Exterior wood

I want to use 3/4"Baltic birch for my side walls. I don't plan on skinning the exterior because I want the birch wood look, not crazy about marine plywood finish (luan). I have been told Baltic birch is not an exterior rated plywood. What are my choices? Really don't want to go through the process of laminating with fiberglass. I'm sure I can seal the wood with varnish but am worried about extreme temperature fluctuations here in New England causing delaminating of the plywood. 
