Cold Floors
We own an old timber cottage. It's sitting on concrete peers on a sloping block, so wind can get under the house and chill the floor boards down.

Insulating under the floors
To try and take the chill off the boards, we have put insulation under the floor boards. It's a fairly simple process as follows:

1. Staple builders string (the same stuff brickies use for their string lines) under the floor joists at intervals suitable to support the fibre glass insulation batts. You need a good stapler! I was stapling into hardwood joists. So I used staples with 6mm or 1/4 inch legs
2. Place the insulation batts between the floor joists resting them on the builders string. If neccessary cut out sections of the batts to allow them to get around obstacles without squishing them. If the joists are too wide apart to snuggly fit between the floor joists, cut extra strips of batt to fill big voids
3. Staple Sisalation (needs to be the perforated stuff that allows water vapour through it) to the underside of the joists.
4. Tape the joins and any perforations (e.g. Bathroom drains, water & gas piping etc). I bought the silvered tape at Bunnings. Sometimes this part makes you feel like you are packaging an elephant
5. Then use Pryda Foil Fix nails to permanently fix Sisalation to the bottom of joists at roughly 500mm centres. The staples may tear through the sisalation or pull out but the pryda foil fix nails should stay put.

The Benefits
Less Chilly
Taking the chill off the boards. As we did the insulation in stages, we tested the insulations effect by taking our socks off late at night and checking the floor temps from both the insulated and uninsulated areas. While the floors weren't warm in the insulated areas the chill had definately reduced. (Yes I know it's unscientific but I didn't have a infrared temperature taking gun).

Quieter
Our uninsulated floors allowed noise to travel from outside to inside our house and vice versa. Putting insulation under the floor reduces noises. My wife says she can no longer hear me stomping around inside when she's in the Garden.

Reduced Dust Incursion
Our floorboards are old and have gaps in the form of broken and missing tongues due to our house being relocated after the 1955 Maitland floods. This allows dust up through the floor into the house in places. The insulation and sisalation has reduced this considerably.