I have built a room off of my walkout basement for my Spa. The room is 10x12 with a 12' vaulted ceiling with 2 18x30 double insulated skylights. The east wall is all aluminum framed double insulated glass, north wall is framed with 1 window, insulated steel door and R-13 insulation. South wall is similar with 1 window. The west wall is the outside basement wall. Ceiling is insulated with R-19 fiberglas, with fanfold foam covering it as a vapor barrier. There is good airflow between the underside of the roof and the insulation with soffit vents and full ridge vent. All walls are R-13 fiberglas, also with fanfold foam. The ceiling is finished with tongue & groove pine treated with thompsons water seal. The walls are all finished with Tongue & Groove Cedar planking.

This room is not heated except for the incidental heat from the spa which keeps the temp at 50 when outside temperature is 18.

I have installed a 110 cfm vent fan in the ceiling which is controlled by a humidistat which turns the vent on at 65%.

I have been given differing opinions by non professionals on whether or not I should have make up air in this room for the vent fan or not. I have been told that due to the humidity I want to keep a negative pressure to help keep the moisture out of the walls. Other opinion is that I should install a passive barometic type vent to bring in outside air.

The room is sealed from the rest of the house by insulated steel french doors and when using the spa the humidity will get to about 90% but within 15 minutes of replacing the cover the vent has it back down to about 55%.

Do I need to provide make up air or should I keep a negative pressure??

Any other suggestions are welcomed also.