Q. I am trying to plan the best way to ventilate a hipped roof above an attic. Since the ridge is very short, ridge venting is not an option. I was planning to install powered exhaust ventilation. How should I size the fan?

A.Bill Rose, architect and building researcher at the University of Illinois in Urbana-Champaign, responds: Although many building codes require attic ventilation, there are more important factors than ventilation when it comes to designing an attic and roof system that works well. First, be sure the following points have been covered:

  • seal any cracks where air can move into the attic or cathedral cavities from below, especially at mechanical chases and the access hatch;
  • insulate well above the living space;
  • keep mechanical equipment and ductwork out of unconditioned spaces;
  • divert rainwater well away from the foundation in order to keep the foundation from contributing excessive humidity to the house; and
  • flash the roof correctly.

And ventilation? For hip-roof homes, I’ve had occasion to recommend soffit plus mushroom vents, soffit only, and no vents at all, depending mostly on the predisposition of the client. Clients concerned about strictness of code enforcement and shingle warranties should have ventilation by the book, and should have simple designs to match the simplicity of the venting requirement. Clients with complex conditions — dormers, cathedral sections, flat-roof sections, shed sections — as well as clients in fire-prone regions, should investigate construction without vents. I suspect that soffit-only venting may give some relief of occasional water without the strong suction of high vents or the possibility of rain or snow entry, and I’d encourage more research and experimentation with this approach. I’ve never had occasion to recommend powered venting. It’s noisy and expensive to operate, and I haven’t seen evidence of a payoff. If you still want to know how to size an attic exhaust fan, my answer is: as small as possible.