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floor insul & vapor control above unconditioned humid space - mountain climate

proto

Residential addition project, climate zone 4c, but above 5000', heavy snow in season (300+psf design snow load) pacific nw

Goal: addition w/ living space w/ radiant floor above an unconditioned indoor/outdoor room  AND with hot tub outside/adjacent/very close by such that humidity is likely high at this soffit. Finished soffit surface cedar decking/timbers.

Space below is basically a porch w/ large glazed sliders that open to the seasons. Walls are icf where solid.

Question for you pros, esp you mountain-folk archs chad/jlc/whistler: we will over insulate the soffit...but what sort of vapor control?

[ and i've already hired an architect ;) ]

your thoughts appreciated

 
Oct 3, 22 1:55 pm

Well then ask your architect!  Sorry, I had to.  :)

I would go with a standard WRB similar to the exterior construction assembly.  

Oct 3, 22 2:00 pm  · 
1  · 

Also I wouldn't insulate the overhang beyond a foot or two.



Oct 3, 22 2:07 pm  · 
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proto

initial thinking floor sandwich

Oct 3, 22 2:05 pm  · 
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I would have the finished floor be a decking type system that is removeable. I wouldn't install any radiant flooring - it won't fully melt the snow and will just create ice. For roofing I'd use a low slope adhered membrane.

Oct 3, 22 2:10 pm  · 
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proto

that system ^^^ is for the living space above

the floor of the indoor/outdoor room is likely sloped structure with a porcelain paver system as the level finish

Oct 3, 22 2:14 pm  · 
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The radiant slab heat will work. It will take a long time (several days) to heat up and cool down. You're going to need a secondary, on demand heating system to handle any fast drops in temps.

Oct 3, 22 3:00 pm  · 
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proto

this area


Oct 3, 22 2:20 pm  · 
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The dashed line is the WRB. It would stop the water vapor from the hot tub.

Oct 3, 22 2:58 pm  · 
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Wood Guy

Most WRBs are vapor-open, anywhere from 13 to 60 perms. They won't stop and hardly even slow water vapor movement.

Oct 4, 22 1:53 pm  · 
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atelier nobody

I would make the ceiling/soffit and wall finishes as vapor permeable as possible, then a small vented and drained air gap, non-moisture-sensitive continuous insulation, then an impermeable air-/vapor-/wrb between the continuous insulation and the framing. More insulation in the framing cavity optional. Avoid any materials that would create another vapor barrier on the conditioned side of the main AVW barrier - you don't want to create a "sandwich" where moisture can get caught between two barriers.

Oct 3, 22 3:27 pm  · 
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Non Sequitur

I was going to recommend exactly this, but got distracted. We’ve done similar types of assemblies between high humidity and cold spaces in spas/pools/sauna/gyms. If I recall correctly, We had multiple layers of air barriers.

Oct 3, 22 3:42 pm  · 
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proto

this is the thrust...i've got multiple seasons, all of which have active/significant humidity & do I button it tight or let it breathe? as if i were installing a steam room in the lower level?

...if i'm understanding, basically do a kind of horizontal rain screen assembly?

Oct 3, 22 3:51 pm  · 
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whistler

That seems to be a appropriate strategy. We often avoid soffit venting above dryer vents etc too to avoid the hot moist air getting into the attic spaces. One contractor alerted us to that issue many years back when he had to investigate some mold growth and it was clear as day given the location of the dryer / soffit venting. It wasn't catastrophic but a good heads up. I also don't think the Hot tub will be as big an issue as the use is not 24 hrs a day and there should be an insulated lid on the unit when not in use plus it's outside to open air flow. Non the less the strategy is valid.

Oct 3, 22 4:14 pm  · 
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Non Sequitur

Wrapping one wall like a condom is that that bad… unless you’re proposing a 360d moat of hot tubs.

Oct 3, 22 4:21 pm  · 
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proto

too much?

Oct 3, 22 4:27 pm  · 
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The spray foam you're showing would act as a vapor barrier. I would install rigid insulation and then add a vapor permeable WRB that is sealed to the structural member.

Oct 3, 22 4:47 pm  · 
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Maybe?


Oct 3, 22 6:12 pm  · 
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atelier nobody

Look at you being all architectural and sketching it out...

Oct 4, 22 2:26 pm  · 
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I try my best.  Also I sketchy better than I talkie . . . .

Oct 4, 22 2:50 pm  · 
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JLC-1

I wouldn't put a soffit over the hot tub, how are you going to look at the stars at night?  We finished this one right when the pandemic hit, it doesn't have an opening right next to it, when the cover is open it sits exactly under the gutter. The crossed door you see it's the electrical panel.

Oct 3, 22 6:18 pm  · 
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That is a good point. You're not going to need the extending roof to keep the people in the hot tub warm. It could keep snow off of it but unless it's enclosed on all four sides you'll still get drifting.

Oct 3, 22 6:30 pm  · 
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proto

more to protect from roofalanches than anything...otherwise gotta set it well away from the structure

Oct 3, 22 6:57 pm  · 
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JLC-1

that's what that double row of snow fence is for, with added heat tape in lower part of roof and gutter. I think you're making too much of something that's negligible in terms of vapor. This house sits at 9000 ft.

Oct 4, 22 10:01 am  · 
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proto

I see the mitigation on the image you show…on the structure I’ve got, we have a 15:12 roof and heavy forest & relatively close properties. So snow accumulates and little space to push it away from the buildings, so covered works for us. [Some winters you can walk from roof to roof.]

I’d be happy if vapor impacts are truly minimal & I can ignore. That’s the part I”m not clear on and just want to see what the simplest mitigation might be.

Oct 4, 22 11:27 am  · 
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justavisual

In Europe they would add 3cm XPS in the overhang

Oct 4, 22 9:28 am  · 
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Why?

Oct 4, 22 10:08 am  · 
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justavisual

In our damp climate (lots of misty days) the builder claims it helps prevent condensation buildup that might otherwise rot the wood structure. Time will tell... I have however seen overhangs with similar buildups and no insulation start rotting after about 20 years. So there is something in it?

Oct 4, 22 12:55 pm  · 
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I'd think having a properly vented overhang would be the key to stopping rot due to heavy water vapor. I don't think the insulation would do much. I suppose the insulation could act as a vapor barrier if it's installed in the correct location and sealed.

Oct 4, 22 1:09 pm  · 
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