Tyvek is not a single product. There is a range of different Tyvek fabrics that are used for a wide range of purposes. A lot of the white suits that one sees in use in spray booths or when agricultural chemicals are being applied are made of one form of Tyvek. The material you see being applied between the sheathing and siding on houses is another form of Tyvek, called House Wrap. You can obtain Tyvek that is, for all practical purposes, waterproof. Or, you can obtain Tyvek that is water-resistant/breathable. Some Tyvek is as light as 1.3 oz per square yard; some, including House Wrap is nearer 2 oz per square yard. So, when you talk about Tyvek, bear in mind that it is not all the same; you need to be specific about the Tyvek to which you are referring.