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Is my wall design OK?

user-959871 | Posted in General Questions on

In the article, “Calculating the minimum thickness of rigid foam sheathing”, one paragraph states,

“What if I live in one of the warmer climate zones?
If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don’t have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem.”

Would that “any foam thickness” also include no foam at all?

I am at the point of needing to install my windows and then put on the vinyl siding, and I’m not sure if I can get by without using any foam over the sheathing (don’t want to foam it if I don’t really need to).

I am in zone 3 (mid-western Arkansas) and my walls currently are 2×6 with 1/2″ Zip wall panels which are OSB coated with whatever that water repellent coating is that Huber puts on the panels; the panel seams are taped with the Zip tape.

I didn’t plan to use a housewrap because Huber says that the Zip panels after being taped make a sufficient air barrier. They also said that a layer of building felt over the sheathing was not necessary because of the coating on the panels.

Question 1:
Do the above two statements seem logical?

I will be using Roxul 5.5″ bats between the studs and 5/8″ drywall on the inside with latex paint.

Question 2:
For my climate, can I get by with no foam without risking condensation on the inside of the sheathing ?

(The Roxul supposed to hold and release moisture better than fiberglass, if that makes a difference.)

(The house will be heated with a wood stove, if that makes any difference either. I know wood stoves generally make for dryer inside air than central heat.)

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jim,
    Q. "Would that [statement that] 'any foam thickness' [will be safe in a warm climate] also include no foam at all?"

    A. Yes. You don't have to worry about moisture accumulation in OSB during the winter in Arkansas. However, a wall without any foam sheathing will not perform as well as a wall with foam sheathing (even in Arkansas), because of thermal bridging through your studs. To put it simply, a 2x6 wall without foam sheathing isn't as well insulated as a 2x6 wall with foam sheathing.

    Q. "Do the above two statements [about the Huber Zip system] seem logical?"

    A. Yes, the Huber Zip system creates an effective air barrier (although you still have to worry about air leakage at the bottom and top edges of your wall, as well as air leakage through your floor system and ceiling). And yes, Huber has demonstrated that you can skip the installation of housewrap if you want. (However, conservative builders worry about the longevity of the Zip system tape, and often install housewrap as cheap insurance, in case the tape gives out in 30 years.)

    Q. "For my climate, can I get by with no foam without risking condensation on the inside of the sheathing?"

    A. Yes.

  2. Expert Member
    ARMANDO COBO | | #2

    Jim,
    Doing the “math” with ASHRAE Fundamentals for Texarkana, when the interior temperature is >70°F or higher AND the Relative Humidity is >50%, you increase the chance to develop condensation on your wall sheathing. For Fort Smith, when the T>70°F and RH>40%, you increase the chance to develop condensation on your wall sheathing.
    Relative humidity is more important than temperature … so make sure you keep it controlled with ventilation and/or dehumidification. I would recommend R3 rigid insulation minimum in Texarkana and R5 in Fort Smith, and as Martin said, the wall will perform better.
    I should add this applies in the three coldest months, Dec-Jan-Feb.

  3. homedesign | | #3

    Martin,
    Armando's "condensation warning" is similar to advice/warnings given in Lstiburek's
    Builder's Guide to Mixed Humid Climates

    Yet... your Advice for Arkansas is "No Worries"
    Can you explain why you say "No Worries" ?

  4. GBA Editor
    Martin Holladay | | #4

    John,
    The source of my answer was a phone conversation a few months ago with Joe Lstiburek. I don't have a copy of the Mixed Humid book handy.

    So tell me: If condensation is a worrisome feature of walls in southern climates, how many of these walls are rotting? How are builders handling the problem?

    I'm skeptical. Even badly built walls don't usually rot up north -- and down south, the number of cold days is far fewer, and the temperatures aren't as cold, making condensation less likely.

  5. homedesign | | #5

    Martin,
    I am "Skeptical" myself.
    I have never seen, heard or read about wall sheathing(OSB or Plywood) being damaged by "condensation" in North Texas.
    The Mixed-Humid Climate Guide (and Armando)give me the impression that ...
    if Builders do not include spray foam and or Rigid insulation in the wall assembly ......
    the homes will be at "risk " ...
    UNLESS not-so-high(40 to 50%) humidity is maintained during the winter months

    Armando,
    Do you know of any published examples of sheathing damage in Zone 3?
    How about Zone 4?

  6. Kopper37 | | #6

    Jim,

    I've built and remodeled in North-Central Arkansas (near the Greers Ferry Lake area).

    I worked on one house where I saw sheathing rot that could have been related to winter time moisture sorption / damage. It had other problems too, so I'm very skeptical of this being the only driver. This particular house had T-111 siding for both interior walls and exterior sheathing, so there was absolutely no air barrier, and no weather resistant barrier on the outside. The owners heated with an unvented propane stove (creating a high humidity environment). And it was insulated with poorly installed fiberglass batts.

    Having said that, I think there are FAR more important issues for the Arkansas climate:

    * Rain (~ 50" annually where I was, and I don't think it's much different in your neck of the woods)
    * Termites
    * Summertime solar vapor drive

    Address these first, otherwise you'll be "straining a gnat and swallowing a camel."

    It doesn't sound like you are trying to build a high r-value wall. It sounds like you are moving from a 2x4 wall to a 2x6 wall with decent airtightness, and that you want to do that as well as possible. It sounds like you are worried more about durability than energy efficiency.

    If that's the case, then I think that you should focus on building strategies that make rain and termites a top priority, worry less about wintertime moisture sorption.

    If you want to look at improving energy efficiency, then we're on a different topic.

    Also, I would definitely include housewrap or building paper on top of the ZIP system. Like Martin said, it's cheap insurance. That, and I wouldn't want to depend on tape to act as my flashing system for windows, doors, and other penetrations. Not with those November rains . . .

    Good luck!

  7. homedesign | | #7

    If you decide to add rigid Insulation outside the ZIp
    Be sure to check with Huber
    I believe they require a drainage plane between ZIP and Rigid

  8. Expert Member
    ARMANDO COBO | | #8

    John,
    I can tell you that a good friend of mine in Frisco, TX is starting litigation on his 3-year-old house because of rotted sheathing on two sides of the house. There are many cases in NM, OK and KS on houses I’ve worked on with same issue. I guess I like to be safe and not sorry in CZ3, and for the record, I explain the issue to my clients and on the plans it goes as “optional” specification if the client prefers that way. In CZ4 it goes automatically in the specs. It’s all about safe and cheap insurance and good building practice for my clients.

  9. homedesign | | #9

    Armando,
    I am confused about the "Risk" because it seems that Martin AND Joe Lstiburek do not believe that we (not-so-cold climates1-4(except marine-4)) NEED to worry about moisture accumulation in OSB sheathing during the winter.

    Can you describe the wall assembly that "failed" in Frisco, TX?
    The type of insulation and WRB?
    What was the air barrier strategy?

  10. GBA Editor
    Martin Holladay | | #10

    Armando,
    You wrote, "I can tell you that a good friend of mine in Frisco, TX is starting litigation on his 3-year-old house because of rotted sheathing on two sides of the house."

    Perhaps my earlier post was unclear. I don't dispute that rotted sheathing occurs in the South. What I am saying is that in the South, as in the North, the vast majority of these problems are water-penetration problems (wind-driven rain).

    The second biggest category of rotted sheathing problems are basically air barrier problems (interior moisture piggypacking on exfiltrating air).

    WAY down the list, comprising a very small number of cases, are wintertime diffusion problems. And these very rare problems are a northern phenomenon, not a southern phenomenon.

  11. Expert Member
    ARMANDO COBO | | #11

    I don’t disagree with you, in general, about condensation in the south, but condensation issues do happen in Mixed-humid-dry climate like Dallas…(the homeowner asked me not to comment on his case yet). NE Texas is probably one of the toughest climates I design since half of the time is dry and the other half is wet, and it does get cold. Most of the folks in Dallas keep their interior temperature around +74°F in the winter and high humidity (in this particular case). Can anyone guarantee that their clients will keep their indoor temperature and humidity lower, as they should? I make sure my houses have controlled humidity of no more than 35-40%.
    See attached spreadsheet I developed long a go. All I need to do is change T, RH & DP and it tells me if I need to use exterior rigid foam or not on my wall and roof assemblies. I really like this sheet to explain why we should install rigid insulation “down in the south” on top of the roof decking when we do conditioned attics with 5.5” open cell foams… very common!

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