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Community and Q&A

Air sealing plumbing/skylight penetrations in insulated over-roof?

jchas | Posted in Green Building Techniques on

Looking at doing a chainsaw reno on a 1942 1-1/2-storey gable house. Region 4 Coastal.

Proposed stack-up (bottom-to-top) on the roof is:
– existing drywall
– existing 6-mil poly
– existing R-28 fibreglas batts over the inhabited part (poorly installed) with no ventilation chutes, no eaves vents
– existing 1×6 skip sheathing (from original cedar shake roof)
– existing OSB roof sheathing
– new BlueSkin VP160 vapour-permeable peel-and-stick
– new 8″ of Roxul comfortboard IS, staggered
– new 2×4 strapping laid on side, aligned with rafters
– new plywood sheathing
– new tar paper
– new asphalt shingles or metal roof

Given the ventilation channel between the Roxul and the shingle deck, there would be air leakage around the roof penetrations if they were sealed/flashed to the shingle roof deck in the traditional way.

How should we air seal the roof penetrations down at the BlueSkin layer yet still provide flashing up at the shingle plane?
– skylights?
– 4″ plumbing stack?
– brick chimney?

Thanks!
…/j

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    Q. "How should we air seal the roof penetrations down at the BlueSkin layer yet still provide flashing up at the shingle plane? skylights? 4" plumbing stack? brick chimney?"

    A. You can use the peel-and-stick BlueSkin to seal air leaks at the skylights and the plumbing stack.

    However, I don't think you can use a peel-and-stick to air seal around a brick chimney. You'll have to revert to the tried-and-true method used to air seal at a chimney: use metal flashing, with the seams in the flashing sealed with high-temperature silicone caulk.

    Once you've installed the metal flashing as an air barrier, you can adhere your peel-and-stick to the metal flashing.

  2. charlie_sullivan | | #2

    I would add that up at the metal or shingle roofing layer, you'll still need regular flashing for all of these things for shedding rain. That sounds trivial but personal experience and anecdotal evidence suggests that finding people who do this properly is way more difficult than it ought to be. It has nothing to do with tricky modern airtight building techniques, but just finding people who bother to use established best practice.

  3. jchas | | #3

    Thanks for the help guys. Just to make sure I'm clear on what you're saying...

    Skylights:
    - Extend the curb right up so it's 3-1/2" proud of the topmost sheathing plane
    - blueskin the entire outside of this new curb
    - install skylight on top of new higher curb and flash as with a traditional roof deck
    ** How do you handle the venting of the cavities above/below the skylight locations?

    4" Plumbing Stack
    - use blueskin around the base of the stack where it penetrates the current sheathing layer
    - install traditional lead flashing/counter flashing where it emerges from the new higher sheathing
    **Is pipe movement an issue? Would a rubber gasketted flashing work better/worse at the level of the original roof sheathing (allowing some movement)? Or is it better to stick with all the same material (blueskin)?

    Chimney
    -Air seal at current sheathing level with metal/high temp caulk (4" gap between wood and chimney?)
    -Flash at new roof level in the traditional way
    **Can roxul (being fire retardant) be installed tight against the brick?

  4. GBA Editor
    Martin Holladay | | #4

    John,
    Q. "How do you handle the venting of the cavities above/below the skylight locations?"

    A. According to the standard advice given on GBA, if you have a skylight, you shouldn't be attempting to build a vented roof assembly. You want an unvented roof assembly (because the skylight precludes venting). For more information, see How to Build an Insulated Cathedral Ceiling.

  5. GBA Editor
    Martin Holladay | | #5

    John,
    Q. "Can Roxul (being fire retardant) be installed tight against the brick?"

    A. People do it, but you're not supposed to. The code insists on an air space (usually 2 inches) around the chimney.

  6. jchas | | #6

    Thanks Martin. I was thinking about the venting around the skylights some more. What if they vented horizontally into the two adjacent rainscreen bays (by not having the 2x4 strapping completely meet the top and bottom skylight curbs)? So the air in the skylight cavities would be able to go around them in an hourglass pattern? I seem to recall a similar approach to rainscreening around window bucks in walls. Workable?

  7. Dana1 | | #7

    On masonry chimneys clearance to combustibles needs to be 2" in some places, 1" in others, but where the chimney passes through assemblies suchs as floors & roofs non-combustible air barriers are code. Installing completetly non-combustible high melting point insulation such as rock wool isn't expressly allowed, but not particularly dangerous as long as it's not insulating the combustible materials near the chimney a lot hotter in the even of a chimney fire or failure.

    http://www2.iccsafe.org/states/seattle/seattle_residential/PDFs_residential/Chapter%2010.pdf

    http://premiumaccess.iccsafe.org/document/code/343/5670979

  8. GBA Editor
    Martin Holladay | | #8

    John,
    Lots of people have tried the method you suggest (trying to convince air to flow sideways around a skylight). These methods don't work all that well -- research suggests there isn't much air flow when these methods are attempted.

    That doesn't mean your roof will fail. The higher the R-value of your insulation, and the more airtight the construction details, the more likely that your roof will perform well.

    However, if your ceiling is a little leaky, or your R-value is low (or if your R-value is provided by fiberglass batts, which are notoriously bad), you may end up with problems or ice dams. That's why GBA recommends an unvented approach for roofs with skylights.

  9. charlie_sullivan | | #9

    "That's why GBA recommends an unvented approach for roofs with skylights."

    That might merit qualification with "if you must have a skylight." For example, one article here is titled "Why I hate, hate, hate skylights." For a more balanced discussion, see:
    https://www.greenbuildingadvisor.com/articles/dept/building-science/pros-and-cons-skylights

  10. GBA Editor
    Martin Holladay | | #10

    John,
    For further arguments against skylights, see Martin’s Ten Rules of Roof Design.

  11. jchas | | #11

    Great roof design article. Unfortunately, the house already has skylights and, after living with them for 15 years, it'd be a tough sell to cover them up. :-)

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