Here is a sketch of my ideal sauna, drawn in ideal conditions

on scratch paper in a dimly lit changing room, with a couple beers, taking a sauna with an architect and contractor friend, while at my island lake cabin:

1. Compact: 8′x12′ overall dimensions.  Most building codes don’t require a building permit for structures less than 100 sq. ft.
2. Efficient: a 6′x8′ hot room is a nice size.  Big enough to accommodate as a one person sauna to a six person sauna, yet small enough to heat quickly and efficiently.
3. Divine proportion: the golden ratio of architecture, harmony within nature, plus all your building materials come in dimensions that offer minimal cutting and waste (to burn in your sauna stove).
4. Two rooms: Critical in cold climates.  Just as grocery stores have double doors as an energy saver, your sauna should have a changing room.  It’s impossible to have an efficient sauna if the door from the hot room opens to the outside.  Close the door!
5. Dual benches: Consider pushing your internal wall a few inches, 6’4″ bench length is a magic dimension.  One person can stretch out, laying on the sauna bench or three people can sit comfortably.  Duel benches allow for good flow.  One can alter the two bench plan, and there is a good argument that having an “L” bench instead, modesty, privacy wise.
6. Versatile: A great sauna party by night can also be a nice home office escape or kids play area by day.  Storage above your hot room, and allow yourself to make a cabin style patio, that flows out from your changing room.  Turn your 8×12 sauna into a backyard escape.
7. Odds & Ends: A few tips to consider:

• Put a dimmer light in your hot room.
• 7′ internal ceiling in hot room, never higher.  Less unneeded cube mass to heat.
• Before cedar paneling your sauna/changing room, wire for sound and Ovi/Nokia mobile device technology.  Plus, I’m a fan of music in the sauna.

Here’s a 3-D picture of a pretty good mobile sauna.

• 8×12 structure
• 6×8 sauna room*
• 6×8 changing room
• dual benches on either side.
• stove centered along the back wall.
• 2×4 framed construction
• 5/12 roof pitch
• put a window in your sauna if you have a nice view.

I have been in over 100 saunas, and have sat in every one and have considered what’s good, bad and ugly with the design and construction of each sauna.

Some saunas are two man saunas, and don’t allow for a good sauna party.  Some saunas are bigger than a four man sauna and take too long to heat up, use too much wood, or the loyly (steam rising from tossing water on the rocks) never reaches you.  With divine proportion, according to the Greeks and confirmed by Da Vinci (thanks Matt), this is my ultimate sauna plan.

This design uses North American standard measurements, minimizing waste in construction and allowing for full cuts of material.  This design gives you a changing room, a critical space for privacy, having a beer between rounds, and a buffer for temperature extremes in cold climates.

TIP:  Frame the interior wall with 2×2′s vs. 2×4′s.  It is non load bearing, and the extra 2″ sure is valuable!

EXTRA TIP:  * The interior wall can be nudged slightly to allow for a slightly bigger sauna room (say 6’2″x7’4″ interior dimensions).  Four people can fit nicely along a 6’2″ bench, and it’s nice to lay along your sauna bench.  Remember, though, you’ll be having dual benches, a comfy sauna and ample changing room is the goal.   Having a bunch of people over for a sauna party?  don’t sweat it!  click here for the sauna party equation.