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Would redoing the roof earlier than planned change insulation advice and be cost effective overall?

tswanick | Posted in Energy Efficiency and Durability on

Zone 5/6 line in small NH town with no code officials. Retrofit for 1820 two-story Cape 1-story kitchen/dining sandwiched between main house and two-story attached barn that is ¾ converted to living space, ¼ garage (garage poorly insulated with sliding barn door that leaks to air and critters). Granite/rubble foundation, poured concrete floor with good drains but bulk water ingress in the springtime, so planning gutters and likely interior drain on uphill side and sump also to mitigate radon.

Converted space has cathedral ceiling (no purlins, 5″ rafters which all butt together with no ridge beam), board sheathing with gaps to felt paper, dormers, soffit vents, no gable or ridge vent, batting and T&G. T&G and FG batts will come out, although one vendor said to rake out FG and dense pack and air seal over the T&G despite leaving unvented: “we do this a lot; they’ve done this in NE forever.” Other vendor advised to spray foam this roofline and dormers and fireproof them. This is the worst section for ice dams.

Wish we could dense pack the roofline with smart membrane, as we’d love to not do something as permanent as spray foam to an antique cape. SF seems the only way to code R value without 475’s method of extending rafters to 10″. We love that rock wool can be used to dense pack.

Middle attic has 4″ purlins between 6″ king rafter beams, board sheathing, no dormers, roof vents as only way to ventilate (between two two-story house sections). 8″ FG under floorboards that is only storage in house. Advised to SF roofline by one vender, vac out and SF joists by other. Concern is future wiring/perforation of SF seal, so leaning toward the roofline.

Main house attic has 6″ king rafters and purlins, small gable vents, dormers with the tiny gable vents,no soffit vents, loose rock wool between joists. Second story slopes are blown with rock wool.

Main house stays almost 20 degrees cooler than the 80-90s outdoor summer temp. now, for most of day, being kept shut with insulated window shades. (All opened all night for cooling like my mother and grandmother did.) These hot days, basement is wet on the floor from in spots not from rain.We run fans down there to keep air moving and have a heat pump water heater.

Advised to SF the slopes and dormers, and blow in to code on attic flat. One vender said enlarge gable vents due to no soffit vents.

We have paused to consider that when we bought our forever home in 2014, it was projected to need new roofs in 5-10 years. Shingle over felt may be replaced by metal, possibly removing barn dormers for future solar.

We are somewhat paralyzed with the fear of permanent (not green/unhealthy) SF but mostly because of this beautiful antique cape. We even wonder if moving up the redoing of the roofs along with the insulation project could change the insulation advice to allow funds to shift from expensive SF to dense pack rock wool and cover more of the combined roofing/insulation job, albeit sooner than originally planned (just to avoid SF – are we crazy)?

I’ve subscribed and read numerous Q&A threads and have read several of the basic intro articles (such as insulating cathedral ceilings). Very much appreciate any ‘if it were mine to do’ advice, in light of the “permanence” of the project. Thank you, Terry

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  1. GBA Editor
    Martin Holladay | | #1

    Your post is long. I suspect that you have provided more information than necessary. Your question seems to be something along the lines of, "We own a difficult-to-insulate old farmhouse and barn with lots of cathedral ceilings that are impossible to vent. But spray foam makes me nervous. What should I do?"

    Unless you are able to create ventilation channels in every rafter bay from a soffit vent to a ridge vent -- and it sounds like you can't -- you have only two options:

    1. Install an adequately thick layer of closed-cell spray foam on the underside of the roof sheathing, or

    2. Install an adequately thick layer of rigid foam above the existing roof sheathing, followed by a second layer of roof sheathing and new roofing.

    Details on these approaches are provided in my article, "How to Build an Insulated Cathedral Ceiling."

    Good luck.

  2. tswanick | | #2

    Thanks for distilling everything swirling in my head, Martin! If some of the original 200 yr old sheathing boards are tapered from the mill leaving cracks, open to the felt paper/shingles (flexible to the touch), is this an issue for installing spray foam without installing exterior foam/sheathing/new roofing? Also, currently there is some mold on the interior of the sheathing which the SF would cover. Is this more likely from conditioned air movement into the leaky attic, (will SF halt this?), or from bulk water/damp basement whose resolution may not be timed before the SF? Should drainage happen first? Thanks again

  3. seabornman | | #3

    Just a note of caution. Having renovated an 1820 house myself, the rafters are often undersized and widely spaced. Fortunately they were (usually) old-growth wood that is stronger and more resilient than current wood. No problem when waste heat from the house melts off snow relatively quickly. Good insulation will make snow load higher and last longer, so a quick review of structure by engineer may be called for. A metal roof has the advantage of shedding that snow more quickly.

  4. Expert Member
    RICHARD EVANS | | #4


    The mold on the under side of the roof sheathing could be from conditioned air from the house- as you stated. Once that warm, humid air comes into contact with the cold sheathing it condenses there. Closed cell spray foam would mitigate this because it is both an air barrier, and less importantly, a vapor retarder. Unlike fiberglass batts or other fluffy insulation (which are not air barriers), closed cell foam will prevent any warm, moist air from ever hitting the sheathing.

    Given what you have told us though- specifically that you 1.) have concerns about the spray foam and 2.) will likely be getting a new roof next year- I would opt for exterior rigid insulation. (Martin's option 2)

    Two layers of 2" or 3" polyiso over the roof deck would warm up that board sheathing and allow for 'fluffy stuff' to be dense-packed between the irregular rafters. You would be approaching R-40 with this set up. The polyiso could also be taped and provide something of an air barrier for you. (unlike the board sheathing.) This is also a 'greener' option as polyiso has much more benign blowing agents compared to Closed cell foam. The only challenge would be finding someone to do it and how to deal with the dormers and fascia details. Reclaimed polyiso is cheap and is readily available in MA and VT. (search for polyiso within craigslist- the one in VT will deliver for free.)

    Good Luck!!

  5. Expert Member
    RICHARD EVANS | | #5


    You asked about adding spray foam to board sheathing and any issues that may arise from the gaps between the boards. Martin wrote a great piece on this a couple of years ago. Its more intended for walls but I think there are takeaways for your project:

    Also- here is a link to the reclaimed/ cosmetically blemished polyiso in Vermont that will deliver to most of NH for free.

  6. tswanick | | #6

    Joel you’re point is well taken! I can arrange a consult. Good advice as we definitely have undersized timbers. We have been working on correcting the mistakes made when dormers were added (some would say, that’s the mistake). They cut sections out of the purlins and did not support those now-severed beams. It would be much easier to fix them from above and not gut it like we did for one section, and like we would have to do in order to open all the slopes for spray foam. Which makes Rick’s nudge toward Martins option #2 more compelling. Thank you, Rick, for the advice and the links on point. I’ll consult a roofer next (after the carpenter ant specialist!) and maybe put funds into the exterior to save us some work and relieve my worries about SF. Thanks, all, for sharing your thoughts.

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