Looking for a good detail for using 1" rigid on exterior

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Permalink Reply by Scott cragun on November 25, 2010 at 9:28pm

I've used XPS rigid foam on both new construction and retro-fit applications, the amount of rigid foam ( R value) will depend on the climate zone you live in, colder climate zones require a higher percentage of the balance between exterior insulation and interior insulation to be on the exterior, for example I live and build in a maritime climate in Alaska, in our area we use a 1/3 - 2/3 rule for exterior insulation with a minimum of 1/3 of the total R value of the wall assembly being on the exterior side. I have used several different types of moisture barriers or house wraps, but usually rely on Tyvek Stucco wrap at the wall sheathing and cover the foam with Tyvek or Typar house wrap. Some will say this not neccessary but I know I sleep well at night knowing that there are several layers of protection between the elements and the wood of the structure.

Scott

Scott

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Permalink Reply by William Wesson on January 4, 2011 at 7:45am

Possibly overkill, but the Building Science Corp white paper might help:

 

http://www.buildingscience.com/documents/reports/rr-1012-residentia...

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Permalink Reply by Jim Peck on January 5, 2011 at 6:24am

Interesting Conclusions in the "White Paper".

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Permalink Reply by Ed Voytovich on January 5, 2011 at 7:34am

In Central NY we use 2" of XPS to keep the dew point at heating design temperature (-2°F) outside the wall assembly.  These homes are quite comfortable, get excellent blower numbers, and are well prepared to fend off the universal solvent.

 

Still, my general policy and universal recommendation is always the same:  "foam is good, more foam is better, and a s***load of foam is best of all."  In my opinion, the concept of "overkill" does not apply until the building achieves passivhaus -level performance.  Anything less is a compromise.  Energy consumed now to save energy indefinitely into the future is a wise investment, although it is one tough sell to a nation that likes to buy lottery tickets.

 

By the way . . . speaking of dew point, I understand that there is an app available for psychrometric calculations for the iPhone and one on the way for Droid. 

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Permalink Reply by Sloan Ritchie on January 5, 2011 at 9:16am

I discovered this comprehensive answer to the design question here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-...

thanks for all your suggestions!

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Permalink Reply by Leo Klisch on January 5, 2011 at 11:12am

Yes, the dew point is the issue but also has to do with cold air leakage through cracks around the foam panel joints and windows that ends up creating cold spots inside the wall for potential small spots of condensation inside the wall if inside moisture gets through gaps in the inside vapor barrier. If these are small, I suspect they can completely dry seasonally.

Leo K. PE

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Permalink Reply by Al Heath on January 5, 2011 at 3:00pm

I just finished a deep retrofit and used 2" polyisocyurate interiorly firectly over old rock, strapping, new rock.

 

This site has the best free articles around. 

 

http://www.buildingscience.com/documents/information-sheets/high-r-...

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