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Unvented vented roof with exterior rigid insulation

jetlaggy | Posted in Green Building Techniques on

Hey All,

I am doing an energy retrofit on a Zone 5 simple box gable roof. Part of this calls for redoing the roof. I am sold on the exterior rigid foam idea to cut down on thermal bridging and also since my roof slope is very low and the eaves are shallow.

Code calls for R38 but I want to shoot for about R50. The planned assembly going upward is:

1. R20 of fluffy between upper truss cords
2. Structural sheathing with taped seams
3. Air barrier
4. First layer of 3″ polyiso rigid foam
5. Second layer of 3″ polyiso rigid foam with staggered and taped seams
6. Roof underlayment

My question is in venting the underside of a metal roof to add to the performance since ice damming can be a problem around here.

It seems you would need to use horizontal purlins to add a gap between the metal roofing and the underlayment. However horizontal purlins seems to be contradictory to air flowing up to the ridge and water possibly damming.

Could anyone make any recommendations on this scenario?

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Jet,
    It's not possible to install metal roofing with 7-inch fasteners, so you'll need either a second layer of solid sheathing above the 6 inches of rigid foam, or (if allowed by the manufacturer of the metal roofing product that you are specifying) you will need 1x4 or 2x4 purlins above the rigid foam.

    An air gap under the metal roofing is not required, but if you want one, you can always include one. An air gap between 1x4 or 2x4 purlins (installed parallel to the ridge) provides benefits, even though there is no soffit-to-ridge airflow. If you insist on soffit-to-ridge airflow, you can install 2x4s perpendicular to the ridge, with each 2x4 above a rafter or roof truss. Then you can install a layer of OSB or plywood above the air channels.

    That makes for a nice roof, but it's a little expensive.

  2. User avatar
    Armando Cobo | | #2

    The purlins do not need to be continuos, we space them with 2"-6" gaps between them to allow air and water pass though. Peter Pfeiffer, an Architect from Austin, TX, likes to install the purlins at 45° allowing air and moisture to "move" up the roof to a vented ridge cap, I think is a great idea but needs more quality control.. Maybe you can Google his approach.

  3. jetlaggy | | #3

    @Martin - Thank you again for the great information. Most of what i've learned is from this site. Since my siding is also vertical I need to do the 2 layers of perpendicular furring to achieve a proper rain screen so it sounds like the same can be done on the roof as well. Is the top layer of OSB necessary or can the roofing just go directly on the outermost layer of purlins?

    @Armando - I have seen this approach in many of Matt Risinger's projects (in Texas) and while it certainly makes a lot of sense I was curious if anyone had negatives on this approach. I'll definitely need to consult the roof panel manufacturer to see if they recommend installs on diagonals purlins. It'd certainly save a lot of labor and materials.

  4. Malcolm Taylor | | #4

    Jetlaggy,

    If you install the purlins on a diagonal, you need to figure out how to vent the ones that don't make it to the ridge but end up terminating on the gables (the two triangular areas at each end).

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