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Attempting chainsaw/PERSIST retrofit… Help?

toshiyano | Posted in GBA Pro Help on

In June, I will be attempting a chainsaw retrofit of a 50-year old country cabin that faces ESE.

On the roof, we have vinyl shingles nailed into (I believe) tar papered 1×6 sheathing over 2 x 4 framing, and a chimney protrusion for boiler venting from the basement; that roof is on top of an unconditioned attic with about about 4-6″ of blown-in cellulose on top of fiberglass batting on the attic floor. Hangover for eaves and rakes is approximately 18″. There’s also a dormer on the front for two fo our bedroom window.

On the walls, there’s log cabin clapboard siding nailed directly into 2×4 framing, with carpenter bee infiltration in the clapboard, blown-in cellulose in 2/3rds of the walls and ancient fiberglass batting in the other third.

The basement is unfinished and uninsulated, but at least it’s dry – a cement slab and cinder block walls with probably a 1/2″ of stucco inside and out; there are holes in the wall for a water, septic and dryer out, and oil, phone, and data in; and a pipe in the slab for water from the well. The mechanicals (oil burner, hot water heater, water pump, electric) and the washer/dryer are all in the unfinished basement.

The first and second floors are t+g pine over uninsulated 2×4 framing. We have a pellet stove on the first floor. There are also hot-water baseboards throughout the house which we don’t want to use (oil is expensive).

There’s also a would-be solarium/actual drafty porch that is currently outside of what passes for the envelope of the house; however, it is under uninsulated roof framing on the front of the house and above uninsulated 2×4 framing joined to the front of the house; it is supported by 4×4 beams sunk into concrete in the earth. Ideally, I’d like to bring it into the envelope.

20 of the 23 windows are double-hung, single-pane and wood.

Anyway a lot of issues…

I’d like to do an energy audit. Then: tear off the siding and vinyl roofing, and cut the eaves and rakes; then I’d like to cover the house with OSB, Ice & Water, 4″ inches of polyiso, housewrap, furring and cedar t+g on the walls; for the roof I’d like to keep the 1×6 sheathing and then cover with Ice and Water, 6″ of polyiso, furring, OSB, roofwrap, and zinc panels. I’d like to spray foam the attic rafters, and put HRV ventilation in the attic. I’d also like to spray the basement rafters and put rigid foam or drywall on the ceiling, but I don’t have much height to work with. Then I’d like to do another energy audit. Ideally I’d replace the boiler and hot water heater with an oil buring on-demand water heater. I’d like to replace all the windows with Marvin Integrity.

Does that sound like a good plan? Where can I do less? Where do I need to do more. Can I do this in a month or two with a couple carpenter friends and a spray foam guy or do I need to plan for a longer build time using more paid labor?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I do this in a month or two with a couple carpenter friends and a spray foam guy?"

    A. Probably not.

    Here's some feedback:
    1. T&G boards make lousy siding. Choose clapboards or vertical shiplap boards. Just my 2 cents.

    2. If you have a boiler, water heater, and plumbing in the basement, you want the basement to be inside your home's conditioned envelope. That means you need to insulate the basement walls, not the basement ceiling.

    3. If you are putting 6 inches of polyiso on the roof, why not put 6 inches of polyiso on the walls?

    4. Beware of sandwiching board sheathing between two foam layers. If you are going the PERSIST route (with exterior Ice & Water Shield), I wouldn't install any spray foam on the interior of the sheathing.

  2. user-659915 | | #2

    You learn something new every day on GBA. I had to Google vinyl roof shingles and oil-burning demand water heaters to be sure they exist.
    Martin's advice, as ever, is sound.

  3. homedesign | | #3

    Why not make wall polyiso equal to roof polyiso?
    Why not ask Joe Lstiburek the same question?

  4. toshiyano | | #4

    Martin, thank you for your advice. I was looking at ship lap as well, but I figured T&G would have a tighter seal. Is the problem that the tongue gets damaged too easily? As for the basement, I'd love to bring it inside the envelope but a) doing so is beyond our means at the moment and b) even if it were in the budget, the current state of the basement makes the project a nightmare - the walls are covered with plumbing, electrical, etc. and there is no room to insulate the slab unless we tear it up and dig. However, given all that, if insulating the basement is an absolute must, what could I lose from my project list to make it happen? As for the polyiso, a) I can't afford 6" all around and b) from what I've read it seems the roof should have more insulation than the walls. I will take your advice on the spray foam in the attic. If we need more insulation, would fiberglass batting between the rafters suffice?

    John, that is a nice detail - where can I find the rest of it? Looks like he puts tar paper and plywood on top of board sheathing before the peel-and-stick - that seems like a lot of sheathing!

  5. homedesign | | #5
  6. toshiyano | | #6

    thank you, the article is most helpful.

  7. user-659915 | | #7

    "I figured T&G would have a tighter seal"
    The tighter seal is the problem: it's just tight enough to trap moisture, but not nearly tight enough to act as an air barrier. We've found any siding that lays flat against the sheathing (t&g, german siding) is prone to mold in any but the most sheltered conditions (e.g. inside a porch) - if you're determined to use it for esthetic reasons use a rain screen detail and take extra care with roof overhangs.

  8. toshiyano | | #8

    I'm not determined to use it by any means. But as I tried to explain in my plans, there *is* a drainage plain/rain screen under the siding created by the furring strips on top of the polyiso. There's also ice and water shield acting as an air barrier on top of the sheathing. I will defer to the experts and use shiplap instead, but I still don't understand the issue here...

  9. GBA Editor
    Martin Holladay | | #9

    Q. "I still don't understand the issue here."

    A. The issue is moisture held in the tongue-and-groove joint. Clapboards work so well because their shape creates triangular air spaces behind each piece of siding, because they are shaped for drainage, and because there are no tight joints to trap water.

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