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Bath Fans and the Tissue Test

Mike Holmes – Contractor, Reality TV Personality, Canadian, and all around “good guy.”  I must admit, I have watched a lot of Mike over the years.  Holmes on Homes, Home Free, Holmes Inspections…all good TV shows.  However, Mike recently posted a social media post that made me pause and reflect.  This post relates to bathroom fans and their ability to move air appropriately out of a bathroom.  When buying a house, Mike recommends using the “Tissue Test.” In his words, put a tissue next to a bathroom fan that is running and “Tissue test: if the fan holds the tissue, you know it’s pulling air. If not – replace it.”  As a builder who has seen several bathrooms and bathroom fans, I can tell you that Mike’s advice is not quite correct – yes it will tell you if the fan is working, but that isn’t the question you should be asking.  What you should be asking is whether the fan sized appropriately for the space.  Most architects and designers do not specify fan sizes and most builders get cheap fans that are not sized appropriately for the space.  Bathroom fans like any other ventilation device are designed to move a certain amount of air over a certain time.  The Home Ventilation Institute (HVI) recommends that fans in a bathroom should be sized to replace the air 8 times per hour when running[1].  HVI goes on to provide some “rules of thumb” for small bathrooms and larger ones, but these assume certain ceiling heights and volumes. I think it is much easier to calculate the fan size required.

Now as an example, I recently built a custom home with a very large master bathroom.  The size was approximately 14’ wide x 15’ long with 10’ ceilings. So:

Most fans that are supplied by Home Depot or Lowes have a flow rating around 90-120 cfm.  So, in this case, we decided to install two in-line hidden bath fans we purchased online, each with a flow rate of 190 cfm, for a total rating of 380cfm.  Why so large you may ask?  Don’t forget there will be pressure losses due to the ductwork (that discussion as well as the need for make up air is reserved for an entirely different post).  In the end, our result couldn’t have been better: There are no steam issues and there is no condensation on the bathroom ceiling.

Now, this was a new custom home, but the principles are the same for renovations and existing homes.  Size fans based on the volume of the bathroom.  The tissue test just doesn’t cover the complexity of the issue in my opinion.  Other issues you should consider: Fan noise rating, where the exhaust is ducted (you don’t want all that moisture in your attic), and fan aesthetics.



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