Just fitted this oak window for Joachim.
Natural oak on the inside. White on the outside.
Gaps packed with lime mortar-soaked oakum. Outside.
And topped off with lime mortar (NHL 3.5/sand). This is the best method for sealing between the window frame and masonry, and the window frame and oak lintel. It creates a weatherseal, while allowing the window frame to breathe.
The mortar is covered with wet hessian to slow down the curing.