Fresh Air Ventilation Needed at Home?

As a preface to this post, I have been working as an engineer for the last 6 1/2 years.  My work is primarily focused on waterproofing buildings, but we also get involved with designs accommodate thermal and moisture transmission through the walls of the building.  I also have a little first hand experience, at my first apartment living in snow country.  This post primarily focuses on spaces located in cold winter climates, tropical climates have a unique situation that I am not familiar with in dealing with high humidity levels.

When I first moved to the Boston area, I learned that you can tape up plastic over the window openings on the inside to reduce the heat loss through the windows, we would still spend a couple hundred $’s heating our 2-bedroom apartment.  The plastic layer sort of creates an insulated unit similar to dual-pane windows.  Of course when you cover the windows, which may be the only source of exchanging the interior air with fresh outside air for many apartments, moisture from daily washing dishes, cooking, and showers can build up in the apartment.  Before covering the windows with plastic, we had so much condensation it caused mold growth on the rubber gaskets of the windows.

You will see moisture condense on the windows (when not covered by plastic) but you cannot see if water is condensing in the walls.  If you do put up plastic which prevents the moisture from getting to the window to condense, that doesn’t mean you don’t have a problem hidden in the wall.  This water in the wall can be just as harmful as  leakage from the outside.

There are few methods of solving this problem.  Open your windows/doors to allow the moist interior air to change with dry exterior air.  This will require more heating to warm the cold air coming in from outside.  Another alternative is to increase the interior temperature until the condensation does not form, again high heating costs.   Lastly, expensive renovation of the wall construction and heating system can be designed to work together for these conditions.  If you wish to monitor this condition in your living space, you can pick up a Hygrometer  (moisture meter) and then ventilate as necessary to keep the Relative Humidity (RH) below 70%.  Keeping it under 50% would be ideal.

As new designs try to be more green (saving on material and energy use) modern buildings and homes have less fresh air ventilation.  They are designed to operate with the minimum ventilation as required by code, but interior spaces typically still are not monitored for RH levels to prevent condensation and related problems.  I’m not suggesting that there is a problem with green design, I think it’s headed in the right direction.  I just believe there are many lessons to be learned as occurs with the development of any new technology.

Additional resources on the subject:


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