Window Condensation Calculator

Insulating shutters or window treatments that fit inside the window are a very cost effective way to reduce heat loss from windows.  We have a section on insulating window treatments that provides a raft of ways to do this.  I got an email from Dave in the UK pointing out that these kinds of insulating window treatments can also cause condensation on the inside window surface, and that this can be a problem.

I gave Dave some kind of vague and not very helpful answer on humid vs dry climates and the like.  Dave, being more of an engineer than me, was not satisfied with generalities, and came up with this very nice Window Condensation Calculator that can be used to predict exactly how much of an insulating treatment you can add before getting condensation -- yea Dave!

An insulating treatment installed on the inside of a window increases the likelihood of condensation forming on the window because the added insulating value of the treatment lowers the window surface temperature.  If the window surface temperature is lowered enough, it goes below the dew point of the room air, and condensation begins to form.  The lower the window surface temperature and the looser the window treatment fits, the more condensation you will get. 

There are basically two ways to address the problem:

1. Use a window treatment that fits snugly enough that very little air circulates between the window surface and the insulating treatment.
   
   
2. Choose a window treatment with an R value low enough that the window surface will not go below the room air dew point.  

Dave's calculator tells you what  the highest R value window treatment you can use and still avoid condensation on a loose fitting thermal shade -- very nice!
Dave's condensation calculator is based on Wolfgang's Dewpoint Calculator -- also very handy.

It turns out that this critical R value depend on:

• The room temperature and relative humidity
  
• The outside temperature
• The R values of the existing window and the insulating window treatment.

The room temperature and relative humidity allow you to calculate the dew point, and the two temperatures and 2  R values allow you to calculate the surface temperature of the inside of the glass.

Condensation is less likely with lower room humidity, warmer outside temperature, higher R value windows (e.g. double glazed), and lower R values for the insulating shutter.  You also get less condensation if the insulating shade fits tightly enough to reduce air flow between the shade and the window.   So, by playing around with these values, you should be able to find something that works in your situation.

Climate has a big influence -- our area is so dry that even though we routinely get below zero F temperatures and have high R value thermal shades, we rarely get condensation -- occasionally on VERY cold mornings we do get some beautiful frost patterns on the inside of the window glass.

Anyway, thanks to Dave for going the extra mile.

Gary