Vapor barriers and wall design

[Arky217]

At the buildingscience.com site there is a paper titled 'Vapor barriers and wall design'. Here is a link to the paper.

http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

I have read, re-read, and re-read this but still can't figure out if my wall design for my climate is ok, or if I need additional insulation on the outside of the sheathing, and whether or not I need a class II or class III vapor retardant paint over the drywall.

The climate is western Arkansas, a mixed-humid climate.

The wall from the outside in consists of:
Vinyl siding, Huber Zip system wall sheathing, 6" batt of Roxul insulation, drywall, vapor retardant latex paint.

I will be heating with wood and don't have any idea what the inside RH will be in the colder months, otherwise I might be able to use their formula and figure out what the dew point will be.

Also, the house will be air conditioned in the hot months and I do know the the outside humidity gets rather high sometimes.

I would rather not have to put any foam board over the sheathing to keep the inside surface from reaching the dew point, but will if it's really necessary.

Can anyone shed some light on this ?

Thanks,
Arky

[Alan Gage]

Sure is lots of good stuff on the Building Science site but sometimes I come away more confused than when I went in. I was just reading the one you linked to last night and it had be scratching my head a bit.

Have you looked at their "enclosures that work" section? Here are the ones for a mixed humid climate:

http://www.buildingscience.com/doctypes/enclosures-that-work/etw-building-profiles/bsc_climatemap_view?climates=mixed-humid#climatemap_anchor

Alan

[Alan Gage]

More info on rigid insulation and guidelines here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

Alan

[JRR]

My thinking is similar to "Energy Nerd's".  Thinner wooden walls are better than thicker wooden walls.  Most of the insulation should be exterior foam, none or little in the wall cavity.  Pure masonry walls with exterior foam may be the best of all worlds ... little chance for rot.  Works for all climates.

Too bad foam is so expensive.

[Arky217]

Well, according to greenbuildingadvisor.com,

"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.",

I guess I'm ok without any foam. I'm in zone 3.

That's their opinion anyway.

Arky

[JRR]

Does "any foam thickness" include "zero" ... no foam?   I'm no expert, but if I build again ... I intend to err on the side of safety.  I want exterior foam and as much as I can reasonably afford.

[Don_P]

Condensation is a dew point problem. Look at the interior temperature and relative humidity and find the dew point. If a surface drops below the dew point and that interior humidity contacts it, condensation occurs.

[Arky217]

Condensation is a dew point problem. Look at the interior temperature and relative humidity and find the dew point. If a surface drops below the dew point and that interior humidity contacts it, condensation occurs.

Does "any foam thickness" include "zero" ... no foam?   I'm no expert, but if I build again ... I intend to err on the side of safety.  I want exterior foam and as much as I can reasonably afford.

Well, I submitted this question on GreenBuildingAdvisor.com:

#########################
Is my wall design OK?
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 2x6 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.)
######################

And got the following two answers:

#################
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.
ANSWERED BY MARTIN HOLLADAY, GBA ADVISOR

#############

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.
ANSWERED BY ARMANDO COBO
###############

I'm leaning on not using any foam, and just attaching the vinyl siding directly to the Zip osb sheathing.
I'm not concerned about a higher R value wall, just about moisture damage to the Zip panels or the the wall studs or plates.
I know the wood heat causes the interior RH to be lower, so I think that I'll be ok.
Anyone think that I may be making a mistake ?

Thanks,
Arky