm trying to remember....I think maybe the strapping was 16" oc. in any case it sloved the problem as far as the dormers etc not getting vented.