Ventilation and wet towels

Insulation, airtightness and ventilation. Here in Australia, when it comes to the performance of our buildings, we mainly think about insulation. We are only just beginning to have the discussion about airtightness but we still don’t talk much about ventilation. As I have previously discussed with my Canadian building science friend, Gord Cook, Insulation, airtightness and ventilation go hand in hand. They all need to be considered together. An insulated house needs to be airtight, an airtight building needs to be ventilated. Click here to re-visit my previous post about this.

A ventilation stragtegy is all about control. A leaky house suffers from air infiltration. This air infiltration happens at different rates and moves according to varying conditions. Sometimes a leaky house leaks a lot, sometimes it doesn’t leak much. Sometimes it leaks through the holes from inside to outside and sometimes it leaks through the same holes from outside to inside, (Building Performance Lesson 4 – Air out equals Air in) and the holes are in places that we are unaware of. None of this leakage is under our control. The answer to controlling air is to first contain it. Containing the air is achieved with an airtightness strategy. Once we have contained the air we can then introduce a ventilation strategy to control the flow of the air.

So, what’s this got to do with wet towels? Stick with me for the following scenario.

After you have a shower you dry yourself with a towel, you then hang the towel on the towel rail and leave the exhaust fan running for a while to get rid of the airborne water vapour. We generally consider everything to be ok once the bathroom mirror is free of condensation. 24 hours later we repeat the process and dry ourselves with the dry towel………Let’s just think for a minute. How did the towel become dry? Where did the water from the wet towel go?

Remember Building Performance Lesson 3? Moisture moves from more to less. The moisture in the towel evaporates into the surrounding air and as an airborne gas moves via the path of least resistance to wherever the air takes it. In a leaky house this is generally through a hole in the building envelope where it can potentially do damage, (rot, mould etc.). In a tight house the water vapour doesn’t have as many holes to escape through so it finds the first convenient condensing surface, quite possibly on a window. This concept isn’t restricted to towels, it applies to any moisture generating activity in the house including bathing, cooking, laundry, and breathing just to name a few. This is where ventilation comes in.

The capacity for continuous ventilation that expels moisture laden air and introduces fresh air is the strategy here. One such system can be a bathroom exhaust, (Panasonic FV-24 or similar). These fans are very quiet and can be set to run continuously using less than 4 Watts. The fan exerts a slight negative pressure within the building envelope and provides a controlled path for the stale air to leave the building. This is quite a reasonable strategy for a building envelope with an air permeability of about 2.0 – 4.0 @50Pa. This will sort out the towel-drying issue and will also sort out the stale air from the rest of the house. Is this a perfect strategy? No, but it is a relatively inexpensive and effective way to incorporate ‘whole house’ continuous ventilation.

A balanced ventilation, filtration & heat recovery system such as an ERV or HRV is the optimal system. These systems are a must for a very tight house and a requirement for a certified Passivhaus. Unfortunately, at this stage of the game in Australia, these units have a large price tag and we are limited for choice. The more we focus on the airtightess of our building stock the more commonplace, (and hopefully more cost-effective) these units will become.


  • An insulated house needs an airtightness strategy.
  • An airtight house needs ventilation.
  • Infiltration is uncontrolled.
  • Ventilation is controlled.
  • Continuous ventilation will help with your household moisture issues……including your towels.

By the way. there is more to indoor air quality than just moisture. I limited this post to the moisture issue purely to keep it simple. Later we can talk about other indoor air quality issues such as Carbon Dioxide, Carbon Monoxide, VOC’s, dust, toxins, pollens etc.

For the purpose of demonstraing towel moisture, below is a bath towel weighed prior to use.

Below is the same bath towel weighed after use. 94 grams of moisture that needs to evaporate in order for the towel to dry.20190908_0848294679572407065560643.jpg

WarningDon’t show these types of photos to people at family gatherings. Aparently weighing your bath towel is not “normal.”