Quality #1 Northern White Cedar.  
Kiln Dried
1-5/16" Thick
3-5/16" Face covering
 
6', 7', and 8' lengths available
 
Please note: notches for barrel sauna front & back walls are NOT included.  This is easily doable for DIYers with a router and a straight bit.
 
At this time we do no offer "clear" knot free wood.  Based on customer reviews, heat retention in knots is unnoticeable to the touch.