Question
We live in a wonderful 100y/o country home with no A/C.  The Southerly breezes during the spring and summer cool the house well but the winter is another story.  The prevailing winds come from the SSW until about Oct then change to WNW for the winter.  How far away from the house should we plant a windbreak of Emerald Green Arboravitae?  Thanks for your help

Answer
Hey, Rick -

The goal is to create an air pocket between the trees and the building, to get the wind to 搑ide over?the top of the building.

Picture of the basic windscreen idea:
http://s222.photobucket.com/albums/dd267/nowuntill/?action=view=windscreen.gif

How far distance to plant from the house?  Well, it depends on a number of factors:  Is it a one- or two-story home? How close will you plant the screen, and will they be a single 搑ow of soldiers?or are you going to mix in other trees & shrubs to increase the barrier effect? What is the anticipated wind speed?  How big will the tree抯 搒kirt?get? Big trees need lots of room.  These factors and decisions are not all science, but mostly artistry & learning from others mistakes.

If you have the room, I抎 layer the barrier, and if it is a single-story structure, back about 20 to 30 feet.  Arborvitae (Thuja occidentalis 'Emerald Green') may not be the best plant for the task, as it gets easily wind-burned in the winter in high-desert applications ?They need supplimental winter watering.  It may do just fine in your part of the country, growing 6?9?a year in the midwest.  

T.o. 慐merald?is THE most popular form, but only gets to about 5?wide x 15?tall.  There are other varieties of Arborvitae available, with different growth characteristics*, but may not be available in your area:

'Techny' (also known as 'Mission') is popular for hedging and screening, this broad pyramidal plant grows slowly to 15' tall. It holds its deep green foliage all year and considered one of the hardiest forms available.

'Nigra' has fine dark green winter color, this symmetrical, pyramidal form reaches 15' to 25' tall and 5' to 8' wide.

'Wintergreen' (also known as 'Hetz Wintergreen') is a superior pyramidal form which grows to 30' tall and 10' wide, this plant holds its dark green foliage color even through cold winters. It may also display better shade tolerance than other forms, but is not as fine-textured in foliage and habit.

'Lutea' (also known as 'George Peabody') is a large-growing pyramidal form that can exceed 30' tall. The most notable feature of this fine old form, is the bright yellow foliage that is strong most of the season.

(*source: University of Connecticut)

If it is a two-story structure, I抎 consider dense, rigid evergreen trees like the Colorado Blue Spruce (Picea pungens) or Douglas Fir (Pseudotsuga menziesii).  These larger trees should be set back from the house at least 30 feet.

I抎 even consider a low mound (2-3 feet high x 6-7 ft. wide) to plant them on, if you have the room.

good discussion &  how to implement a screen:
http://www.bobvila.com/HowTo_Library/Energy_Efficient_Windbreaks-Trees_and_Shrub...

Hope it helps. ~Marc