Window Condensation Problems

I had a past client call the other day with a question about their windows. This summer they had some more work done on their home in Bath,

including new siding and more new windows.  Recently, as the temperatures outside got lower, they had been getting condensation inside the new windows. They’d never had condensation on either the original windows or the larger triple pane ones they had installed as part of our project together, so they wondered if the new windows were faulty.

I told them the problem probably isn’t with the windows, but with the humidity level in the house.  The triple pane windows didn’t have condensation because their interior surface stays warmer than the glass of the double pane windows they just installed.  The 1950’s windows they had just replaced didn’t get condensation as easily because they let more fresh air into the house through their leaky construction and installation.  I’m guessing they had more air sealing done as part of the siding work too.  So now the moisture from showers, cooking, houseplants, and everyday activities stays in the house longer; driving up the relative humidity and creating condensation on those nice new windows.

Two catch phrases in building science are “build tight, ventilate right”, and “the house is a system”.  My clients had made a lot of progress on getting their home more air tight, and I’m sure their energy bills will be lower this winter.  But since they had changed the part of the system of their house that brought in fresh air (even though it was brought in by accident through leaks, without any way of controlling it), they need to change their behavior to take out the extra moist air from showers and cooking. I expect running the bath fans and range hood longer than they used to will help.  If a house is tight enough, it may be necessary to have a dedicated ventilation strategy such as a heat recovery ventilator (HRV or ERV), a fresh air intake at the furnace, or an exhaust fan that runs on a timer.  Ventilating right also means not having a super-sized kitchen hood without providing make up air, because that has been known to cause dangerous back drafting at the furnace.

Most of us can’t upgrade everything in our homes all at once, so doing one project at a time is the most practical way to improve our efficiency.  Just remember that a change to one thing can affect a number of others.  So check in with a home performance professional or your architect to learn more about how the pieces fit together.