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Rigid foam overcoat method on my own house

Matthew St Martin | Posted in Plans Review on

Hello,
Been reading this website on and off for the last 7 years or so. I really appreciate the knowledge base here.

It is time to re-roof my own house, Denver, climate zone 5b. I plan on incorporating something similar to the JLC August 2014 article, “Project Overcoat” with a couple of caveats.
Starting at the bottom: Existing 2×4 rafters 24″ OC with R13 paper faced FG stuffed (from an attic conversion in 90’s), 1×6/1×8 skip sheathing, 4″ tall 2x sleepers on edge screwed through 1x and into 2×4 rafters, (2) 2″ Fiberglass faced poly iso insulation in 4″ cavity, 1″ XPS (for thermal break), 2x sleepers laid flat screwed through XPS into 4″ sleepers, 15/32″ OSB, synthetic roof paper, and asphalt shingles.

My SE likes the 4″ sleepers for strength (granted its a thermal break) and I will run them long and build new fascia/soffit from them, similar to cutting rafter tails in the air after the roof is stacked. I will cut away the existing sloped soffit/tails.

Now, questions:

Will adding 1″ of XPS add enough of a thermal break?

Do I need a bituminous membrane (Grace) over the existing 1×6/1×8 (or over a layer of OSB applied directly on top of the 1×6/1×8) for air control? The attic space is finished with drywall on the ceiling/knee wall, no can lights, just fixtures mounted to the peak.

What else am I missing?

Thanks,
Matt

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Matt,
    How will you make the 4-inch-deep rafters to hold the 4 inches of polyiso? I imagine that you will have to rip those out of 2x6s. That's a lot of work.

    I don't like the idea of cutting rigid foam into narrow strips for a cut-and-cobble job, but I understand that the existing 2x4 rafters are undersized.

    Maybe you could install 2x4s or 2x6s above the existing sheathing to satisfy your engineer -- don't rip them to 4 inches -- and stuff the cavities with mineral wool or fiberglass. Then you can install a continuous layer of R-20 rigid foam above that.

    It's a good idea to include an air barrier above your existing sheathing, but you don't need to use peel-and-stick. You can install a synthetic roofing underlayment with taped seams if you want.

  2. Matthew St Martin | | #2

    Thanks Martin. I appreciate your response. I would set the rip guide on my skillsaw to 4" and rip the 2x6s. I suppose I could re use the off cuts for sleepers on top of the 1" foam.

    I understand completely the "cut and cobble" rigid insulation from a thermal break point of view.
    If I strengthen the rafters, then put foam on top, I will have to draw up a multiple fascia detail to accommodate that thickness, not sure how it will look at this point.

    I like the idea of the synthetic felt and tape, any suggestions on brands of tape, and acrylic or butyl?

    thanks,

    Matt

  3. D Dorsett | | #3

    Putting high-R/inch foam between R1-R1.2/inch framing is a waste of good foam, since the thermal bridging cuts the average performance by a large double-digit fraction. In a 4" deep cavity separated by 2x timbers 24" o.c. the difference in performance between R4/inch insulation and R6/inch insulation is worth about 1/2" of continuous EPS over the top of it.

    As long as there is a soffit to ridge vented 1.5" gap between the new nailer deck and the insulation layers below it's fine to use a low permeance membrane air barrier over the existing structural roof deck, but as Martin says, it isn't really necessary. If you DON'T have the vented air gap you can't use anything with a permeance less than 0.5 perm or you'll have a moisture trap.

    Try this stackup, from the top down:

    ...shingles

    ...#30 felt

    ...new 1/2" OSB nailer deck (it's not structural.)

    ...4" continuous fiber faced roofing polyiso (temperature derated value ~R20 in your climate)

    ...R15 rock wool in new 2x4s glued & screwed in directly above the original rafters

    ...original roof deck

    ...R15 rock wool between original 2x4 rafters

    ...MemBrain air barrier & vapor retarder cualked at edges, and seams taped (seams only where supported by framing)

    ...Ceiling gypsum

    That gets you to about R50 center cavity, but due to the R20-ish thermal break over the rafters it'll outperform a code-min R49 by quite a bit. The original strutural roof deck is totally protected from interior moisture drive by dew point control of the exterior insulation, and the new nailer deck is protected by the combination of ~0.5 perm facers on the polyiso, the ~1 perm vapor retardency of the original roof deck, and the MemBrain. The polyiso/rock wool interface is at an R-ratio sufficient to eliminate moisture accumulation as well. You may want to install a 1/2" OSB layer above the new 2x4s to better support the polyiso but 2lb roofing polyiso isn't as fragile as the 1lb foil-faced goods. That layer is dew-point protected by the R-ratio, but also becomes another ~1-perm vapor retarder into the stack.

    The nailer deck needs to be through-screwed to the new 2x4s 24" o.c. with pancake head timber screws, penetrating them at least 1.5". With 4" foam and 1/2" OSB above & below the foam means you'd need a minimum length of 6.5" on those timber screws. (7" is probably better) If you skip the OSB between the foam & 2x4s you can use 6-6.5" screws. In buckets of 250 you're looking at about 50 cents per screw.

  4. Matthew St Martin | | #4

    Thanks Mr. Dorsett.
    I do not plan on removing the existing ceiling sheetrock, so I will not be able to install the MemBrain material. I am not 100% sure if the FG insulation in between the existing rafters is paper faced or no facing.
    I do plan on venting the whole assembly, 1-1/2" airspace although I recall Dr. Lstiburek saying 2" is better.
    I am open to opinions on the air barrier membrane, just don't want to spend the $ on Grace if I don't have to. Funny how when it's your own money how important each specification becomes.
    Again, my major consideration would be how to design a soffit (probably sloped) and fascia detail to accommodate this additional thickness.

    Matt

  5. GBA Editor
    Martin Holladay | | #5

    Matthew,
    This illustration from the Building Science Corp. web site might give you some ideas on your soffit and fascia.

    .

  6. Randy Bunney | | #6

    D Dorsett wrote:
    ...R15 rock wool in new 2x4s glued & screwed in directly above the original rafters -

    Creative thinking in "re-purposing" existing rafters

  7. Matthew St Martin | | #7

    Martin,
    Thanks, that does give me some ideas. I am thinking that Frieze Board would cover the old rafters at the heel. The wall will grow the height of a plumb cut of 2x4 rafters on an 8:12 (after chopping off the overhangs), and the frieze will look good/like it was always there. The new 2x4 "over rafters" will form the bottom of the sloped soffit, and I can attach a taller subfascia to stop the foam.

    Dana, where is the vented space in your assembly above? I am thinking 2x sleepers on top of 4"of Polyiso, with long pancake screws, then the new 1/2" OSB deck on top of that. Or am I mis reading?

    Matt

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