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Thermal wall value, condensation point, and air/vapor sealing above a bathroom?

Irishjake | Posted in Energy Efficiency and Durability on

I live in NH – zone 6A, average cold temp 6F in January (not last year – slush sucks!)…….
Heating degree days are between 7800 – 8200

I am getting ready to buy and then do a whole house renovation (foundation, walls , to roof) on a 1858 old farmhouse – pics here (I am closing on the property in the next couple weeks):

I will be utilizing the REMOTE or PERSIST wall/roof building/insulating approach and have questions about interior dewpoint at inside of exterior sheathing, and cathedral/ceiling air/vapor sealing.

The existing wall construction from the inside > out is:
Horsehair plaster – lath – 3″ rough sawn stud – horizontal 1″ strip sheathing (barnboard) – lap siding (lead paint).

The existing roof construction from the inside > out is:
Horsehair plaster – lath – 2″x 6″ rough sawn rafter – horizontal 1″ strip sheathing (barnboard – asphalt shingles – corrugated metal roof.

I am using either Poly-iso or EPS from Insulation Depot OR Roxul Comfortboard on the exterior walls and roof.

The walls are currently designed to be built utilizing the existing 3″ studs and will be built from the inside > out as such:
paint – drywall OR polyurethane – wood paneling (beadboard, barnboard, etc.) >
.5″ furring strip to extend stud depth to 3.5″ >
Roxul 3.5″ R15 batt or Johns Manville R13 Batt or blown-in cellulose >
3.5″ stud > sheathing >
vapor/air sealing barrier (either Drainwrap or ice/water shield) >
R24 of Roxul Drainboard (2 layers of 3″ R12 offset) or R 24-30ish of either Poly-Iso or EPS (2 layers of ??? R24-30 offset)>
Ventilation channel made up of either 3/4 plywood or 2×4 12″ -16″ O.C>
Hardiepanel Lap siding (prepainted)

Rigid insulation (continuous) value – R24 (nominal)
Sheathing – R .62
Vapor barrier – R .17
70 degrees F inside
2 degrees F outside (wanted to use a really cold day for f-factors aka murphy)
Inside RH% 40

Based on the above values I figured my condensation point on the inside of my exterior sheathing would be 41.89162.
here is the calculation I did:
T = 2 + [24/40.24 x (70-2)] + [.62/40.24 x (70-2)] + [.17/40.24 x 70-2)

Is my math right?
How will the inside insulation effect my condensation level?
Do I have enough insulation on the exterior ( I am slightly below the 2/3 to 1/3 ratio that is adequate for some of the Alaskan test ratios)?
Seems like I am doing something wrong because when I use the calculator from
and type in 70 degrees F for temp, 40 % for inside RH%, I get a dew point of 44 degrees F. Which means based on that calculator that I will have condensation on the inside of sheathing, based on my calculation)
Does Roxul on the exterior or interior effect any insulation values or vapor drying values differently than Poly-iso or EPS?
What insulation would you use? What would you use if money matters? What would you use if environment matters more?
Do I need interior vapor/air barriers at my bathrooms, kitchen, laundry rooms?

For my roof the existing rafters and barnboard above will be left exposed for aesthetic reasons.
The roof above the existing barnboard and rafters I have currently designed from the inside >out to consist of R60 of insulation:
diagonal barnboard/planer-offs from a local mill or thin plywood sheathing>
?????Vapor barrier or no vapor barrier?????? next
(what about above bathroom? does that change the immediate previous question?)>
R15 worth of Roxul batt insulation or fiberglass batt within rafter cavity>
2×10 rafter (amazing that after 160+ yrs the 2×6 rafter 2′ O.C. held up!)>
5/8″ OSB or plywood sheathing>
Vapor barrier (Triflex, 30 lb asphalt or Ice and Water shield???)>
R40 of Roxul, OR Poly-iso or EPS from InsulationDepot with offset layers extended out over wall insulation and framing>
2×4 vertical strapping for ventilation channels>
plywood or diagonal barnboard/planer-offs>
Standing seam roof
Minimum 4.9 kW of PV panels!

Should I have (do I need a interior vapor/air barrier) at the whole roof or ceiling assembly?
My ceilings are primarily going to be cathedral ceilings.
Do I need one above my upstairs bathrooms?
My ceilings and walls in the bathroom are going to consist of tilebacker/hardieboard to reduce mold growth on the drywall paper.

Thanks for any input………..!!!!!!!!!

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  1. GBA Editor
    Martin Holladay | | #1

    As your calculations have shown, your proposed assembly is near the danger point. What that means is that you might get condensation (moisture accumulation) in your sheathing if you run your house with high indoor humidity, but you will probably be OK if you run your house with low indoor humidity.

    Personally, I wouldn't want to build a house that was so close to the tipping point. You are on the wrong side of the 2/3 and 1/3 rule (the old Canadian rule of thumb that you need to have at least 2/3 of your R-value on the exterior side of your vapor barrier).

    Ideally, a PERSIST house has empty stud cavities. That means you need to resist the temptation to fill your studs with insulation, and plan for more foam on the exterior of your house. That's my personal preference. (If you haven't seen it, here is a link to my article on PERSIST: Getting Insulation Out of Your Walls and Ceilings.)

    Your proposed details have a few other problems. First of all, if you end up installing Roxul between your framing members, you need a better interior covering than just tongue-and-groove boards. You always want to have an air barrier on the interior side of any air-permeable insulation. The easiest way to do this is to install gypsum drywall before you install your interior boards.

    Finally, you mention the possibility of installing an interior vapor barrier (presumably, polyethylene on the interior side of the Roxul) as a possible solution to your condensation worries. Don't do this. You are mixing and matching two approaches (and frankly, the interior polyethylene approach was never a great idea in New Hampshire). You already have a vapor barrier -- the Ice & Water Shield on the exterior side of your wall sheathing -- and you certainly don't want to install a vapor-barrier sandwich. Once you decide to go the Ice & Water Shield route, it's vitally important to be sure that all of the materials on the interior side of your Ice & Water Shield are vapor-permeable, so that the wall can dry to the interior. In other words -- don't install any vinyl wallpaper, either.

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