Sunday, January 17, 2010

Double walls and suspended roof

The green gussets show the secondary roof hanging off the original roof rafters

Another view of rafters with gussets to secondary roof
that acts sort of like a suspended ceiling that will
eventually be filled with cellulose insulation

A close up of roof system

Last week Joe and I finished building in the secondary, or double, walls and roof in the house.

The purpose of this work is to create a 12" deep wall and 21" roof cavity along all the exterior surfaces of the house. The photos show these being built, but I wonder if the concept is clear. Imagine you built a wall stood it up and called it good. In this case you then build another wall exactly the same size and stood it up a few inches apart from the first so that the total depth of the wall is 12" from one side to the other. The same thing is happening at the roof except in this case the second "wall" is a ceiling being hung off the original rafters to create a total depth of 21" from outer surface to inner, once the sheetrock is in place.

Once all the plumbing and electrical work is done, well then staple into all the interior wall and roof framing a retaining fabric that will hold in the cellulose insulation that will fill these walls and roof bays.

The whole point here is to reach our R-value insulation goals. Dense packed cellulose has a R-value of 3.8 R per inch, so with a 12" thick wall we'll have a calulated R-value of 45.6. The roof, at 21" will have an R-value of 79.8.

A secondary benefit of building the walls and roof this way is that there is very little physical contact between the outside and inner walls and roof. This means there is very little opportunity for thermal bridging between the two (thermal bridging is cold moving in pathways from the outside to the inside. In typical construction, the 2x4's or 2x6's in the wall are thermal bridges for the outside to the inside.) The green straps in the photos are connecting gussets between the inner and outer walls and are the only point of connection between the inner and outer surfaces.

Another advantage of this way of building is that we do not have to drill through the studs in the walls to run the electrical cable or water lines as is typical in standard construction.

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