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Please help me make an informed decision on how to construct and insulate my addition’s walls.

GBA Editor | Posted in GBA Pro Help on

I am adding onto my home here in south Louisiana (25 miles west of New Orleans) and am also retrofitting the existing structure. The original structure is a 2×4 wall with blackboard sheathing on the outside between the bricks. I would like to remove the entire outside envelope (not 2×4’s), spray the stud bays w/ closed-cell foam, then cover that with 1/2″ foam board, then 3/4″ plywood, housewrap, 1/2″ foam board and James Hardie lap siding.

On the new walls starting from inside: 1/2″ sheetrock, 1/2″ plywood, 2×6″(with closed-cell spray foam between stud bays), 1/2″ foam board, 3/4″ plywood, housewrap, 1/2″ foam board and James Hardie lap siding. This new addition is a master bedroom (17×17), master bath (12×17), and dining room (15×17) with double-pane low-E glass vinyl windows.

Let me explain my design; the plywood on both inside and outside is to essentially make the whole wall a shear wall (remember that I live in prime hurricane country and our new code requires shear walls at corners and windows). The closed-cell foam is to stop humidity transfer into the house (this is a high humidity area most of the year). The foam board is just to get some extra insulating properties and help with noise issues (apt. complex behind new addition).

I live in a hot, humid, rainy and dangerous area.

A few builders I talked to said they thought the extra 1/2″ plywood and foam board was overkill, and a couple thought it made sense and would be a negligible cost addition. I like them both for the ability to hang cabinets, and pictures on the inside and peace of mind. Also, I have upgraded my attic insulation to 8″ thick paper-faced batts in the existing attic. Should I use the same in the new addition our do something different?

I would like to know on scientific basis if you all agree with my design, and if not what would you change and why. I do not care about the cost factors as much as I do about heat and humidity transfer, the potential for mold growth and the structural aspects this system entails.
Thanks,
Blake

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Blake,
    If you want plywood on the interior of your walls, go ahead. It's your house.

    However, the sandwich you propose for the exterior of your walls -- 1/2-inch rigid foam over the studs, followed by plywood and then a second layer of 1/2-inch rigid foam -- is eccentric. If in fact the entire wall is a shear wall, it's important to check your design with an engineer. Shear walls usually have an exterior layer of OSB or plywood sheathing nailed directly to the studs, with no intervening foam.

    It's great that you want exterior foam. But why slice it up into two 1/2-inch layers and separate the foam with plywood?

    I would sheathe the exterior of the wall with plywood for strength. Then install as much rigid foam as you want -- apparently you want 1 inch of rigid foam on the exterior, although you could certainly install more if you wanted. Then install vertical 1x3 or 1x4 strapping, screwed through the foam to the studs, to create a rainscreen gap between your foam and your Hardie lap siding.

    A rainscreen gap will help the wall dry quickly after a rainstorm; will make the paint last longer; and will greatly reduce the chance that water will enter your wall cavities.

  2. Expert Member
    Armando Cobo | | #2

    Blake,
    The homes I’ve designed in NO, MS and South TX have the following wall design: ½ sheetrock, 2x6 @24” o.c. studs w/ cellulose insulation R20, ½ OSB/Plwd, taped ½”-1” rigid foam, house wrap, cement board or lap siding.
    For structural 1 OSB/Plwd is fine, but you must use Simpson’s Strap Holdowns from concrete-rim joist/truss-stud installation as well as hurricane ties for roof truss-wall installation and roof bracing.
    For roof framing I used Raised-Heel or Energy trusses, cellulose insulation R38, baffles and vented attic.

  3. Robert Hronek | | #3

    Blake

    You need to decide where your air and water resisent barriers need to be. For the addition on the exterior of the 2x6's install the plywood. In another postingsection here Martin wrote about spray applied WRB's in place of house wrap. You might consider that to get your vapor and air barrier. On the outside you can place as much rigid insualtion was you want. This would give you additional insulation and act as secondary water barrier. Any wind driven rain now has to get by the foam and the WRB.

    You have established the WRB/air barrier on the outside of the plywood. The you can insualte the wall cavities with open cell or cellulose.

    Now on to attic. Will you be having HAVC in the attic? I am not a fan of ducts or equipment in an unconditioned attic. Since we are talking about a addition to an existing stucture it complicates the matter. I assume you have at least part of the HVAC in the attic. At the minimum air seal at the ceiling level, seal the duct work and cover with lots of cellulose. Ideally you would bring the attic into the condtioned space.

  4. Robert Hronek | | #4

    One other thing, have you thought of LP Smart Siding. It weighs less, costs less, cuts like wood. The LP site has a video showing a small sledge hitting both cement and Smart Siding and the smart siding was more durable. I have also heard of people doing a water test soaking each and the Smart Siding held up better. Though I think most siding perform well if properly maintained and allowed to dry- back vented

  5. Blake | | #5

    Thanks for all the info. I have no problem doing the plywood, then foamboard then wrb etc. I do have a couple of questions. Is using closed cell to accomplish a better air seal and insulating quality a good idea for my locale or should I use open cell? As far as the cellulose that Robert spoke about, is this a lose fill blown in product? Also, my ductwork, and Havc is located in the attic( which has soffit vents) and I will not be moving any of this equipment. I will be removing the old round metal ductwork and replacing with high quality insulated flexable ductwork. Also to save money I plan to use paperfaced batt insulation in the attic like I stated earlier.
    Does anyone have any issues with these ideas or any new ideas to add?

  6. GBA Editor
    Martin Holladay | | #6

    Blake,
    You asked, so I'll answer.

    "Does anyone have any issues with these ideas?" Yes.

    If you have HVAC equipment in your attic and attic ductwork, you should convert the attic to conditioned space by insulating between your rafters. Leaving these components in an unconditioned attic wastes tremendous amounts of energy.

    Your choice to use "paper-faced batt insulation in the attic" is a poor one. I assume you are talking about fiberglass batts. Almost any other insulation choice would perform better.

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