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New house question

Bill_C | Posted in Energy Efficiency and Durability on

After a lifetime in the Southwest, we are building our first home in a Zone 6 area (Northwest Montana near the Canadian Border). I stumbled across this website accidentally, and have read everything I can regarding insulation/ barrier techniques, but was hoping for some clarification.

The house is partially constructed now with windows and house wrap installed, but no siding, insulation or drywall yet. The walls are 2×6 sheathed by OSB wrapped in un-taped Tyvek. The plan is to install House Slicker rainscreen and then wood siding to allow drying to the outside. On the inside, the plan is to blow in fiberglass in the wall cavities, with either a MemBrain or Air Tight Dry wall as an air barrier. Which is a better choice? In either case, is the interruption of the wall at the second floor line sufficiently handled with caulk/ sealant at the rim/ plate joints or is there another superior method? Is this the best assembly given the current state of construction?

At the roof, half the house is vented attic and half is cathedral, with dormers. On the dormers and unvented sections, the plan is to fill the bays with closed cell foam and use ADA drywall bottoms. On the vented portions, would it be best to fill the ceiling joist bays with Closed Cell foam or blown in fiberglass? Would a MemBrain below the insulation be a good idea to prevent any airflow in case of gaps, settling or shrinkage?

Many Thanks!

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 6 you'll need the MemBrain, which will behave as a Class-II vapor retarder during the winter season, but becomes vapor open should the moisture levels in the wall cavities rise to mold-inducing levels. If you use drywall alone you'd have to make the air-tightness perfect (tough to do), and use a vapor-barrier latex to limit wintertime moisture diffusion. Unlike MemBrain, vapor barrier latex impedes drying, increasing the drying time by about an order of magnitude. (The amount of moisture than can leave with MemBrain in a week when the humidity is high takes 6-10 weeks through vapor barrier latex.)

    Filling rafter bays or stud bays with closed cell foam is unnecessary, environmentally damaging, and expensive. A better use of those funds would be to install sufficient rigid foam on the exterior side of the roof deck/ sheathing for dew point control, and use fiber insulation in the stud /rafter bays, with only standard latex paint as the interior vapor retarder. In a zone 6 climate that takes 50% of the total center-cavity R above the roof deck, or 33% of the total center-cavity R on walls.

    eg: For an R49 code-min roof it takes R25 above the roof deck with R24 in between the rafters. (R25 is ~50% of R49)

    eg: For a wall with R23 rock wool in the 2x6 wall cavities, it takes R12 on the exterior (R12 is ~33% of R23+ R12 = R35.)

    The IRC has set prescriptive minimums based on the framing size for walls:

    But they only prescribe the exterior R indpendent of framing for roof decks:

    The very high center cavity R obtained by closed cell foam is severely undercut by the dramatically lower R-value of the framing fraction, wasting most of that performance. A 2x6 wall with 5.25" of closed cell foam in the cavities is only about R2 "whole-wall-R" higher than R20 cellulose cavity fill. That thermal performance boost can be bought much more cheaply with 3/8" fan-fold XPS siding underlayment. The framing fractions of cathedralized ceiling is lower than that of walls, so it's more like an R4 whole-assembly improvement, but that's a ridiculously expensive R4.

    Code-min walls for zone 6 is R20 + 5, which is 2x6/R20 plus R5 continuous insulation. But simply putting R5 foam on the exterior would not be sufficient dew point control for zone 6- you would really need R11+ for R20 cavity insulation. If you can, adding 3" of EPS over the housewrap would allow you to use R23 rock wool or damp sprayed celluose in the wall cavities without interior side vapor retarders, which would maximize resilence, and would put you ~R7.5 ahead of code-min performance. While using 2" polyiso would get you there from a labeled performance point of view, polyiso would have to be derated to no more than R5/inch in that stackup, in which case it comes up short, as would 2" XPS.

    Like closed cell spray foam, XPS is climate damaging, but it also loses performance over decades, eventually hitting the same performance as EPS, so from the long view you'd have to go 3" XPS too. EPS is cheaper per R, and does not experience that decades-long performance decline. Both EPS and polyiso are comparatively benign products next to XPS or closed cell polyurethane, largely due to their lower impact non-HFC blowing agent (pentane.)

  2. GBA Editor
    Martin Holladay | | #2

    There are lots of issues raised by your post. Dana has done a good job responding to many of them. In general, rigid foam above the roof sheathing makes much more sense than spray foam between the rafters. For more information on this topic, see How to Install Rigid Foam On Top of Roof Sheathing.

    One other point: it seems that you may be confusing air leakage problems with vapor diffusion problems. You wrote, "Would a MemBrain below the insulation be a good idea to prevent any airflow in case of gaps, settling or shrinkage?" You should keep in mind that the main reason that builders install MemBrain is to control vapor diffusion, not air leakage. If you want a good air barrier, there is usually a better way to reduce air leaks than installing MemBrain.

    For more information about these issues, see:

    Questions and Answers About Air Barriers

    All About Vapor Diffusion

  3. Bill_C | | #3

    Thanks for the responses. I should have made more clear that the house is completely dried in- the roof is complete, all doors and windows are in and flashed. So I would prefer not to remove the roof to install exterior insulation or redo all the doors and windows. I am seeking the best alternative for an interior system.

    So- on the walls, would blown in FG with Membrain or ADA be the best option? What is the best atlternative to handle the second floor intersection with the exterior walls?

    On the roof, I see MemBrain is specified as an air barrier, but I am not clear what better way there is to seal the attic. Would simply using ADA on the ceiling be better?

    Is there a better alternative for unventable rafter/ dormer bays than the spray foam with ADA bottoms?

    Thanks again.

  4. GBA Editor
    Martin Holladay | | #4

    For your walls, Dana already suggested that MemBrain would perform better than the Airtight Drywall Approach.

    Where the second floor intersects the walls, you will need to insulate the rim joist area. Here is a link to an article on the topic: Insulating rim joists.

    Here is a link to an article that explains all of your options for insulating a sloped roof assembly: How to Build an Insulated Cathedral Ceiling.

    If the designer specified the use of MemBrain as an air barrier for the roof assembly, the designer may not understand the purpose of MemBrain.

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