A scissors truss is a roof truss that creates a sloped ceiling instead of a horizontal ceiling. Unlike parallel-chord trusses — a type of truss that creates a sloped ceiling that is just as steep as the roof — a scissors truss creates a ceiling that has a gentler slope than the roof slope. You end up with a steep roof and a not-so-steep ceiling — in other words, a ceiling that is almost a cathedral ceiling but not quite.
A scissors truss creates a cramped attic that is very tight at the eaves and somewhat more generous in height near the ridge. It’s not the type of attic that you want to spend much time in, since it is far too cramped to negotiate easily.
Builders sometimes specify scissors trusses in hopes that the trusses will create a cathedral-like effect with less hassle than rafters or parallel-chord trusses, which can be tough to insulate well.
Surprisingly, some builders install scissors trusses before they have finalized their insulation plan. Then they post a question on the GBA site: “How do I insulate my scissors trusses?”
Scissors trusses can be tricky to insulate, because loose insulation doesn’t generally like to be installed on a slope unless it is fully encapsulated by rigid materials (due to the risk that the insulation will slide downhill). Whatever insulation plan you come up with, talk to your insulation contractor before you order your trusses, because there is no use in specifying an insulation method that your local contractor won’t agree to.
A variety of insulation materials can be used to insulate scissors truss roofs, as long as: