# Flash and Batt in 2x4 wall system zone 5



## rferoni (Apr 13, 2013)

Hey all,

Looking to get a home built in climate zone 5. The builder we like has been building his homes for the last 12 years using the flash and batt system. Builder sprays 1/2" closed cell spray foam inside wall cavity with an R13 batt. Typical 2x4 wall with osb sheating and Tyvek on exterior. 

I've researched this to no end and it just does not seem like a good idea in our climate zone. Someone tell me I don't need to worry! We really like the homes. 

Opinions? 

Thanks,
Ron


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## Nealtw (Jun 22, 2017)

rferoni said:


> Hey all,
> 
> Looking to get a home built in climate zone 5. The builder we like has been building his homes for the last 12 years using the flash and batt system. Builder sprays 1/2" closed cell spray foam inside wall cavity with an R13 batt. Typical 2x4 wall with osb sheating and Tyvek on exterior.
> 
> ...


 I have not seen a house built with 2x4 outside walls for 30 years. 

And we are a little warmer out here.


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## APA (Jul 13, 2018)

A lot of guys are doing 15+5 where they put an inch of R5 foamboard on the outside and r-15 batt on the inside. That is the only way I would do a 2X4 wall now. Most are and have been using 2X6 walls here.


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## SPS-1 (Oct 21, 2008)

Does he put a vapor barrier in that assembly ? Where ?

Seems that the system as mentioned by APA is becoming popular. The exterior foam board is taped air tight and it becomes the vapor barrier. But for this to work, the exterior foam has to be thick enough that you don't get any condensation on the interior face of the foamboard. In Michigan, I don't think R5 (mentioned by APA) is quite sufficient to ensure no condensation on the inside face. I certainly don't see how R3 (half an inch of PU sprayfoam) will avoid condensation on the inside face.

But not sure if half an inch of closed cell sprayfoam is going to be a vapor barrier. But if he puts a conventional poly vapor barrier on the warm-in-winter side, what would be the point of the sprayfoam?

I am no expert, but I don't like it either.


.


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## rferoni (Apr 13, 2013)

Is Tyvek considered a vapor barrier? From exterior to interior the wall system is cladding(mostly vinyl siding)-Tyvek-OSB-sprayfoam-R13 batt-drywall.

Ron


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## SPS-1 (Oct 21, 2008)

I will stick to my previous opinion.


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