Spar thickness (1x2 VS 2x2)

Do i have to use 1 x 2 spars or can i use 2x2 ? i havent measured the thickness of insulation yet for the roof. Can i go with the 2x2 and use house insulation like R19 instead of the rigid foam stuff?? has anyone tried that??
Also, can i use pine (2x2) for the hatch spars or should i go with poplar like the generic benroy plans suggest because of the harder wood.
Thanks. Keith

Also, can i use pine (2x2) for the hatch spars or should i go with poplar like the generic benroy plans suggest because of the harder wood.
Thanks. Keith