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Please advise: Polyiso exterior in mixed humid coastal climate

shister | Posted in General Questions on

This site is wonderful…and I’m hoping I can get some of you to weigh in on the wall assembly we are designing for an upfit of a really old single wide trailer at the coast. I’m wondering about the polyiso and how warm humid air on the inside dries out during the winter.

# Proposed Wall Assembly

#
Based on all our reading and research, here’s what we’re thinking for an exterior wall assembly.
* Interior finish: 1/4″ plywood paneling, painted with non-vapor barrier paint, then
* R-6 ecobatt insulation, compressed in-between the 2×2 studs and puffing out between 1×4 nailers, then
* 1/2″ polyiso foam exterior-facing foil-lined sheets as a continuous sheath (air/vapor/thermal barrier), then
* 3/8″ coroplast / air gap (rain screen), then
* 1/2″ LP Smart Siding panels (back-primed, structural siding) as the final layer.

What are we missing? What could be done better?

Here are some additional details that might help understand the context.

Thanks in advance for your feedback!!!!

Christy

# Climate Conditions

#

HUMIDITY & DEW POINT
Relative humidity 50% – 93%, rarely <30% or up to 100% Driest mid Feb; relative humidity <61% three days out of four Most humid late Sept: >89% three days out of four.
Dew point typically 28°F – 76°F, rarely <12°F or >79°F
Early Mar – Mid May and early Oct to mid Dec are most comfortable

TEMPERATURE
Typically 36°F – 85°F, rarely <26°F or >89°F
Warm season late May – late Sept, average high temperature > 79°F
Cold season early Dec – early March, average high temperature <59°F RAIN Warm season 42% chance precipitation on a given day (49% thunderstorms, 20% heavy rain, 19% light rain, 12% moderate rain). Cold season 34% chance precipitation on a given day (35% light rain, 28% moderate rain, 26% heavy rain, 6% thunderstorms). Hurricane season Jun – Nov

# Current Construction Situation

#
Single-wide trailer with 2×2 framing. Has been gutted down to studs and retrofitted with hurricane straps. Currently, we have:
* Floors: Plywood, R-19 ecobatt insulation, enclosed cinderblock crawl space
* Ceiling: 1/2″ gypsum, R-19 ecobatt insulation, old metal roof, R-13 rigid foam, new metal roof
* Walls: 1/4″ plywood paneling, studs, 1×4 horizontal nailers, corrugated aluminum siding

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Christy,
    While the weather data you shared are interesting, it's more helpful (in general) to know your climate zone or location.

    You are planning to install rigid foam on the exterior side of your wall. Your 1/2-inch polyiso has an R-value of about R-3 (perhaps a little bit less). That's about 33% of your wall's total (nominal) R-value, assuming that the insulation between your studs has an R-value of R-6. This ratio of the R-value of exterior rigid foam to the R-value of the entire wall will work if you live in Climate Zone 5 or anywhere warmer. However, if you live in Climate Zone 6, it won't work. Fortunately, from the weather information you shared, it doesn't sound like you live in a cold climate.

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    If you go ahead with your plan, pay attention to airtightness. It's possible to use rigid foam with taped seams as an air barrier, but only if you are meticulous with your installation details.

  2. shister | | #2

    Thanks, Martin. We are in climate zone 3.

    I had read your article on minimum thickness and found it helpful - in fact, most of our insulation thinking has been guided by your work. I am so grateful!

    I think there is still a basic concept I'm not grasping. Obviously condensation from warm interior air meeting with cold exterior air is one possible source of moisture in the wall cavity. It sounds like this is not an issue per your response. What I don't grasp is if interior moisture permeates our wall cavity through the interior wall finish, what happens to the moisture it if it doesn't condense? I am imagining that the polyiso will trap it inside, and I guess that is okay?

    Thanks so much for the advice. I feel a lot more comfortable as a result of your guidance.

    C.

  3. GBA Editor
    Martin Holladay | | #3

    Christy,
    Q: "If interior moisture permeates our wall cavity through the interior wall finish, what happens to the moisture if it doesn't condense?"

    A. During the winter, warm interior air is able to hold more moisture than cold exterior air. That's normal. The air we breathe always contains moisture -- usually more during the summer than during the winter, and usually more in Florida than in Arizona. But there is always moisture in the air. It's supposed to be there.

    So the answer to the question -- "What happens to the moisture if it doesn't condense?" -- is that it stays in vapor form. Again, that should be considered normal.

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