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Recommended taping of plywood and foam layers

dburgoyne | Posted in Green Products and Materials on

I am building a small casita (500 sq. ft.) in Northern California in a mild climate with hot summers. My wall framing is 2×4 w/ 3/8 continuous plywood shear (also insulated w/ R-13 fiberglass batt insul.), which I will cover with two layers of 1/1/2″ Polyiso (Firestone) insulation (has fiberglass reinforced face). I will cover everything with Tyvek housewrap and fasten 1×4 battens creating a rainscreen, and attach lath and plaster as an exterior finish to the battens.

I have read about the importance of taping plywood, and lapping layers of insulation, but am looking at four levels of taping, which I am wondering if is all necessary.

1. Taping plywood joints – this will be covered and held in place by polyiso, do I really need to buy expensive tape with superior adhesion & permeability if it will be held in place like lettuce in a sandwich?

2. Taping layers of polyiso insulation (2 layers of 1 1/2″ totaling 3″) – same sandwich question, and wondering about redundancy?

3. Tyvek housewrap – I read its not really necessary to tape horizontal laps, but vertical laps.

Do I really need to tape at all four layers, and where is it most beneficial? What types of tape are really needed in these conditions?

-Dan Burgoyne

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  1. exeric | | #1

    Answering as a non professional also living in a hot northern California climate, (might even be close to you), I would leave out the taping of the plywood and concentrate on taping both layers of foam. The taping of the Tyvek is just part of all construction and isn't something exceptional at all, like the other taping you're describing.

    If you are as concerned about air infiltration as it sounds then I would forego the batt insulation and rent a cellulose blower and densepack the cellulose yourself. It's easy and cheap as long as you pay attention to detail. A side benefit of cellulose dense packing of walls is that it really lessens airflow and also dampens outside noise if you live in an urban environment.

    In a California environment one can practically get to Passive House insulation and air sealing standards with the building techniques that would entail. Of course that would also have to include the important wall/ceiling and wall/floor air and insulation junctions.

  2. dburgoyne | | #2

    What kind of tape do you recommend for the foam? Do you really think there's not much benefit in taping the plywood under the foam?

    I already bought bundles of fiberglass insulation, thinking it was not as important to worry about a small difference in cavity insulation efficiency, if I'm adding 3" of polyiso outside.

    I'm also insulating my slab edge perimeter with 3" of XPS, and have two layers of 1/4" sill sealer (one below and one above a termite shield), and plan to foam seal the joints between the top plate and roof framing.

  3. exeric | | #3

    I'm not an expert on the different tapes and their applications. I'll defer to others on that and hope that they chime in here. As far the need for taping the plywood, a lot of times you will see descriptions of "belt and suspender" multiple points of air sealing in passive house construction. That's because it really hard to get to 0.6 ACH 50 blower door tests. They have to do those multiple levels to get that certification. But from a practical point of view if you aren't going for the certification then you won't see a discernible difference in house performance between 0.6 a d 1.0 ACH 50. The "sensible"
    difference to you or me would really only be in the certification difference. You are really already getting multiple sealing with 2 overlapping staggered rigid foam layers that are individually taped. And that still doesn't count for the impossible to stagger joints at windows and doors where the real problems are and which taping the plywood can't affect.

  4. GBA Editor
    Martin Holladay | | #4

    You are overthinking this. You are building a 500-square-foot tiny house in Northern California (a "mild climate with hot summers"). Your utility bills will be tiny.

    Don't worry quite so much about air sealing. Are you even planning to install a mechanical ventilation system? My guess is that your windows will often be open.

    Here's my vote: tape one layer. You can choose any layer you want. Everything will be fine.

  5. dburgoyne | | #5

    Thanks for the feedback. Which layer would be best to tape? I am planning to install a 3/4 ton ductless mini split, primarily for air conditioning during hot summers.

  6. GBA Editor
    Martin Holladay | | #6

    Tyvek tape is cheaper that the European tapes, so if you want to keep costs down, tape the Tyvek layer. Since you are aiming for a reduction in air leakage, tape both horizontal and vertical seams.

    If you are willing to spring for a roll of Siga Wigluv tape, use it to seal the seams in the outermost layer of rigid foam. That will give you a more effective exterior air barrier than taped Tyvek.

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