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Advanced wood frame parapetitis


kurt
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10 year old house, drywall to the bottom of the roof joists, cavities 8" faced batt, OSB sheathing, no roof vents (not recommending they'd have stopped the problem), and pretty much ignorance on all things Joe L.

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What was the pitch on the roof? Shingles or roll roofing? Did the people below grow lots of plants or is this area over a bath?

New thinking would have the underside of the sheathing sprayed with closed cell foam and no ventilation, correct?

Or, did your tell them to install the high/low vents, air channels, and more insulation against the warm side of the house. And, vent the the baths and kitchen and maybe install a whole house ventilator.

Ezra Malernee

Canton, Ohio

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Yep, flat roof, OSB roof sheathing, TJI's and LVL roof structure.

No venting system, but I don't know how you'd do it even if you wanted to; the joists run varying directions withe blocking, skylite penetrations, different levels, etc. You'd have to vent every joist cavity.

Nothing particular other than a sealed joist space and no air or vapor barrier.

There's huge fungal growths on the LVL's and TIJ's. I'm not sure the structure can be saved.

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Hi,

We have 'em here. They put vent stacks in the center and use a fascia vent at the perimeter to get air under the surface. Seems to work. Of course, we don't have winter temps like Chi-Town; so I'm not sure if it would work in your climate.

ONE TEAM - ONE FIGHT!!!

Mike

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They'll use like an 11-7/8 I joist with 9-1/2-inch thick high performance fiberglass batts so there will be an air gap above; then vent the ends of the I-joists, use spaced blocking to create a gap between the top of the rim board and the parapet curb, apply a layer of mesh over the slot, install more spaced blocking, hang a wide fascia on that and bring the metal coping over the top of the curb and down the face of that fascia, leaving a gap behind the fascia for air to flow behind and enter the roof cavity. A long tower or parapet topped with a vent cap extends down the center of the roof perpendicular to the line of the joists. Once the roof surface heats up, convection takes over as air starts leaving the tower and pulls in fresh air at the fascia.

I've emailed you a very, very rough sketch. Don't you dare snicker at my lousy drawing skills.

ONE TEAM - ONE FIGHT!!!

Mike

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OK, got the sketch, and I get it. My email response didn't get it, now I do.

This might work, but understand, there's joists going in different directions all over the place hung on LVL's with hangers. It's more complicated than the sketch accounts for.

Ezra, steel decking and bar joists aren't necessary for flat roofs if they're designed correctly. I look at hundreds a year that are just fine. It's about design more than materials.

The real problem with this house is it was a transplanted SoCal architect building for the desert, only he happened to be @ the 42nd parallel. Stuff he did there might work, but here, it's a mold farm.

It's a building science case study of how to do it exactly wrong for a particular climate.

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Yeah,

Just imagine a long narrow box with tapered sides sticking up above the roof at the inner ends of any joists that dead end in the center and at the center where any joist bays continue from one side to the other. A curved or gable cover covers the top with screened mesh down both sides under shelter of the vent overhang so animules don't get in there. It has to be high enough to place the strip vents above any snow accumulated during a storm so that ventilation isn't impeded.

ONE TEAM - ONE FIGHT!!!

Mike

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They'll use like an 11-7/8 I joist with 9-1/2-inch thick high performance fiberglass batts so there will be an air gap above; then vent the ends of the I-joists, use spaced blocking to create a gap between the top of the rim board and the parapet curb, apply a layer of mesh over the slot, install more spaced blocking, hang a wide fascia on that and bring the metal coping over the top of the curb and down the face of that fascia, leaving a gap behind the fascia for air to flow behind and enter the roof cavity. A long tower or parapet topped with a vent cap extends down the center of the roof perpendicular to the line of the joists. Once the roof surface heats up, convection takes over as air starts leaving the tower and pulls in fresh air at the fascia.

I've emailed you a very, very rough sketch. Don't you dare snicker at my lousy drawing skills.

ONE TEAM - ONE FIGHT!!!

Mike

I'd like to see that sketch, if you don't mind.

Marc

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Thanks! Nothing communicates like a drawing. It looks innovative.

Marc

How does the air know where it's supposed to go?

- Jim Katen, Oregon

My guess is that it's pulled towards the vent tower which functions as a flue, unless the wind currents overpower it. Heat energy is added to the air at the underside of the deck and perhaps at the flue itself. Kurt seems to suggest that it works, that's the bottom line.

Marc

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Thanks! Nothing communicates like a drawing. It looks innovative.

Marc

How does the air know where it's supposed to go?

- Jim Katen, Oregon

It doesn't. It's just like every other vent scheme out there. The stack might be the only part that could reliably drive a convection current, but I doubt it's big enough.

How much of that structure has to come out? If it's far enough apart, say 60-65% of the sheathing removed, it just might make better sense to strip it all and foam it.

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It doesn't. It's just like every other vent scheme out there. The stack might be the only part that could reliably drive a convection current, but I doubt it's big enough.

How much of that structure has to come out? If it's far enough apart, say 60-65% of the sheathing removed, it just might make better sense to strip it all and foam it.

Let me ask you if you agree with this: (In this case) foam would reduce, but not eliminate the amount of ventilation needed. Regardless of how much foam you install, some ventilation will always be needed for as long as any moisture from the conditioned space is able to reach the underside of the roof deck.

Sound correct?

Marc

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Thanks! Nothing communicates like a drawing. It looks innovative.

Marc

How does the air know where it's supposed to go?

- Jim Katen, Oregon

It doesn't. It's just like every other vent scheme out there. The stack might be the only part that could reliably drive a convection current, but I doubt it's big enough.

How much of that structure has to come out? If it's far enough apart, say 60-65% of the sheathing removed, it just might make better sense to strip it all and foam it.

I have no idea how well it will work in a climate as cold as Kurt's; all I can tell you is that I know it works well here because I've been seeing it for years and I've actually gotten into the roof plane beneath some of these, expected to find the underside of the roof covered with fungi and found that they were clean and dry despite our inaccurately-reputed-to-be-damp ([;)]) climate.

ONE TEAM - ONE FIGHT!!!

Mike

P.S.

Normal rainfall history for the area for the month of September is less than 2-inches - by September 20th we'd had more than 4 inches this year.

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Marc,

Yes, but, the idea behind closed cell foam is that it seals and fills all the voids thus leaving nowhere for the moisture to go. They will of course need to use the exhaust fans in the building to clear the humidity, and quite possibly improve them so that they can. The roof plane does not get vented.

I personally haven't done any significant retrofit foam applications, but I do know of a few projects done by a very reputable roofing company. The most notable was on a $500K chalet at a ski resort near here. The roof cover and sheathing were removed, Poly ISO sheets were cut to fit between rafters and glued in place with a bead of low expanding foam around the perimeter (because much of the ceiling finish was T&G cedar), the remaining cavities were then sprayed from above. The new roof cover consisted of full ice and water shield and Certainteed Grand Manor shingles above 2' of copper standing seam at the eaves. When viewed from the chairlift, the copper is free of snow and ice and the shingles are nicely blanketed with snow. If a steep chalet can be retrofitted so can that flat roof.

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