What I had:
An unfinished basement (20' X 26') with a 6' ceiling in my older (~1925) colonial. Sewer pipes protruding from the floor, steam pipes below the joists. A free-standing toilet (what were they thinking?), laundry hook-ups, gas boiler and water heater. Cement floor.
What I did:
1/2 basement became play area for kids, 1/2 HT/listening room. No wall dividing it, but kids don't play there when I am using the room. 1/2" drywall on all walls, 3/8" drywall nailed right to the ceiling joists, recessed lighting between joists. Boxed in the main beams and steam pipes (some of these extend a foot below the 6' ceiling). Built 3/4 bath where the free-standing toilet was. Commercial grade carpet right over the cement floor (damp basements are not a good place for carpet padding, and I needed to conserve every inch of headroom). Built walls around laudry area and boiler area. Wired a lot of outlets from one 15 amp breaker

, put in new windows (the basement little ones). Ran roof antenna and Dish cable into basement. Built a storage chest over the exposed sewer pipes. Used heating vent registers to cover and provide access to all electrical junctions, doorbell electronics, antenna cabling & splitters, pipe spigots. Ran speaker wire (16 gauge) to small shelves where I thought bookshelf surrounds would go. Put window a/c unit in window farthest from HT. Applied some foam panels to reflection points to tame excessive brightness in room. Bought a pair of high-back leather chairs with ottomans.
What I did wrong and need to do:
Used flat white paint, will have to re-do in flat black or charcoal. Used one 15 amp breaker. I would like 2 X 20 amp circuits dedicated. Door at top of stairs is solid but with a large window. Need to fill that in and add weather-stripping for noise control (I can hear everything from the kitchen that this door leads to). Portable heater not enough to heat in winter, and a/c unit is coupled to drywall, creating loud 60Hz hum. I am eventually getting a heat pump/cooling unit like this:
www.mrslim.com, ($2500-3000). Chair-backs too high for surround; eventually will get lower-back seating. Drywall is not a problem, but some of the recessed lighting fixtures resonate at certain frequencies. Some type of foam stripping should tame this. Boiler noise can be distracting. I need to replace slat bifold doors with solid core door. This must be custom due to 6' ceiling. Also, I am concerned with ventilation of boiler & water heater if I do this. In wall wiring now in the wrong place (now using tower surrounds, not bookshelfs), but I am not snaking cable. I should have used 12 gauge cable anyway.
On the plus side, the room is dark, and with the kiddies two flights up, I can crank my system and not bother any sleepers in the bedrooms.
