Advice on hip roof venting and underlayment choice
Ranch house 50′ x 30′
Hip roof 6/12 pitch
Attic has been air sealed and insulated. Brief details:
All existing fiberglass batting removed.
Air sealing completed.
The existing 2×8 rafters were birdsmouth cut at the top plate leaving about 4″ between the sheathing and top plate. Not enough room for soffit ventilation and proper air sealing and insulation in this critical area. So closed-cell spray foam was installed in lifts building up to 9″ thick.
18″ of cellulose installed elsewhere.
Time now for reroofing before adding PV.
The existing roof has a ridge vent and two “mushroom” vents not far from the ridge. The mushroom vents will go away. The ridge vent will be replaced during reroofing.
I’m thinking that adding ridge vents along the hip rafters will allow some convection and help keep the attic temperature down. The entire length of the 4 hip rafters wouldn’t need to be vented (sheathing cut into), just the lower portions. Enough to pressurize the attic, so slightly more vent area on the lower hips than at the ridge. Of course the ridge vent cap would need to be installed the full length of the hip in order to maintain proper roof flashing details.
At the eaves where there is spray foam, I’m thinking that a vapor permeable roof underlayment is more appropriate here than a traditional vapor impermeable underlayment (ex. ice and water shield) because the risk of ice dams is low and if water does get to the sheathing the thickness of the spray foam at the eaves will greatly limit drying to the inside. So the entire roof could have a vapor permeable underlayment.
I appreciate thoughts, comments, advice, etc.
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