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1.5-inch to 2-inch exterior insulation / fiber-cement siding in Memphis

Beaver_Solar | Posted in Energy Efficiency and Durability on

Been lurking for too long! Hello glad to B here. I have T1-11 over 1″ polyiso on 1979 two story 2×4 stick home. I’m looking at > for my Pella casement windows. I thank you in advance for any advice or critique. The T1-11 is currently my WRB. I appreciate Mr. Holliday explaining why long bolts through semi-rigid insul-board is “special” due to shear. My focus is max bang for least bucks while minimizing R bridging in 1.5-2 inch exterior insulation foam. If we can use non-solvent adhesives like Henry 444 FRP with polymer nails like to “glue and screw” a sandwich together, we are only limited by 3.5″ max/ framing guns. Every extra furring etc. in this sandwich adds to labor and materials. ?ROI?
I found fastening tables supportin’ 4″ foam on wood framing when direct siding attachment weight =3 psf or less with nails. Hardie panel is less than that and Raptor beats nails for pullout in wood! If our first array of mechanical fasteners imbed 2×4, then WRB and VB becomes foam?
Might be kool to over drive composite nails and squirt adhesive into hole…

I apologize for the jargon in the sake of brevity, but I have been thinking about this since 1978 when I asked the people in Plano TX when I could get their heat sinking siding across the MS river! I didn’t like their answer so I changed the rules!
Anyone not understanding R bridging can grab your snorkel to find fifty+ ways to kill your R factor @ Builders Research Report on Fastening Systems for Continuous Insulation
it’s worth one explanation, “For instance, where previously one carpenter could hold the siding with one hand and hold the nail gun pressed up against the siding in the other, now one hand must hold the screw to the tip of the screw gun, another must hold the screw gun itself, and at least a third hand is needed to hold the siding in place.”

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  1. GBA Editor
    Martin Holladay | | #1

    Your telegraphic style is hard to understand. (We don't charge by the word to post questions, so you might want to consider posting your questions and follow-up explanations in complete English sentences.)

    I don't see any questions, except perhaps "ROI?"

    I think that stands for, "return on investment," but I may be wrong. To answer your question, I need to know:

    1. What are you investing in? Glue? Screws? Furring strips? Exterior insulation?

    2. What returns are you considering? Probably energy savings, but I'm not sure.

    3. What time frame do you consider when making your investment decisions?

    Even if you answer my three questions, I doubt if I can provide a "return on investment" estimate...

  2. GBA Editor
    Martin Holladay | | #2

    On re-reading your post, I discovered a second question: "If our first array of mechanical fasteners imbed 2x4, then WRB and VB becomes foam?"

    A. I don't know what "imbed 2x4" means. Probably you meant to write, "If our screws are long enough to embed in the 2x4 studs..."

    I don't know what "...then WRB and VB becomes foam" means.

    Perhaps it means, "then my water-resistive barrier and vapor barrier become foam," but that doesn't make any sense to me. Maybe you meant to write, "If our screws are long enough to embed in the 2x4 studs, then we can use the rigid foam layer as the water-resistive barrier." Is that what you meant? Is that a question?

    If I guessed right, then maybe this article will help you: Using Rigid Foam As a Water-Resistive Barrier.

  3. GBA Editor
    Martin Holladay | | #3

    Yes, you can use rigid foam as a water-resistive barrier, as I explained in the article I linked to, but I don't really recommend the practice. It's your house, so if you want to use rigid foam as a WRB, go right ahead.

    No, I don't think it makes sense to depend on an adhesive for structural support, although you can always talk to an engineer if you want. I would trust an engineer's advice. The standard way to install rigid foam on the exterior of a wall is with cap nails; if furring strips are used to create a rainscreen gap, then it's essential to follow standard recommendations for the number and spacing of the fasteners used to hold up the furring strips. For more information, see Fastening Furring Strips to a Foam-Sheathed Wall.

    T-111 is a type of siding. It's not a WRB. Code requirements for WRBs are very specific.

    You wrote, "I see no Return On Investment (ROI) in using Furring Material." I suppose you're right. Furring strips aren't designed to save energy, so there is no "return."

    Furring strips create a rainscreen gap, which is good practice. A rainscreen gap can help prevent wall rot -- so if you consider the cost to repair wall rot, I suppose it could be argued that avoiding the cost of rebuilding your wall is a type of "return," even if the rainscreen gap doesn't lower your energy bills. For more information on this issue, see All About Wall Rot.

  4. Beaver_Solar | | #4

    Mr Holladay, thank you for your response and I apologize for my misspell of your name.
    VB was error. I meant Vapour Retarder VR; not Vapour Barrier. Your assumptions about my telegraphic style are correct. "If our screws [fasteners] are long enough to embed in the 2x4 studs, then we can use the rigid foam layer as the water-resistive barrier [WRB]?" Major Question 1.

    And yes I meant to reference your WRB posts. And after rereading it again, I am convinced that a VR may need to supplement or replace tapes, I was really surprised that aluminum (Al) tape on Al foil backed boards was not best performer in your backyard test.

    I think I need in my hot humid climate with a pool, 1.5 to 2 inches of exterior insulation. Iff (if and only if) Raptor poly nails will drive into and fuse into the studs, why can't an approved adhesive applied to the current Water Resistive Barrier (WRB)- my T1-11 plywood contain and support exterior insulation? I see a long list of polyisocyanurate and polystyrene compatible adhesives. Consider this sequence starting at my outside wall:

    1. outside wall =Vertically grooved plywood,

    2. add approved adhesive, Insulation board,

    3. Poly Fastener (with washer) imbedded in stud thru Insulation board(s) per Table 1a below,

    4. repeating 2 and 3 requires longer fasteners but no additional helper or Furring,

    Furring strips are the difference in Table 1a and 1b. I see no Return On Investment (ROI) in using Furring Material Iff we can trust>>

    5. add adhesive, fiber-cement board or panel and longer Fasteners. Comments? Shear happens and this may be where wood beats metal!

    Major Question 2. While minimizing R bridging, have we missed any assumptions or misread tables relative to shear out to four (4) inches from wood supporting 3 pounds /square foot or less?

    Major Question 3. If cost and availability of 2 x(3/4) and 2 inch board is similar and I can't trust tape, where is best location for VR? Reason I ask is 2 inch will require less labor and
    I'm ready to complete at least part of the upgrade when I get my sheer and R together.

  5. Beaver_Solar | | #5

    Martin, OK, Triage, {All about wall rot} was as eye opening as backside of framing hammer! As you can see in my window detail picture, Houston we have a moisture problem! You wrote, “Why did this wall rot?”
    There is just one answer to this question: “Because the rate of wetting exceeded the rate of drying.” Part (Critical) of this project is diverting ingress of water and re-sheathing afterwards with better than T111 and adding R15+.
    I admit to my aversion to Furring Material as an extra expense and beg your consideration of insofast that you reviewed in 2014 because it has built in Furring (or an alternative). Any thoughts about rotating this product so that the furring does not line up with the 2x4 studs. I ask because of Idea from REMOTE , "Transfer Stud locations on successive layers of foam board." Thank you for your help.

  6. GBA Editor
    Martin Holladay | | #6

    Here is a link to my review of the InSoFast product, which you have apparently read: Two New Exterior Insulation Products for Walls.

    That article explains the strengths and weaknesses of the InSoFast approach. InSoFast hasn't yet obtained approval by many siding manufacturers for their approach, but I have no reason to doubt that you can install T-111 on the exterior side of the InSoFast panels if you follow the fastening recommendations provided by InSoFast.

    You wrote, "Houston, we have a moisture problem." In light of that fact, your biggest challenge is providing flashing for your window rough openings. If I were you, I would concentrate more on solving that challenge.

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