This is for a buddy of mine living in magnolia tx. House is 11 years old and was very under insulated. Is a 2x6 framing on 16" center's with osb sheathing, just regular fiberglass batts was all he had. House has some flood damage from all the rain so sheathing and stucco are being removed. Ok so the question he's wanting answered is where to put rigid foam sheathing when the stucco contractor says he won't apply stucco over foam? Stucco contractor says to put it under sheathing. Then how much to eliminate any chances of it getting wet from condensation? He's going with either cellulose or blown in fiberglass. Thoughts on this. Thanks

With proper air gap with weep screed & top venting of the stucco and the proper weather resistant barrier over the OSB there shouldn't be any problem with the OSB in a US climate zone 2/SE-TX climate.

If the foam is very thick there may be structural issues that need to be attended to where hurricane resistance is required. Any rigid-foam + 2x6 batts wall is well above the 2x4 /R13 code min for that climate zone. A 1" Huber ZIP-R solution might be easiest & cheapest overall. A 16" o.c. 2x6 wall with 1.8lb fiberglass and 1" ZIP-R would have about twice the thermal performance of a code min wall.

In a flood zone he's better off with fiberglass or rock wool than cellulose. If it floods again cellulose would wick the moisture all the way to the top plate and would hang onto substantial amounts of moisture until the place was gutted. With fiberglass it wouldn't wick as high, and most of the moisture would leave with the outgoing tide.

The average mid-summer outdoor dew point in your area is about 73F.  With properly vented stucco for much condensation to occur on the OSB it would have to be at a temperature average of 73F or less during the height of the cooling season, which isn't very likely.  Without the foam there is some potential for temperature striping at the studs that cool if somebody insists on turning it down to 65F indoors or something, but with the foam that temperature striping is much subdued, it almost disappears.  But stucco can deliver huge spikes of moisture when heated by the sun. If air gap between the stucco & OSB isn't vented top & bottom that high-dew point super-saturated air lingers, and average temperature at which the OSB takes on moisture can be quite a bit higher than with vented stucco.

So 1 inch should be sufficient? The pro's are doing all the work, so I'm sure everything will be up to code or above.


Thanks

It depends on what the design goals are, but a code-min R13 16" o.c. 2x4 wall with vented stucco siding and half-inch wallboard on the interior comes in at about R9.5-R10 after factoring in the thermal bridging of the studs.

With R20-something cavity fill and an OSB sheathed no-foam wall you'd be at about R13.5-R14 whole-wall, after factoring in the thermal bridging. If you used an integrated polyiso-laminated-to-OSB sheathing like ZIP-R, the 1" version adds another R3 to the stackup, and the 1.5" version adds another R6.

If you went with the 1.5" ZIP-R its the same as adding 1" of polyiso under half-inch gypsum, and you'd have a whole-wall R of about R20, which is 2x code-min or slightly better.

With the 1" ZIP-R it would be about R17 whole-wall, which is more than 1.5x code, if not quite 2x.

http://www.huberwood.com/zipsystem/products/zip-system-rsheathing

They have detail drawings of how to use any of their ZIP sheathing products under stucco siding here:

http://www.huberwood.com/search-tech/stucco

If you're looking for PassiveHouse certificaition or something you might need more, but in a US climate zone 2A location it's not necessarily financially rational. Read the first chapter of this document, and refer to Table 2 as a starting point for long term financial rationality:

http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

They're suggesting that for zone 2 an ~R15 whole-wall performance would still make sense in a location with average energy costs and average labor & material costs. That would be half-inch polyiso on a 2x6 16" o.c. wall with R20+ cavity fill, or a 1" ZIP-R solution. But since the labor cost of 1" ZIP-R and 1.5" ZIP-R is the same, and is lower than a foam + OSB dual-layered solution, there may still be a long term financial case for the 1.5" ZIP-R. Price it out both ways.

What WON'T make financial sense is closed cell foam cavity fill and no foam sheathing, but that too would come in at about R15 whole-wall, or a bit better, due to the very high cost of the foam.

I think some people are confused about what's going on. The house in question isn't mine, was built 11 years ago, framing was 2x6 on 16" center's, typical osb sheathing, stucco applied to code. So about 2 weeks ago we had nothing but rain and some parts flooded, my buddies house had 3 feet of water. Insurance is cutting him a check for repairs. Sheathing and all have been removed up about 4 feet and the studs are being dried out. He's not looking for any awards or anything, just get him a better r value by adding foam sheathing. But the issue was the stucco contractor said he won't apply stucco over foam, he recommended to put it under the osb. Question was how much to be sure there's no condensation forming on the back side of the osb?

Hey Dana1, I copied and pasted this whole subject to my buddy, he's going to look into the zip r sheathing. I cant tell you what he's doing to reinforce the structure, he and I myself thought any wood outside the protection of the foam was a bad thing. The stucco guy told him that's how they do it on the high end homes they build.





Ps, received a message saying build it with sips.