A composite structure, comprising: a first extrusion; a second extrusion disposed adjacent the first extrusion with a space there between, each of the first extrusion and the second extrusion having a channel (12C2, 14C2); a first cap extending between and joining the first extrusion and second extrusion and defining a three-dimensional volume bounded by the first extrusion, the second extrusion and the first cap, the first cap being made from a material having a thermal conductivity less than that of the first extrusion and the second extrusion, wherein the first cap has a pair of insertion legs extending from opposing sides of the cap, each of the pair of insertion legs extending in the same direction, wherein each of the channels is parallel with one of the pair of insertion legs, and wherein the channels face toward the first cap such that each of the pair of insertion legs insert into one of the channels; a foam fill at least partially filling the volume, wherein the channels have extensions (12E, 14E) along a length thereof, the foam fill mechanically keying to the extensions; and a pair of end caps (22L, 22R) bridging the first extrusion, the second extrusion and the first cap and perpendicularly thereto, a first end cap of said pair of end caps applied to one end of the first extrusion, the second extrusion and the first cap, and a second end cap of said pair of end caps applied to a second end of the first extrusion, the second extrusion and the first cap. a first extrusion; a second extrusion disposed adjacent the first extrusion with a space there between, each of the first extrusion and the second extrusion having a channel (12C2, 14C2); a first cap extending between and joining the first extrusion and second extrusion and defining a three-dimensional volume bounded by the first extrusion, the second extrusion and the first cap, the first cap being made from a material having a thermal conductivity less than that of the first extrusion and the second extrusion, wherein the first cap has a pair of insertion legs extending from opposing sides of the cap, each of the pair of insertion legs extending in the same direction, wherein each of the channels is parallel with one of the pair of insertion legs, and wherein the channels face toward the first cap such that each of the pair of insertion legs insert into one of the channels; wherein the first cap has a pair of insertion legs extending from opposing sides of the cap, each of the pair of insertion legs extending in the same direction, wherein each of the channels is parallel with one of the pair of insertion legs, and wherein the channels face toward the first cap such that each of the pair of insertion legs insert into one of the channels; a foam fill at least partially filling the volume, wherein the channels have extensions (12E, 14E) along a length thereof, the foam fill mechanically keying to the extensions; and a pair of end caps (22L, 22R) bridging the first extrusion, the second extrusion and the first cap and perpendicularly thereto, a first end cap of said pair of end caps applied to one end of the first extrusion, the second extrusion and the first cap, and a second end cap of said pair of end caps applied to a second end of the first extrusion, the second extrusion and the first cap.