Drying of excess moisture in building materials in building projects is crucial if healthy buildings are to be obtained. Mostly that requires a preliminary heating system combined with even more machinery like dehumidifiers to get low risks of mould growth during construction. Up till now the understanding of how to obtain and maintain good drying climate different seasons has been poor in Sweden. Often the companies involved use simple thumb rules like x watts per square meter for the machinery not regarding even what kind of building material is used or how airtight and insulated the building envelope is at the moment. This approach produce a great spread in drying cost alternatively a great spread in drying result. You don’t get your risk exposure or cost until after construction when it’s too late to do anything about it. Another problem is that ordinary energy calculation programs aren’t designed for varying air tightness and varying insulation over time. Also they mostly calculate with normalized climate data while a construction project needs to evaluate risk costs for bad luck with weather. An alternative to drying materials in situ is to buy pre dried or robust materials often at a higher material cost. In Sweden this approach haven’t been used as much as you would think considering optimizing project cost. A qualitative and quantitative model for evaluating drying cost with developing building envelope and varying outdoor climate is presented