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Need advice on outside insulation under new vinyl siding

George Baloga | Posted in General Questions on

Hi, intending to re-side house with vinyl. 2×4 walls w/plastic under sheetrock and batt fiberglass in walls. Plywood sheathing on exterior. Worried about leaks and trapping moisture. High wind/rain (coastal) area, zone 5 (CT). After stripping existing failing siding, plan to Tyvek house and flash well. Current siding provides R4 (plastic coated fiberglass impregnated foam – looks like board and baton from 1980). Thinking of using Progressive Foam’s expandable polystyrene product called Neopor. 1″ with R4 and high water vapor permeability. Should allow vapor pass thru in case dew point (a kin to condensation) problem occurs in walls (moisture due to cold meeting hot). With plastic on inside, walls can’t vent/dry to anywhere but outside. Anyone have any experience with this product and my logic? Am I thinking straight? Thx! Can also respond to [email protected].

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  1. Nate G | | #1

    With the curse of interior poly, you should choose an exterior insulation product that is vapor permeable. Rigid mineral wool would be perfect. Unfaced EPS could work in low thicknesses, but you're fighting the material, since you want thicker stuff to get better thermal performance, but the thicker you go, the lower its permeability. With mineral wool, you can add as much as you want.

  2. GBA Editor
    Martin Holladay | | #2

    There are two issues here:

    1. What is the minimum R-value of the exterior rigid foam required to keep the wall sheathing above the dew point during the winter?

    2. Does the fact that the house has interior polyethylene mean that installing exterior rigid foam is inadvisable?

    The answer to the first question is that the rigid foam needs a minimum R-value of R-5 in your climate zone. For a thorough explanation of this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing. I don't think that you can depend on vapor diffusion through EPS to keep you out of trouble if you violate the minimum R-values suggested in this article.

    Concerning the second question, here is my standard answer:

    Many energy experts worry that it may not be a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly weren't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

    That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

    If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

    If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

    To summarize, here are four caveats:

    1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter.

    2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

    3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding. (In your case, you are planning to install vinyl siding, which is inherently well ventilated, so no further provisions for ventilation are needed.)

    4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

  3. George Baloga | | #3

    Nathaniel and Martin, not sure how to respond to your suggestions via this forum. But thank you for your responses. Appreciate it. George

  4. Expert Member
    Dana Dorsett | | #4

    With the pre-existing interior polyethylene you don't really need to worry about the temperature of the exterior sheathing or the R-value of the exterior foam, since it is largely protected from interior side moisture drives.

    Neopor runs 3.5 perms at 1" thickness, which is even more vapor permeable than dry half inch CDX sheathing, and is plenty permeable for letting the sheathing dry should it become wet.

    The key thing to keeping the sheathing dry will be the window & door flashing details relative to the felt or housewrap. Unless you're re-mounting the windows, you will want to have the housewrap or felt between the Neopor & sheathing, and lap the flashing so that it drains onto the Neopor side of the housewrap/felt.

    Having a rainscreen gap (even 1/4") between the Neopor & siding enhances the drying ability, and limits the amount of direct wetting from wind-driven rain getting by the siding from time to time, which WILL happen in coastal CT.

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