Green Building - Truss roof

Truss Roof

Due to cost reasons, most houses are built with truss roofs.  In most cases there is also a material savings since trusses are typically made of 2x4s, while an equivalent stick frame house would like need 2x10s or larger for the top chord.  However few houses use attic trusses, so there is an opportunity loss there since the space under a standard truss roof is essentially unusable.  Depending on their size, roof trusses often need to be placed up where they will be installed by a crane because of being too heavy and awkward for a two man crew to put up there by hand.

When it comes to insulation the downside of trusses is the the bottom chord is often quite thin, so the temptation is to just blow at lot of insulation up there and call it R-60.  Alas, in its uncompressed state both fiberglass and cellulose have a much lower R-value per inch and in addition the first time someone walks between the webs of the truss (for example to run coax) every place you step the insulation compresses and never returns to its former thickness.  In fact, in terms of weight of insulation at the proper density, that 16" they put in the attic is often the equivalent of not even half that.  Batts have their own share of problems, but at least you're more likely to get the rated insulation value.

truss insulation

If the truss terminates at the outer edge of the wall (ie any roof overhang will be site-framed), then you want to use raised heel trusses so that you can get enough insulation out by the wall, otherwise you get a thin spot near the walls.  The alternative is to run the truss long and the box the part sticking out as a soffit.  In this case you either need to put some kind of blocking to keep insulation from going beyond the outside of the wall, or possible just let it go and not worry about it.  In the diagram at right, the pink area indicates where the insulation goes

 

Summary:  The big advantage is price and material efficiency, but at the expense of usable space.  Attic trusses are a decent compromise if the pitch of the roof is steep enough and the space is desirable.


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