Eliminating Thermal Bridging (Residential)

I came across an article that describes a slightly different way of framing a standard 2×4 wall that eliminates thermal bridging.  There is nothing extraordinary here, no technological innovations, but the more I thought about this framing addition, the more it made sense.  It will add to the first cost of the construction slightly (materials and labor) but it will save on yearly air conditioning costs, paying itself back over time.  I am a firm believer that a building, especially a home, should function passively when it comes to energy so that smaller MEP systems can be fitted without the need to expensive renewable that will effectively never pay themselves back.

The “Mooney Wall” adds extra interior furring and spray insulation.  Picture a standard 2×4 wall with added 2×2 16″O.C. but running in the perpendicular direction (see pictures in article).  These 2×2’s extend beyond where the 2×4 ends since they are mounted and screwed into the structural studs.  Once the wall is sprayed solid with insulation, thermal bridging is virtually eliminated.  The various parallel paths are as follows:  Exterior sheathing and cladding -> A) full depth insulation, B) insulation meeting the back of a 2×2 C) through a 2×4 stud then through insulation -> interior finish.  There is no direct flow through a stud directly to the interior finishes.  This assembly focuses on conductive flows through these materials.  The convective and re-radiated flows would be very similar to the standard 2×4 construction.  There is a decent amount of added mass so the re-radiation, if any, would be reduced.

Still the weakest point of this assembly, conductively, is when the flow goes through the 2×4 stud and then through 2″ of spray insulation (C above).  This would be worse than 4″ of insulation thickness meeting a 2×2 stud or even the full 6″ of insulation (assuming a 6″ nominal insulation layer from the 2×2 mounted on the 2×4).  The only thermal bridging left is when the 2×2 is mounted onto the 2×4 at various locations.  Instead of the standard 2×4 framing when you get linear slices of bridging, this solution gives you point sources of thermal bridging.  The founders claim an R19 to R21 value for the wall versus R13 to R14.  Such a simple way to solve such a common issue.

http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm

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