Moisture problems

Recently I have been called upon to advise on moisture problems in a number of houses. As I have mentioned in previous posts, a high performance, insulated house needs to be airtight. An airtight house needs to be ventilated.

The issue we have is that the more airtight the building envelope is the higher the internal humidity will be. This humidity needs to be addressed either manually, (windows) or mechanically, (fans).

At this point many people argue that airtightness is causing the moisture problem. However, airtightness doesn’t cause the problem, it just moves the location of the problem.

Without airtightness the moisture problem is a lot worse as it occurs out of sight inside the structural assembly. This causes structural decay, a reduction in the thermal performance, and the nasty one – hidden mould.

Airtightness prevents the moisture problem from occurring inside the structural assembly but it moves it to inside the rooms of the house where we can see it, and it obviously still needs to be addressed. This is where ventilation, heating, dehumidification etc. can be discussed.

We are not alone with our moisture problems. Here is an except from a recent article by ‘Building Knowledge Canada’:


Today’s energy-efficient homes are built tightly to seal out cold in Winter and keep in the cool in Summer. Even code-built homes are tighter than they have been historically. Thanks to this, it’s possible a new home may be damaged by lack of ventilation or by excess moisture.

Through laundry, bathing, washing dishes and just breathing, as many as 20l of moisture may be added to the indoor environment on a given day.

Most builders have likely received one of those dreaded phone calls: “Why do my windows have water and mould on them?” or, “My hardwoods are cracking and I don’t know why!” or even the panicked “My basement is leaking!”

Humidity control is a significant issue in new homes, especially as we push the energy performance of homes closer to Net Zero Energy. From swelling of engineered hardwood, musty smelling basements and window condensation to short cycling air conditioning units; understanding humidity and specifically relative humidity is important to all homebuilders, their HVAC partners and happy homeowners.

So, as you can see, internal humidity is an issue that occurs and needs to be understood. Our current building codes don’t adequately address moisture, nor do our energy efficiency thermal performance simulation tools. So it is up to us to educate ourselves, (designers, consultants, builders, and homeowners) in the field of building physics, and apply good strategies in order to deal with the moisture.

Have a look at my ‘build tight, vent right’ blog where I go into this in a little more detail.