[Greenbuilding] Using Framing Cavities for ERV duct chases

[Eli Talking]

Conventionally, ERV ducting uses metal round pipes and flex ducts for delivering supply fresh air and return air.  However, as noted in earlier messages, I am planning an exterior wall and roof mounted insulation, leaving the framing cavities available for other purposes.  I expect there may be good reasons why this is not a good idea.  However, I do not know them.  

Heating to be achieved with direct air supplied wood stove and passive solar.  Immediately we are not installing active dehumidification.  However, we are contemplating some form of desiccant where desiccant can be recharged by solar or wood during the summer months, therefore, giving benefit for latent heat recovery of ERV in addition to sensible heat recovery of HRV.   

On the plus side, these frame cavities reach to all parts of the house and exist without extra construction. They provide a much larger area when compared to round ducts.  50si for 8” round duct.  A 2x6 stud cavity with 1.5 inch subtracted from width for band joist between floors leaves a 4”x 22.5”=92si, a 2x10 joist cavity is 14x9=126si, much larger than the 8” round duct. The air speed of a given flow that significantly affects sound is louder with smaller cross section area.  I know that the shape of stud and joist cavities is not as efficient.  However, I would think that the additional size would make up for the shape.  Air grilles could be balanced to get the appropriate air to all the spaces served.  With an ERV, our temperature differences are not as great as ducts intended for air conditioning and heating.  

Looking in our Virginia Residential Code which is for the most part based on the International Residential Code says the following:  

Use of gypsum products to construct return ducts or plenums is permitted, provided that the air temperature does not exceed 125F and exposed surfaces not subject to condensation.  

I could line the framing cavity with gypsum drywall and call it a duct for returns.  

Stud wall cavities and spaces between solid floor joist to be used as air plenums shall comply with:

These cavities or spaces shall not be used a plenum for supply air.  
Stud cavities shall not convey air from more than one floor level.

These rules pretty much kills the concept.  However, I want to know why.  All air ducts pass air through assemblies that create some vulnerability to flame spread.  If cavities were lined with gypsum and air sealed, they should be fire protected.  The supply air is where the need is greatest to get to all the different spaces with easy access to top or bottom grille as is best for functioning.  

I guess I could make a sheet metal box for stud cavities and use conventional round ducts in joist space to comply with code.  

Achieving cost affective solutions is critical to finding acceptance in the market.  Wherever we can, we need to look for ways of achieving the functionality with lower cost and materials.  

I look forward to hearing from list participants. 

Eli 
www.conservationarchitect.net 



Stud cavities shall not convey air from more than one floor level. 
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