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Where to put the smart vapor retarder?

ckolloff | Posted in Energy Efficiency and Durability on

We are remodeling the second floor of our 1956 Cape in Williamsburg VA, climate zone 4. I need advice about where to incorporate the MemBrain smart vapor retarder in the roof and wall insulation stack up.

Roof stackup: Asphalt shingles- Durabuild synthetic underlayment-1×8 plank roof decking- approximately 1.5 inch airspace (ProVent running soffit to ridge vet)- 3.5 inches of Roxul R15 between the 2×6 rafters- 5.5 inches of Roxul R23 between 2×6 “mooney wall” furring running perpendicular to the rafters- sheetrock.

In the roof, should the vapor retarder go between the two layers of Roxul mineral wool or on the living space side between the 2nd layer of insulation and the sheetrock?

Wall stackup: Vinyl siding- Tyvek- old wood clapboard siding- gypsum sheathing- 3.5 inches of Roxul R15 in the 2×4 wall- a second 2×4 wall must be constructed to address some existing structural issues and can be insulated with an additional layer of Roxul R15 if appropriate- sheetrock.

For the exterior walls, is there any real benefit to insulating the second wall (bringing the assembly to R30) when the roof/ceiling is only at R38? If yes, should the vapor retarder go between the two layers of Roxul mineral wool or on the living space side between the 2nd layer of insulation and the sheetrock? By putting it between the walls, it would eliminate the need to seal around electrical boxes contained in the second wall.

Relevant notes: second floor living space is 2 bedrooms & 2 bathrooms with mechanical ventilation. HVAC ductwork is inside the insulated building envelope. No combustion appliances currently installed on the second floor- possibility of a small gas clothes dryer in the future. Rigid foam above the roof decking and/or spray foam below is not an option.

Thanks for your time and advice! –Chris

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  1. Dana1 | | #1

    With a vented roof deck you don't need a vapor retarder in a zone 4A climate.

    If you are going to install a smart vapor retarder anyway, it's best to put it a the gypsum/rock wool layer, not between adjacent rock wool layers, since that's where the relative humidity will be the lowest on average. The higher the RH of the proximate air, the higher the vapor permeance, an conversely.

    A 2x4/R15 + 2x4/R15 Mooney Wall has a whole-wall-R between R20-R25, which is still financially rational over the long term in a zone 4A climate.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana -- you're worrying too much about vapor diffusion. These assemblies don't need a smart vapor retarder (although a smart vapor retarder will do no harm). Pay attention to airtightness, and you will be fine.

    One other observation: You really can't install vinyl siding over existing clapboard without an intervening layer to create a planar surface. Most vinyl siding installers install a thin layer of fanfold insulation to make it possible to install the vinyl siding over old clapboard.

  3. ckolloff | | #3

    With the 1x8 plank roof sheathing I do not see how I can get the roof sealed up... Unless I were to seal between each individual plank which is not realistic given time and budget constraints. My hope was that the vapor retarder would help with the sealing to a large degree.

    Thought? Advice?

  4. GBA Editor
    Martin Holladay | | #4

    The main purpose of a smart vapor retarder is to limit (or allow) vapor diffusion. In most cases, it isn't intended as an air barrier.

    If you are worried about air leakage, you need to make sure that your roof assembly has at least one, and ideally (in your case) two, air barriers.

    The main air barrier is your ceiling drywall. Pay attention to penetrations (for example, electrical boxes -- you want as few of these as possible, and you want to make sure that any you have are sealed against air leakage).

    It's also a good idea to install your ventilation baffles in an airtight manner. For more information on this topic, see these two articles:

    Site-Built Ventilation Baffles for Roofs

    How to Build an Insulated Cathedral Ceiling

  5. ckolloff | | #5

    What about adding a half in layer of taped and sealed rigid foam board between the roxul and gypsum instead of the MemBrain?

  6. Dana1 | | #6

    What about it? It's not much of a thermal break on the framing, and it's a lot more expensive than MemBrain.

    If it's unfaced EPS or XPS it's a class-III vapor retarder (about as vapor retardent as standard latex paint). If it's foil-faced goods it's a true vapor barrier, which limits your choices on what you can use on the interior.

    There is no reason to install a vapor retarder on a vented roof assembly in zone 4. Standard latex paint is more than sufficient.

    If your goal is an intermediate air-barrier, vapor permeable broad-sheet goods such as housewrap works just fine.

  7. ckolloff | | #7

    I have already read the articles suggested by Martin but with winter approaching and the time involved to create custom site based baffles I had to compromise and use an off the shelf product. Unfortunately events beyond my control forced me to complete this remodel under less than ideal timelines. So I was looking for a reasonable work around to add additional air sealing- my ductwork is above the drywall but below the roof insulation.

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