The most vulnerable part of a wooden nest-box is the roof. Sun, rain and frost will all destroy a wooden roof eventually. Decking oil is a good defence, as is roofing felt, but here is another idea by using Denso flashing tape. The Denso website describes it as "a tough, but conformable, grey aluminium laminate coated with a high melting point bitumen adhesive. The new foil laminate gives greater puncture resistance, improved tensile strength, elongation and tear strength"
Coating the roof of a nest-box may well suffice in keeping both the sun and rain off a box. If the box is kept dry, then frost should cause no harm.
Here, David has used Denso flashing tape to cover the whole box, which, in this case incorporates a double roof, suitable for a south-facing aspect.