Venting a Complicated Cathedral Roof
I am new to the forum. Climate zone 5 (Chicago area.)
The approximately 400 sq ft sun room (an addition)and the rest of the ranch house is roofed with heavy weight cedar shake with standard felt. The sun room has 3 large skylights on the west. I would like to convert to asphalt shingle roof. The sun room has 3 ft wide soffits with vents. There are no ridge or mushroom vents. The roof has a 3/12 pitch and is heated by a separate furnace without a humidifier. I do not know how much insulation is in the cathedral roof. The skylight tunnels are about 10 inches deep. I can see by opening one of the soffit vents that the insulation batt fills the rafter bay and touches the underside of the sheathing leaving no air gap for ventilation.
I have been trying to anticipate problems going from shake to asphalt.
I have read that going to asphalt from shake means that you might have ventilation problems due to the fact that synthetic underlayment and an asphalt roof are vapor barriers.
One of the roofing sales people agrees that I should have more ventilation. I would like to have more but I can’t see how to do it. Not only is there not an air gap under the sheathing but a ridge vent would be significantly shorter than length of the room. On the north side some of the shorter rafter bays could be served by hip ridge vents.
The 3 skylights block many of the rafter bays from the ridge on the west side of the house. The south located chimney blocks some of the east and west rafter bays . The south side of the sunroom joins to the main house , so there are no soffits for vents on the south side of the sunroom. On the north there is another sloped roof that covers the north side and has a window at the gable. This roof does not have a ridge.
The question is what to do when there is a problem venting due to the design of the roof. Even if I compressed the insulation with raft-R mate baffles in some of the rafter bays and installed a ridge vent 30-40% of the roof would still not be fully ventilated. The rafters prevent any free air flow laterally under the roof.
So far in 32 years I have not had any ice dams or known water damage. The sunroom was reroofed 19 years ago and there was no problem with the sheathing.
The salesman suggested I close the soffit vents. I am reluctant to have a closed system given the vapor barrier nature of synthetic underpayment and an asphalt roof. I tend not to believe that a closed system can be maintained. Should I try to make the modifications needed to truly convert to a closed roof? I have no idea if during the original construction a vapor barrier was placed. The salesman said if i don’t close the soffitt vents I should put in a ridge vent even though he acknowledges that much of the roof won’t be served by this vent.
Any opinions on converting from cedar shake/felt to asphalt and synthetic underlayment?
There are no ceiling penetrations in the sunroom ceiling other than the 3 large skylights, I have read that water vapor gets into the ceiling cavity mainly thru penetrations. The relative lack of penetrations and the unhumidified furnace air may help explain why there have been no problems.
Does anybody know of a reliable residential building consultant in the Chicago area?
The shaded areas on the diagram represent areas that would not be affected by the ridge vent
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