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Venting a Complicated Cathedral Roof

heetseeker | Posted in Energy Efficiency and Durability on

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?
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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  1. user-2310254 | | #1

    Is the cathedral ceiling connected to the attic in the main house? Do you have any recessed lighting in the ceiling?

    Be sure to read this article on cathedral ceilings if you haven't already:

    Since you are reroofing, you could install reclaimed rigid foam on top of the sheathing. (See But keeping the skylights might complicate things.

  2. heetseeker | | #2

    Thanks your reply. Yes I have read that excellent article.
    The cathedral ceiling does not connect to the attic space of the main house. No ceiling fixtures other than a smoke alarm..
    One roofer suggests that I put baffles just under the decking of the cathedral ceiling compressing the insulation a little bit and putting smart vents on the hips, ridges and underneath the north window where there is no ridge. Does anyone have experience with a similar solution for a cathedral ceiling?

    Any experience with smart vents?

    Any experience with DCI rafter vents that allow the individual rafter bays in the cathedral ceiling to communicate and allows flow of air around skylights?

  3. heetseeker | | #3

    As far as putting rigid foam on top of the sheathing, I think that would complicate things as far as the skylights. I don't know much about the details of such a process. I suspect I might have to put as much as 4 inches of rigid foam on and put new sheathing on top of the foam.
    I also don't know how that would affect the gutters and valleys.

    1. Expert Member
      BILL WICHERS | | #4

      Rigid foam shouldn’t make things much different with the skylights, you’d just build a a higher curb to get the skylight up where it needs to be, sort of like a window buck but on the roof.


  4. Expert Member
    Akos | | #5

    If you can vent most of the roof, you should be fine. It might be that adding in the venting would cost the same as re-roofing with cedar in the end.

    I think the idea of adding in baffles above the insulation and vents at the ridge at the peak and the ridge along the hip section is a reasonable path. The DCI smart vents look similar to a standard ridge vent, you can use either for venting in both areas.

    To be extra safe, you can also do shingle vents bellow the skylights. This doesn't hurt but isn't always needed especially if your ceiling is well sealed bellow.

    You can also go with a permeable underlayment bellow the shingles. Shingles are still slightly permeable at 0.6 perm, so it would still provide a bit of extra drying capacity.

  5. heetseeker | | #6

    Thanks to everyone who has replied to my post. I value all your opinions.

    I also had thought of using a breathable underlayment. Made sense to me. I did some research online.
    Almost by accident I found an interesting article to share that suggests that a permeable membrane may not provide much value.

    "Repetitive testing on this application showed the
    multi-layered asphalt shingle system—when installed per
    the manufacturer’s instructions—has an average moisture
    vapor transfer rate of 0.65 perms. Demonstrating a measured
    perm of less than 1.0 showed that moisture transfer through
    the multi-layer asphalt shingles is negligible.
    This important fact begs the question: If the asphalt shingles
    act as a vapor retarder on the roof, then what value is added with the introduction of a breathable roof underlayment
    below it? Any moisture within the roof deck or the attic will not be able to move through the roofi ng system, regardless
    of the permeability of the roofi ng underlayment layer. "

    Testing shows breathable
    underlayment provides
    little or no moisture
    transfer value in asphalt
    roofi ng systems.
    Dr. Joseph Lstiburek, Ph.D.,
    P. Eng., ASHRAE Fellow,
    Building Science Corporation
    Dr. Achilles Karagiozis,
    Director of Building Science,
    Owens Corning
    Paul Gassman, P. Eng.,
    Product Engineer,
    Owens Corning

  6. Jon_R | | #7

    See here (a highly vented case) where permeable underlayment made the sheathing somewhat drier. IMO, when you have small amounts of wetting, .6 perms of drying will eventually be significantly drier than 0.0 perms.

  7. heetseeker | | #8

    Thanks for your reply. I am no expert but my reading of the part from the article quoted below seems to indicate that they think the advantage would go to the moisture impermeable membrane in real world situations.

    Page 212 " Larger, less well-constructed, aged roofs that include
    penetrations are more typical of
    actual construction practice; and
    in these cases, the improved water
    ingress resistance provided by the
    self-adhered membrane as compared to the roofing felt would likely outweigh the potential reduction
    in drying capacity."

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