Importance of Mash and Sparge Water Volumes using Batch Sparge

Also, over sparging can extract extra, undesirable tannins from the husks.

@Hopwise addressed the issue of efficiency and mash thickness. But the amount of sparge water will affect your efficiency as well.

The more water you sparge with, the more sugar you'll extract from the mash. The grist will absorb a constant amount of water (around 0.13 gallons per pound of grist). When you add your sparge water and stir, all the sugars are dissolved into the water. If you add a lot of water, the sugars are very dilute. If you add a little water, they're more concentrated. When you drain your mash tun, you're leaving behind jsut the water absorbed by the grain. If the sugars are highly concentrated (i.e. you added a small amount of sparge water), then you're leaving behind more sugar than if the sugars are dilute.

But this doesn't mean that you should add huge quantities of sparge water. The increase in extract is countered by a number of factors. For one, you'll have to boil longer to evaporate all that water you added. Also, you'll have to heat all that extra sparge water. The gains in efficiency are countered by using more energy and spending more time.

I calculate my sparge water volume from the boil time, and evaporation rate. For most beers I boil for 90 minutes. With my setup, this translates to an evaporation loss of around 1.5 gallons. So to get 5.5 gallons into the fermenter I aim for a pre-boil volume of around 7 gallons. You're mileage will, of course, vary.

Mash thickness has a small impact on your beer, but not really enough to stress about. I aim for a constant thickness, regardless of beer style. Some brewers give their stronger beers to have a thicker mash, while their low-gravity beers have a thinner mash.

The logic behind that latter approach is that thinner mashes encourage a more fermentable beer. My opinion is that lowering the mash temp a degree or two has a much bigger impact than mash thickness.

I don't believe that mash thickness will have much effect on your efficiency, as long as you are in the 'normal' range of mash thickness -- about 1-3 quarts of water per lb of grain.

As for how much sparge water you ues, the approach I follow is that First Runnings + Second Runnings (i.e. the sparge water) = Pre-Boil Volume.

You don't have to do it this way, but it's a good solid approach that doesn't waste water. To figure out how much water you'll need requires a small bit of math and knowledge about your system. You'll need to know how much water you'll lose to your mash tun's false bottom (1-2 liters is common) and how much water will be absorbed by the grain (about .9 liters per kg of grain for me).

  Water lost to grain
+ water lost to false bottom 
+ pre boil volume 
= total water needed for mash & sparge.