[Greenbuilding] rigid foam inside Larsen Truss?

[David Wentling]

Historically, we have used the ratio of 1/3 R-value outside of any vapor retarder.  Depending on the vapor characteristics of your foam, the amount of insulation R-value you add, and the framing and sheathing present, will determine if there would be a potential problem in the future. 

There are simple vapor models you can feed wall characteristics into and they estimate where condensation may occur.

You are right to be concern. Perhaps modeling may show just install the foam board on the exterior and apply siding over, skipping the cost of the Larsen truss. Or not.

You do to know what you can not measure.

David Wentling

On Jan 4, 2018, at 9:32 AM, Reuben Deumling <9watts at gmail.com> wrote:

I have a line on 3-1/2" rigid foam panels for pretty cheap. My 1894 house has (of course) 2x4 walls into which I've blown dense pack cellulose. For stage 2, I was going to add a Larsen truss (another 2x4 wall with a big cavity between inner and outer walls) and blow cellulose into the rest for an eventual 11" thick wall. But with these panels showing up I'm wondering if there are reasons not to place these as the middle layer of a cellulose sandwich? Seems like a quick and in this instance cheap way to get a lot of 'r'. But I also don't want to do anything stupid when it comes to moisture. 

Thoughts?
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