Matthew, If you have soffit vents, then you absolutely need to install baffles to prevent wind washing. The key is to get the outside air above the top of the cellulose (assuming the cellulose is on the attic floor). There are products available at home improvement stores or you can do it yourself with cardboard. You want to ensure the bottom edge and sides are fully sealed so no air is able to get underneath the cellulose. If you don't have soffit vents then you don't have to worry about wind washing. Before you install any cellulose on the attic floor, you should have a qualified energy auditor test your home with a blower door for air leakage between the house and the attic floor. You always want to air seal before insulating.
Adding attic insulation
Adding attic insulationAsked by Matthew Tannehill
I am adding insulation to my attic and was wondering if you could explain to me how to go about installing a wind wash barrier and is this necessary ( I will be adding cellulose insulation). Also, do I still need to install baffles if i install wind wash barrier or is it one or the other.