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Rigid insulation on a pitched solid wood roof deck

Robert Damico | Posted in Green Products and Materials on

Have a cathedral ceilinged residence with 4×6 tong & groove hemlock exposed to the interior supported by gluam arches at 15′ on center. Want to reshingle this uninsulated roof with thick nailable rigid insulation on top of the woo deck.

Is there a guide for determining insulation thickness and selecting a vented or non-vented system? Have gotten very mixed answers from architects and engineers, and the manufactures of the systems are of little help. Since non-vented is the leastexpensive, it could offer a better payback. Am in 5,500 degree day climate in New York. – Thanks, Rob

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Replies

  1. Aj Builder, Upstate NY Zone 6a | | #1

    Non vented roof up my way looked nice for a few years. Then small leak. Then they ripped apart roof to find leak. Then they found the whole roof was hiding rot. Then they were looking at redoing all with a vented roof this time. This redo was pricing out at tens of thousands... of dollars.

    Now, make your choice. Pay now... pay later... get lucky... roll the dice.

    This is a real world answer not conjecture. Joe L. also says... better to vent your type of roof especially in colder climates

    BSC.com

  2. Expert Member
    Armando Cobo | | #2

    You may want to read the 2009 IRC, Section R806.4 or 2009 IECC, Chapter 4. If you are in Climate Zone 5, you need R38 insulation on your roof or if you are in CZ 6 then you need R49. One option you may have is to install 2x6 or 2x8 roof rafters on top of your 4x6 T&G hemlock, fit or fill cavities with closed cell rigid or spray foam, then your roof sheathing, WRB and roofing. Other option is to install a WRB on top of your 4x6s, then 6” or 8” of rigid foam, then 1x4 nailers, then roof sheathing, a second WRB and finally roofing. The later will give a ventilated roof assembly. Hopefully you have a way to keep indoor relative humidity below 40%, intermittent or continued ventilation, and develop very good detailing for moisture management and air sealing.
    For more information check:
    https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-insulate-cathedral-ceiling-mineral-wool
    http://www.ecologicinsulation.com/files/JLC_-_Spray_Foam_Insulation.pdf

  3. TJ Elder | | #3

    I like Armando's second suggestion: install 6" to 8" polyiso over the 4x decking, then 2x4 furring on the flats (perpendicular to the eaves), then plywood sheathing, asphalt paper and shingles. The 2x4s create a 1.5" vent space. Use long roofing screws to hold the 2x4s down to the 4x decking.

  4. Keith Gustafson | | #4

    Roof leaks mean you need a better roofer. I believe in venting a sunny shingled foam roof to help save the shingles, but others disagree. This roof system has been used for over 40 years with no venting and little rot issues.
    There is NO reason to build any structure over your roof, screw the foam down directly.
    Use as much foam as you can stand, multiple layers staggered seams. You need plywood on top so that your roof surface can be replaced without destroying the foam, so adding sleepers for venting is not a big additional expense. I vented mine behind the facia, but they make a vented drip edge also.

  5. Robert Damico | | #5

    Armando’s technical references were well taken. The ventilation issue, in spite of some compelling papers on the issue, is not truly conclusive. However the longevity ventilation offers for the shingles and the dual benefits for both seasons are leading me to consider biting the bullet and spending more – the risks are just too compelling.

    Because the 4” roof deck offers such a solid nailing surface, I’d like to add as little redundant structure as possible. There are numerous nailable OSB/rigid insulation systems from the flat roof industry that are now used for the residential/pitched roof sector – so I think I’ll give that a shot, hoping small town contractors will be more reliable installing a more ready-made panel than using hours of labor on putting a package together.

    The fascia/overhang detail for this job will be touchy. Imagine a solid timber overhang extending out 3 feet with the underside being exposed. Cladding/insulating the underside of the overhang will reduce the heat loss from the interior but that’s a lot of overhang underside to deal with. I’ll have to insulate at least a portion of the underside. Also, being a 4 inch thick roof structure with inches of insulation to boot, the fascia will be beafy has heck.

    Your comments are pushing me in a more confident direction – thanks for the reality checks.

  6. Keith Gustafson | | #6

    Things I did on my similar roof:

    Drilled a 1/4 hole at every wood joint right down to the wall and injected a squooosh of expanding foam.

    fixed curbs for skylights, much less likely to leak. Regular deck mount ones are just not made for our style roof

    Hunter panel makes a integrated foam/spacer/osb that they will special order plywood. It was not worth the money to me, and they charged for shipping[as opposed to flat roof iso foam]

    Add a rigid spacer at the roof edge to hang the facia from.

    Also
    Adding foam to the walls where the roof meets is diminishing the 'badness' of the thermal bridging tho not eliminating it. With the consultation of an engineer, one could cut out one t+g board immediately above the wall and replace it with foam

    Mock up your proposed facia so you know what you will be looking at. There are many ways to fool the eye, and a good designer could help. With such big overhangs[as I have] I thought of later creating a double roof line, kind of a roof over a roof that would allow a huge amount of insulation without that funny thick roof edge.

  7. Robert Damico | | #7

    Have to say, I never considered removing a t&g 4x6 where it runs along the top of the wall. Structurally, that's not a problem for the roof loading. Certainly a better premium to pay than wrapping the entire overhang! Don't quite understand the 1/4" holes but do get the challenge of getting that fascia thinner. Thank you

  8. Keith Gustafson | | #8

    There is an air leak at every t+g seam, when you have the roofing off it is the only time you can truly fix it invisibly.

    Removing the board is just an idea, make sure someone smarter than me thinks it is a good idea.....

  9. Robert Damico | | #9

    Got it - Plank removal concept rings true because I'm an architect and therefore know everything................I'll run it by a contractor I know who knows better.

  10. Aj Builder, Upstate NY Zone 6a | | #10

    Many if not most thick roof facias are created by doubling the facia one overlapping the other. It looks like a home that has a thick outsulation cathedral ceiling just as a bear eating beechnuts looks less intimidating than one facing you standing on his hind legs with many teeth to take a picture of... before said bear gives a few bear hugs and minor scratches and bites here and there.

    (if you don't enjoy twisted humor once a day or so... pass on parts... of my serious posts... )

    ;)

    Robert... thanks for the smile your last post brought to the Southern Adirondacks today... Don't fear the tall facia... befriend it my man...

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