Trained Eye


Hot Tub in Basement


Every week I read your articles, as I am a do-it-your-selfer.

I have recently moved my hot tub from the sunroom to the basement to create a kids playroom in the sunroom. As the basement and hot tub are not used regularly, it would be a perfect place to install it. A powerful fan will remove the excess moisture that will appear when opening the tub-cover.

I would like to install the tub in one of the corners of the basement, which is not used at all.  Once installed, and after the tub has been insulated, I have a concern about building up moisture in the lower corner because there is no airflow at all in that area. 

Please advise if you think ventilation is required, or if there is no concern as it is a closed area.


Adding a large fixture such as a hot tub in a basement can dramatically change the humidity level due the release of moisture into the air, when the tub is in use. I think you should not only take into consideration the possibility of condensation around the tub, but the effects on the entire home. A house should be thought of as a single system, with many components or smaller systems within. Moving your hot tub from one area to another will change the conditions in both locations and may create unforeseen problems.

Basements are an area of particular concern for hot tubs due to the temperature difference between the basement and the rest of the building. The coolness of the concrete foundation walls may also be a factor, as condensation is more likely against the cool walls. Condensation occurs when warm, moist air is cooled, or hits a cool surface. The warm vapour emerging from the tub will be more likely to cool quickly in the basement than anywhere else in your home. This may also occur in places not directly around the tub, such as basement windows, electrical outlets and fixtures, and heating ducts. This may lead to moisture damage of building materials and mould growth.

If there is an area around or under the tub that has little or no air movement, there will undoubtedly be some condensation in that spot. If this dampness cannot dry quickly, conditions for mould growth and rot are ideal. Increasing ventilation should be done to minimize the effects of the added hot tub. This may be accomplished in several ways.

For the warm months, central air conditioning should be used regularly or installed, if not already in the home. Central air conditioning dehumidifies the house air considerably in the summer, when ambient relative humidity is the highest. If no heat or return air registers are present in the corner in question, they should be installed to increase air movement in this location. Direct air from the furnace ducting will help dry the humid air in this area year round, and the heat in the winter will prevent condensation. Use of the exhaust fan is a must, and you should ensure that a properly insulated duct, directly vented to the outside of the house is properly installed.

Even if all of these recommendations are followed, there still may be a build-up of moisture and condensation in the house, due to the limited use of the basement. The ultimate solution, if this remains a problem, is to install a mechanical ventilation system. These systems, called HRV’s or ERV’s bring fresh outside air into the home and exhaust house air, on a regular basis. They have built-in heat saving devices that prevent a large loss of heat in the cold months, preventing higher heating bills associated with natural ventilation methods.

Proper installation of an HRV should take into account the exhaust fan for the hot tub area and may have a direct duct connection to this fan to help remove the excess moisture produced. This would be the ideal method of preventing moisture problems associated with the new move, but is costly and may not be needed. Use of moisture-resistant insulation, such as rigid foam or blow-in foam, to insulate the tub and foundation walls, will also help prevent moisture damage. Proper air-vapour and sealing techniques must also be used to prevent warm air leakage into insulated cavities.

You are correct in your concern for localized problems associated with the hot tub in the basement, but the larger picture must also be looked at. This large plumbing fixture will add a tremendous amount of moisture to the air in the home, which may have an overall effect on air quality. Thinking of the entire space inside the home as one partially closed system may help your understanding of the need for increased ventilation, associated with the addition of the hot tub.